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Gray, Henry, 1825–1861.
Anatomy of the human body, by Henry Gray. 20th ed., thoroughly rev. and
re-edited by Warren H. Lewis.
Philadelphia: Lea & Febiger, 1918.
1396 p.: ill. (some col.); 27 cm. Includes bibliographies and index.
Lewis, Warren Harmon, 1870–.
1-58734-102-6.
Gray, Henry. Anatomy of the Human Body. Philadelphia: Lea & Febiger,
1918; Bartleby.com, 2000. www.bartleby.com/107/. [Date of Printout].
2. The Digestive Apparatus
(Apparatus Digestorius; Organs Of Digestion)
The apparatus for the digestion of the food consists of the digestive tube and of certain
accessory organs.
The Digestive Tube (alimentary canal) is a musculomembranous tube, about 9 metres
long, extending from the mouth to the anus, and lined throughout its entire extent by
mucous membrane. It has received different names in the various parts of its course: at its
commencement is the mouth, where provision is made for the mechanical division of the
food (mastication), and for its admixture with a fluid secreted by the salivary glands
(insalivation); beyond this are the organs of deglutition, the pharynx and the esophagus,
which convey the food into the stomach, in which it is stored for a time and in which also
the first stages of the digestive process take place; the stomach is followed by the small
intestine, which is divided for purposes of description into three parts, the duodenum,
the jejunum, and ileum. In the small intestine the process of digestion is completed and
the resulting products are absorbed into the blood and lacteal vessels. Finally the small
intestine ends in the large intestine, which is made up of cecum, colon, rectum, and
anal canal, the last terminating on the surface of the body at the anus.
The accessory organs are the teeth, for purposes of mastication; the three pairs of
salivary glands—the parotid, submaxillary, and sublingual—the secretion from which
mixes with the food in the mouth and converts it into a bolus and acts chemically on one
of its constituents; the liver and pancreas, two large glands in the abdomen, the
secretions of which, in addition to that of numerous minute glands in the walls of the
alimentary canal, assist in the process of digestion.
The Development of the Digestive Tube.—The primitive digestive tube consists of two
parts, viz.: (1) the fore-gut, within the cephalic flexure, and dorsal to the heart; and (2)
the hind-gut, within the caudal flexure (Fig. 977). Between these is the wide opening of
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the yolk-sac, which is gradually narrowed and reduced to a small foramen leading into the
vitelline duct. At first the fore-gut and hind-gut end blindly. The anterior end of the foregut is separated from the stomodeum by the buccopharyngeal membrane (Fig. 977); the
hind-gut ends in the cloaca, which is closed by the cloacal membrane.
FIG. 977– Human
embryo about fifteen days old. Brain and heart represented from right
side. Digestive tube and yolk sac in median section. (After His.) (See enlarged image)
The Mouth.—The mouth is developed partly from the stomodeum, and partly from the
floor of the anterior portion of the fore-gut. By the growth of the head end of the embryo,
and the formation of the cephalic flexure, the pericardial area and the buccopharyngeal
membrane come to lie on the ventral surface of the embryo. With the further expansion of
the brain, and the forward bulging of the pericardium, the buccopharyngeal membrane is
depressed between these two prominences. This depression constitutes the stomodeum
(Fig. 977). It is lined by ectoderm, and is separated from the anterior end of the fore-gut
by the buccopharyngeal membrane. This membrane is devoid of mesoderm, being formed
by the apposition of the stomodeal ectoderm with the fore-gut entoderm; at the end of the
third week it disappears, and thus a communication is established between the mouth and
the future pharynx. No trace of the membrane is found in the adult; and the
communication just mentioned must not be confused with the permanent isthmus
faucium. The lips, teeth, and gums are formed from the walls of the stomodeum, but the
tongue is developed in the floor of the pharynx.
The visceral arches extend in a ventral direction between the stomodeum and the
pericardium; and with the completion of the mandibular arch and the formation of the
maxillary processes, the mouth assumes the appearance of a pentagonal orifice. The
orifice is bounded in front by the fronto-nasal process, behind by the mandibular arch,
and laterally by the maxillary processes (Fig. 978). With the inward growth and fusion of
the palatine processes (Figs. 50, 51), the stomodeum is divided into an upper nasal, and a
lower buccal part. Along the free margins of the processes bounding the mouth cavity a
shallow groove appears; this is termed the primary labial groove, and from the bottom
of it a downgrowth of ectoderm takes place into the underlying mesoderm. The central
cells of the ectodermal downgrowth degenerate and a secondary labial groove is formed;
by the deepening of this, the lips and cheeks are separated from the alveolar processes of
the maxillæ and mandible.
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The Salivary Glands.—The salivary glands arise as buds from the epithelial lining of the
mouth; the parotid appears during the fourth week in the angle between the maxillary
process and the mandibular arch; the submaxillary appears in the sixth week, and the
sublingual during the ninth week in the hollow between the tongue and the mandibular
arch.
FIG. 978– Head
end of human embryo of about thirty to thirty-one days. (From model by
Peters.) (See enlarged image)
FIG. 979– Floor
of pharynx of human embryo about twenty-six days old. (From model by
Peters.) (See enlarged image)
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FIG. 980– Floor
of pharynx of human embryo of about the end of the fourth week. (From
model by Peters.) (See enlarged image)
The Tongue (Figs. 979 to 981).—The tongue is developed in the floor of the pharynx,
and consists of an anterior or buccal and a posterior or pharyngeal part which are
separated in the adult by the V-shaped sulcus terminalis. During the third week there
appears, immediately behind the ventral ends of the two halves of the mandibular arch, a
rounded swelling named the tuberculum impar, which was described by His as
undergoing enlargement to form the buccal part of the tongue. More recent researches,
however, show that this part of the tongue is mainly, if not entirely, developed from a pair
of lateral swellings which rise from the inner surface of the mandibular arch and meet in
the middle line. The tuberculum impar is said to form the central part of the tongue
immediately in front of the foramen cecum, but Hammar insists that it is purely a
transitory structure and forms no part of the adult tongue. From the ventral ends of the
fourth arch there arises a second and larger elevation, in the center of which is a median
groove or furrow. This elevation was named by His the furcula, and is at first separated
from the tuberculum impar by a depression, but later by a ridge, the copula, formed by
the forward growth and fusion of the ventral ends of the second and third arches. The
posterior or pharyngeal part of the tongue is developed from the copula, which extends
forward in the form of a V, so as to embrace between its two limbs the buccal part of the
tongue. At the apex of the V a pit-like invagination occurs, to form the thyroid gland, and
this depression is represented in the adult by the foramen cecum of the tongue. In the
adult the union of the anterior and posterior parts of the tongue is marked by the Vshaped sulcus terminalis, the apex of which is at the foramen cecum, while the two limbs
run lateralward and forward, parallel to, but a little behind, the vallate papillæ.
FIG. 981– Floor
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of pharynx of human embryo about thirty days old. (From model by Peter.)
(See enlarged image)
The Palatine Tonsils.—The palatine tonsils are developed from the dorsal angles of the
second branchial pouches. The entoderm which lines these pouches grows in the form of
a number of solid buds into the surrounding mesoderm. These buds become hollowed out
by the degeneration and casting off of their central cells, and by this means the tonsillar
crypts are formed. Lymphoid cells accumulate around the crypts, and become grouped to
form the lymphoid follicles; the latter, however, are not well-defined until after birth.
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FIG. 982– Sketches
in profile of two stages in the development of the human digestive tube.
(His.) A X 30. B X 20. (See enlarged image)
The Further Development of the Digestive Tube.—The upper part of the fore-gut
becomes dilated to form the pharynx (Fig. 977), in relation to which the branchial arches
are developed (see page 65); the succeeding part remains tubular, and with the descent of
the stomach is elongated to form the esophagus. About the fourth week a fusiform
dilatation, the future stomach, makes its appearance, and beyond this the gut opens freely
into the yolk-sac (Fig. 982, A and B). The opening is at first wide, but is gradually
narrowed into a tubular stalk, the yolk-stalk or vitelline duct. Between the stomach and
the mouth of the yolk-sac the liver diverticulum appears. From the stomach to the rectum
the alimentary canal is attached to the notochord by a band of mesoderm, from which the
common mesentery of the gut is subsequently developed. The stomach has an additional
attachment, viz., to the ventral abdominal wall as far as the umbilicus by the septum
transversum. The cephalic portion of the septum takes part in the formation of the
diaphragm, while the caudal portion into which the liver grows forms the ventral
mesogastrium (Fig. 984). The stomach undergoes a further dilatation, and its two
curvatures can be recognized (Figs. 983, B, and 984), the greater directed toward the
vertebral column and the lesser toward the anterior wall of the abdomen, while its two
surfaces look to the right and left respectively. Behind the stomach the gut undergoes
great elongation, and forms a V-shaped loop which projects downward and forward; from
the bend or angle of the loop the vitelline duct passes to the umbilicus (Fig. 984). For a
time a considerable part of the loop extends beyond the abdominal cavity into the
umbilical cord, but by the end of the third month it is withdrawn within the cavity. With
the lengthening of the tube, the mesoderm, which attaches it to the future vertebral
column and carries the bloodvessels for the supply of the gut, is thinned and drawn out to
form the posterior common mesentery. The portion of this mesentery attached to the
greater curvature of the stomach is named the dorsal mesogastrium, and the part which
suspends the colon is termed the mesocolon (Fig. 985). About the sixth week a
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diverticulum of the gut appears just behind the opening of the vitelline duct, and indicates
the future cecum and vermiform process. The part of the loop on the distal side of the
cecal diverticulum increases in diameter and forms the future ascending and transverse
portions of the large intestine. Until the fifth month the cecal diverticulum has a uniform
caliber, but from this time onward its distal part remains rudimentary and forms the
vermiform process, while its proximal part expands to form the cecum. Changes also take
place in the shape and position of the stomach. Its dorsal part or greater curvature, to
which the dorsal mesogastrium is attached, grows much more rapidly than its ventral part
or lesser curvature to which the ventral mesogastrium is fixed. Further, the greater
curvature is carried downward and to the left, so that the right surface of the stomach is
now directed backward and the left surface forward (Fig. 986), a change in position
which explains why the left vagus nerve is found on the front, and the right vagus on the
back of the stomach. The dorsal mesogastrium being attached to the greater curvature
must necessarily follow its movements, and hence it becomes greatly elongated and
drawn lateralward and ventralward from the vertebral column, and, as in the case of the
stomach, the right surfaces of both the dorsal and ventral mesogastria are now directed
backward, and the left forward. In this way a pouch, the bursa omentalis, is formed
behind the stomach, and this increases in size as the digestive tube undergoes further
development; the entrance to the pouch constitutes the future foramen epiploicum or
foramen of Winslow. The duodenum is developed from that part of the tube which
immediately succeeds the stomach; it undergoes little elongation, being more or less fixed
in position by the liver and pancreas, which arise as diverticula from it. The duodenum is
at first suspended by a mesentery, and projects forward in the form of a loop. The loop
and its mesentery are subsequently displaced by the transverse colon, so that the right
surface of the duodenal mesentery is directed backward, and, adhering to the parietal
peritoneum, is lost. The remainder of the digestive tube becomes greatly elongated, and
as a consequence the tube is coiled on itself, and this elongation demands a corresponding
increase in the width of the intestinal attachment of the mesentery, which becomes folded.
FIG. 983– Front
view of two successive stages in the development of the digestive tube.
(His.) (See enlarged image)
FIG. 984– The primitive mesentery of
a six weeks’ human embryo, half schematic.
(Kollmann.) (See enlarged image)
FIG. 985– Abdominal
part of digestive tube and its attachment to the primitive or common
mesentery. Human embryo of six weeks. (After Toldt.) (See enlarged image)
At this stage the small and large intestines are attached to the vertebral column by a
common mesentery, the coils of the small intestine falling to the right of the middle line,
while the large intestine lies on the left side. 158
The gut is now rotated upon itself, so that the large intestine is carried over in front of
the small intestine, and the cecum is placed immediately below the liver; about the sixth
month the cecum descends into the right iliac fossa, and the large intestine forms an arch
consisting of the ascending, transverse, and descending portions of the colon—the
transverse portion crossing in front of the duodenum and lying just below the greater
curvature of the stomach; within this arch the coils of the small intestine are disposed
(Fig. 988). Sometimes the downward progress of the cecum is arrested, so that in the
adult it may be found lying immediately below the liver instead of in the right iliac
region.
Further changes take place in the bursa omentalis and in the common mesentery, and
give rise to the peritoneal relations seen in the adult. The bursa omentalis, which at first
reaches only as far as the greater curvature of the stomach, grows downward to form the
greater omentum, and this downward extension lies in front of the transverse colon and
the coils of the small intestine (Fig. 989). Above, before the pleuro-peritoneal opening is
closed, the bursa omentalis sends up a diverticulum on either side of the esophagus; the
left diverticulum soon disappears, but the right is constricted off and persists in most
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adults as a small sac lying within the thorax on the right side of the lower end of the
esophagus. The anterior layer of the transverse mesocolon is at first distinct from the
posterior layer of the greater omentum, but ultimately the two blend, and hence the
greater omentum appears as if attached to the transverse colon (Fig. 990). The
mesenteries of the ascending and descending parts of the colon disappear in the majority
of cases, while that of the small intestine assumes the oblique attachment characteristic of
its adult condition.
FIG. 986– Reconstruction
of a human embryo of 17 mm. (After Mall.) (See enlarged image)
The lesser omentum is formed, as indicated above, by a thinning of the mesoderm or
ventral mesogastrium, which attaches the stomach and duodenum to the anterior
abdominal wall. By the subsequent growth of the liver this leaf of mesoderm is divided
into two parts, viz., the lesser omentum between the stomach and liver, and the falciform
and coronary ligaments between the liver and the abdominal wall and diaphragm (Fig.
989).
FIG. 987– Diagrams
to illustrate two stages in the development of the digestive tube and its
mesentery. The arrow indicates the entrance to the bursa omentalis. (See enlarged image)
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FIG. 988– Final
disposition of the intestines and their vascular relations. (Jonnesco.) A.
Aorta. H. Hepatic artery. M, Col. Branches of superior mesenteric artery. m, m’. Branches
of inferior mesenteric artery. S. Splenic artery. (See enlarged image)
FIG. 989– Schematic
figure of the bursa omentalis, etc. Human embryo of eight weeks.
(Kollmann.) (See enlarged image)
The Rectum and Anal Canal.—The hind-gut is at first prolonged backward into the
body-stalk as the tube of the allantois; but, with the growth and flexure of the tail-end of
the embryo, the body-stalk, with its contained allantoic tube, is carried forward to the
ventral aspect of the body, and consequently a bend is formed at the junction of the hindgut and allantois. This bend becomes dilated into a pouch, which constitutes the
entodermal cloaca; into its dorsal part the hind-gut opens, and from its ventral part the
allantois passes forward. At a later stage the Wolffian and Müllerian ducts open into its
ventral portion. The cloaca is, for a time, shut off from the anterior by a membrane, the
cloacal membrane, formed by the apposition of the ectoderm and entoderm, and
reaching, at first, as far forward as the future umbilicus. Behind the umbilicus, however,
the mesoderm subsequently extends to form the lower part of the abdominal wall and
symphysis pubis. By the growth of the surrounding tissues the cloacal membrane comes
to lie at the bottom of a depression, which is lined by ectoderm and named the
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ectodermal cloaca (Fig. 991).
FIG. 990– Diagrams
FIG. 991– Tail
to illustrate the development of the greater omentum and transverse
mesocolon. (See enlarged image)
end of human embryo from fifteen to eighteen days old. (From model by
Keibel.) (See enlarged image)
FIG. 992– Cloaca of
human embryo from twenty-five to twenty-seven days old. (From
model by Keibel.) (See enlarged image)
The entodermal cloaca is divided into a dorsal and a ventral part by means of a partition,
the urorectal septum (Fig. 992), which grows downward from the ridge separating the
allantoic from the cloacal opening of the intestine and ultimately fuses with the cloacal
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membrane and divides it into an anal and a urogenital part. The dorsal part of the cloaca
forms the rectum, and the anterior part of the urogenital sinus and bladder. For a time a
communication named the cloacal duct exists between the two parts of the cloaca below
the urorectal septum; this duct occasionally persists as a passage between the rectum and
urethra. The anal canal is formed by an invagination of the ectoderm behind the urorectal
septum. This invagination is termed the proctodeum, and it meets with the entoderm of
the hind-gut and forms with it the anal membrane. By the absorption of this membrane
the anal canal becomes continuous with the rectum (Fig. 993). A small part of the hindgut projects backward beyond the anal membrane; it is named the post-anal gut (Fig.
991), and usually becomes obliterated and disappears. 159
FIG. 993– Tail
end of human embryo, from eight and a half to nine weeks old. (From model
by Keibel.) (See enlarged image)
Note 158. Sometimes this condition persists throughout life, and it is then found that the
duodenum does not cross from the right to the left side of the vertebral column, but lies
entirely on the right side of the median plane, where it is continued into the jejunum; the
arteries to the small intestine (aa. intestinales) also arise from the right instead of the left
side of the superior mesenteric artery. [back]
Note 159. Consult, in this connection, the following article: “A Contribution to the
Morphology of the Human Urinogenital Tract,” by D. Berry Hart, M.D., F.R.C.P.E., Journal
of Anatomy and Physiology, April, 1901, vol. xxxv. [back]
2a. The Mouth
(Cavum Oris; Oral Or Buccal Cavity)
The cavity of the mouth is placed at the commencement of the digestive tube (Fig. 994);
it is a nearly oval-shaped cavity which consists of two parts: an outer, smaller portion,
the vestibule, and an inner, larger part, the mouth cavity proper.
The Vestibule (vestibulum oris) is a slit-like space, bounded externally by the lips and
cheeks; internally by the gums and teeth. It communicates with the surface of the body
by the rima or orifice of the mouth. Above and below, it is limited by the reflection of
the mucous membrane from the lips and cheeks to the gum covering the upper and lower
alveolar arch respectively. It receives the secretion from the parotid salivary glands, and
communicates, when the jaws are closed, with the mouth cavity proper by an aperture on
either side behind the wisdom teeth, and by narrow clefts between opposing teeth.
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The Mouth Cavity Proper (cavum oris proprium) (Fig. 1014) is bounded laterally and
in front by the alveolar arches with their contained teeth; behind, it communicates with
the pharynx by a constricted aperture termed the isthmus faucium. It is roofed in by the
hard and soft palates, while the greater part of the floor is formed by the tongue, the
remainder by the reflection of the mucous membrane from the sides and under surface of
the tongue to the gum lining the inner aspect of the mandible. It receives the secretion
from the submaxillary and sublingual salivary glands.
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Structure.—The mucous membrane lining the mouth is continuous with the
integument at the free margin of the lips, and with the mucous lining of the pharynx
behind; it is of a rosepink tinge during life, and very thick where it overlies the hard parts
bounding the cavity. It is covered by stratified squamous epithelium.
The Lips (labia oris), the two fleshy folds which surround the rima or orifice of the
mouth, are formed externally of integument and internally of mucous membrane,
between which are found the Orbicularis oris muscle, the labial vessels, some nerves,
areolar tissue, and fat, and numerous small labial glands. The inner surface of each lip is
connected in the middle line to the corresponding gum by a fold of mucous membrane,
the frenulum—the upper being the larger.
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FIG. 994– Sagittal
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section of nose mouth, pharynx, and larynx. (See enlarged image)
The Labial Glands (glandulœ labiales) are situated between the mucous membrane
and the Orbicularis oris, around the orifice of the mouth. They are circular in form, and
about the size of small peas; their ducts open by minute orifices upon the mucous
membrane. In structure they resemble the salivary glands.
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The Cheeks (buccæ) form the sides of the face, and are continuous in front with the
lips. They are composed externally of integument; internally of mucous membrane; and
between the two of a muscular stratum, besides a large quantity of fat, areolar tissue,
vessels, nerves, and buccal glands.
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Structure.—The mucous membrane lining the cheek is reflected above and below
upon the gums, and is continuous behind with the lining membrane of the soft palate.
Opposite the second molar tooth of the maxilla is a papilla, on the summit of which is the
aperture of the parotid duct. The principal muscle of the cheek is the Buccinator; but
other muscles enter into its formation, viz., the Zygomaticus, Risorius, and Platysma.
The buccal glands are placed between the mucous membrane and Buccinator muscle:
they are similar in structure to the labial glands, but smaller. About five, of a larger size
than the rest, are placed between the Masseter and Buccinator muscles around the distal
extremity of the parotid duct; their ducts open in the mouth opposite the last molar tooth.
They are called molar glands.
The Gums (gingivœ) are composed of dense fibrous tissue, closely connected to the
periosteum of the alveolar processes, and surrounding the necks of the teeth. They are
covered by smooth and vascular mucous membrane, which is remarkable for its limited
sensibility. Around the necks of the teeth this membrane presents numerous fine papillæ,
and is reflected into the alveoli, where it is continuous with the periosteal membrane
lining these cavities.
The Palate (palatum) forms the roof of the mouth; it consists of two portions, the hard
palate in front, the soft palate behind.
The Hard Palate (palatum durum) (Fig. 1014) is bounded in front and at the sides by
the alveolar arches and gums; behind, it is continuous with the soft palate. It is covered
by a dense structure, formed by the periosteum and mucous membrane of the mouth,
which are intimately adherent. Along the middle line is a linear raphæ, which ends
anteriorly in a small papilla corresponding with the incisive canal. On either side and in
front of the raphé the mucous membrane is thick, pale in color, and corrugated; behind, it
is thin, smooth, and of a deeper color; it is covered with stratified squamous epithelium,
and furnished with numerous palatal glands, which lie between the mucous membrane
and the surface of the bone.
The Soft Palate (palatum molle) (Fig. 1014) is a movable fold, suspended from the
posterior border of the hard palate, and forming an incomplete septum between the
mouth and pharynx. It consists of a fold of mucous membrane enclosing muscular fibers,
an aponeurosis, vessels, nerves, adenoid tissue, and mucous glands. When occupying its
usual position, i. e., relaxed and pendent, its anterior surface is concave, continuous with
the roof of the mouth, and marked by a median raphé. Its posterior surface is convex, and
continuous with the mucous membrane covering the floor of the nasal cavities. Its upper
border is attached to the posterior margin of the hard palate, and its sides are blended
with the pharynx. Its lower border is free. Its lower portion, which hangs like a curtain
between the mouth and pharynx is termed the palatine velum.
Hanging from the middle of its lower border is a small, conical, pendulous process, the
palatine uvula; and arching lateralward and downward from the base of the uvula on
either side are two curved folds of mucous membrane, containing muscular fibers, called
the arches or pillars of the fauces.
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The Teeth (dentes) (Figs. 995 to 997).—Man is provided with two sets of teeth, which
make their appearance at different periods of life. Those of the first set appear in
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childhood, and are called the deciduous or milk teeth. Those of the second set, which
also appear at an early period, may continue until old age, and are named permanent.
The deciduous teeth are twenty in number: four incisors, two canines, and four molars,
in each jaw.
The permanent teeth are thirty-two in number: four incisors, two canines, four
premolars, and six molars, in each jaw.
FIG. 995– Side
FIG. 996– Permanent
view of the teeth and jaws. (See enlarged image)
teeth of upper dental arch, seen from below. (See enlarged image)
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FIG. 997– Permanent
teeth of right half of lower dental arch, seen from above. (See
enlarged image)
The dental formulæ may be represented as follows:
Deciduous Teeth.
mol. can. in. in. can. mol.
Upper jaw
2
1 2 2 1
2
Total 20
Lower jaw
2
1 2 2 1
2
Permanent Teeth.
mol. pr.mol. can. in. in. can. pr.mol. mol.
Upper jaw 3
2
1 2 2 1
2
3
Total 32
Lower jaw 3
2
1 2 2 1
2
3
General Characteristics.—Each tooth consists of three portions: the crown, projecting
above the gum; the root, imbedded in the alveolus; and the neck, the constricted portion
between the crown and root.
FIG. 998– Maxillæ
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at about one year. (Noyes.) (See enlarged image)
The roots of the teeth are firmly implanted in depressions within the alveoli; these
depressions are lined with periosteum which invests the tooth as far as the neck. At the
margins of the alveoli, the periosteum is continuous with the fibrous structure of the
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gums.
In consequence of the curve of the dental arch, terms such as anterior and posterior, as
applied to the teeth, are misleading and confusing. Special terms are therefore used to
indicate the different surfaces of a tooth: the surface directed toward the lips or cheek is
known as the labial or buccal surface; that directed toward the tongue is described as
the lingual surface; those surfaces which touch neighboring teeth are termed surfaces of
contact. In the case of the incisor and canine teeth the surfaces of contact are medial and
lateral; in the premolar and molar teeth they are anterior and posterior.
The superior dental arch is larger than the inferior, so that in the normal condition the
teeth in the maxillæ slightly overlap those of the mandible both in front and at the sides.
Since the upper central incisors are wider than the lower, the other teeth in the upper arch
are thrown somewhat distally, and the two sets do not quite correspond to each other
when the mouth is closed: thus the upper canine tooth rests partly on the lower canine
and partly on the first premolar, and the cusps of the upper molar teeth lie behind the
corresponding cusps of the lower molar teeth. The two series, however, end at nearly the
same point behind; this is mainly because the molars in the upper arch are the smaller.
FIG. 999– The complete
temporary dentition (about three years), showing the relation of
the developing permanent teeth. (Noyes.) (See enlarged image)
FIG. 1000– The complete
temporary dentition and the first permanent molar. Note the
relation of the bicuspids to the temporary molars. (In the seventh year.) (Noyes.) (See
enlarged image)
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The Permanent Teeth (dentes permanentes) (Figs. 1002, 1003).—The Incisors (dentes
incisivi; incisive or cutting teeth) are so named from their presenting a sharp cutting
edge, adapted for biting the food. They are eight in number, and form the four front teeth
in each dental arch.
The crown is directed vertically, and is chisel-shaped, being bevelled at the expense of
its lingual surface, so as to present a sharp horizontal cutting edge, which, before being
subjected to attrition, presents three small prominent points separated by two slight
notches. It is convex, smooth, and highly polished on its labial surface; concave on its
lingual surface, where, in the teeth of the upper arch, it is frequently marked by an
inverted V-shaped eminence, situated near the gum. This is known as the basal ridge or
cingulum. The neck is constricted. The root is long, single, conical, transversely
flattened, thicker in front than behind, and slightly grooved on either side in the
longitudinal direction.
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FIG. 1001– Front
view of the skull shown in Fig. 1000. Note the relation of the permanent
incisors and cuspids to each other and the roots of the temporary teeth. (Noyes.) (See
enlarged image)
The upper incisors are larger and stronger than the lower, and are directed obliquely
downward and forward. The central ones are larger than the lateral, and their roots are
more rounded.
The lower incisors are smaller than the upper: the central ones are smaller than the
lateral, and are the smallest of all the incisors. They are placed vertically and are
somewhat bevelled in front, where they have been worn down by contact with the
overlapping edge of the upper teeth. The cingulum is absent.
The Canine Teeth (dentes canini) are four in number, two in the upper, and two in the
lower arch, one being placed laterally to each lateral incisor. They are larger and stronger
than the incisors, and their roots sink deeply into the bones, and cause well-marked
prominences upon the surface.
The crown is large and conical, very convex on its labial surface, a little hollowed and
uneven on its lingual surface, and tapering to a blunted point or cusp, which projects
beyond the level of the other teeth. The root is single, but longer and thicker than that of
the incisors, conical in form, compressed laterally, and marked by a slight groove on
each side.
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FIG. 1002– Permanent
teeth. Right side. (Burchard.) (See enlarged image)
FIG. 1003– The permanent
teeth, viewed from the right. The external layer of bone has
been partly removed and the maxillary sinus has been opened. (Spalteholz.) (See
enlarged image)
The upper canine teeth (popularly called eye teeth) are larger and longer than the
lower, and usually present a distinct basal ridge.
The lower canine teeth (popularly called stomach teeth) are placed nearer the middle
line than the upper, so that their summits correspond to the intervals between the upper
canines and the lateral incisors.
The Premolars or Bicuspid teeth (dentes præmolares) are eight in number, four in
each arch. They are situated lateral to and behind the canine teeth, and are smaller and
shorter than they.
The crown is compressed antero-posteriorly, and surmounted by two pyramidal
eminences or cusps, a labial and a lingual, separated by a groove; hence their name
bicuspid. Of the two cusps the labial is the larger and more prominent. The neck is oval.
The root is generally single, compressed, and presents in front and behind a deep
groove, which indicates a tendency in the root to become double. The apex is generally
bifid.
The upper premolars are larger, and present a greater tendency to the division of their
roots than the lower; this is especially the case in the first upper premolar.
The Molar Teeth (dentes molares) are the largest of the permanent set, and their broad
crowns are adapted for grinding and pounding the food. They are twelve in number; six
in each arch, three being placed posterior to each of the second premolars.
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The crown of each is nearly cubical in form, convex on its buccal and lingual surfaces,
flattened on its surfaces of contact; it is surmounted by four or five tubercles, or cusps,
separated from each other by a crucial depression; hence the molars are sometimes
termed multicuspids. The neck is distinct, large, and rounded.
Upper Molars.—As a rule the first is the largest, and the third the smallest of the upper
molars. The crown of the first has usually four tubercles; that of the second, three or
four; that of the third, three. Each upper molar has three roots, and of these two are
buccal and nearly parallel to one another; the third is lingual and diverges from the
others as it runs upward. The roots of the third molar (dens serotinus or wisdom-tooth)
are more or less fused together.
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Lower Molars.—The lower molars are larger than the upper. On the crown of the first
there are usually five tubercles; on those of the second and third, four or five. Each lower
molar has two roots, an anterior, nearly vertical, and a posterior, directed obliquely
backward; both roots are grooved longitudinally, indicating a tendency to division. The
two roots of the third molar (dens serotinus or wisdom tooth) are more or less united.
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FIG. 1004– Deciduous
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teeth. Left side. (See enlarged image)
The Deciduous Teeth (dentes decidui; temporary or milk teeth) (Fig. 1004).—The
deciduous are smaller than, but, generally speaking, resemble in form, the teeth which
bear the same names in the permanent set. The hinder of the two molars is the largest of
all the deciduous teeth, and is succeeded by the second premolar. The first upper molar
has only three cusps—two labial, one lingual; the second upper molar has four cusps.
The first lower molar has four cusps; the second lower molar has five. The roots of the
deciduous molars are smaller and more divergent than those of the permanent molars,
but in other respects bear a strong resemblance to them.
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Structure of the Teeth.—On making a vertical section of a tooth (Fig. 1005), a cavity
will be found in the interior of the crown and the center of each root; it opens by a
minute orifice at the extremity of the latter. This is called the pulp cavity, and contains
the dental pulp, a loose connective tissue richly supplied with vessels and nerves, which
enter the cavity through the small aperture at the point of each root. Some of the cells of
the pulp are arranged as a layer on the wall of the pulp cavity; they are named the
odontoblasts of Waldeyer, and during the development of the tooth, are columnar in
shape, but later on, after the dentin is fully formed, they become flattened and resemble
osteoblasts. Each has two fine processes, the outer one passing into a dental canaliculus,
the inner being continuous with the processes of the connective-tissue cells of the pulp
matrix.
The solid portion of the tooth consists of (1) the ivory or dentin, which forms the bulk
of the tooth; (2) the enamel, which covers the exposed part of the crown; and (3) a thin
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layer of bone, the cement or crusta petrosa, which is disposed on the surface of the
root.
The dentin (substantia eburnea; ivory) (Fig. 1007) forms the principal mass of a tooth.
It is a modification of osseous tissue, from which it differs, however, in structure. On
microscopic examination it is seen to consist of a number of minute wavy and branching
tubes, the dental canaliculi, imbedded in a dense homogeneous substance, the matrix.
FIG. 1005– Vertical
section of a tooth in situ. X 15. c is placed in the pulp cavity, opposite
the neck of the tooth; the part above it is the crown, that below is the root. 1. Enamel
with radial and concentric markings. 2. Dentin with tubules and incremental lines. 3.
Cement or crusta petrosa, with bone corpuscles. 4. Dental periosteum. 5. Mandible. (See
enlarged image)
FIG. 1006– Vertical
section of a molar tooth. (See enlarged image)
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FIG. 1007– Vertical
section of a premolar tooth. (Magnified.) (See enlarged image)
The dental canaliculi (dentinal tubules) (Fig. 1008) are placed parallel with one
another, and open at their inner ends into the pulp cavity. In their course to the periphery
they present two or three curves, and are twisted on themselves in a spiral direction.
These canaliculi vary in direction: thus in a tooth of the mandible they are vertical in the
upper portion of the crown, becoming oblique and then horizontal in the neck and upper
part of the root, while toward the lower part of the root they are inclined downward. In
their course they divide and subdivide dichotomously, and, especially in the root, give
off minute branches, which join together in loops in the matrix, or end blindly. Near the
periphery of the dentin, the finer ramifications of the canaliculi terminate imperceptibly
by free ends. The dental canaliculi have definite walls, consisting of an elastic
homogeneous membrane, the dentinal sheath of Neumann, which resists the action of
acids; they contain slender cylindrical prolongations of the odontoblasts, first described
by Tomes, and named Tomes’ fibers or dentinal fibers.
The matrix (intertubular dentin) is translucent, and contains the chief part of the earthy
matter of the dentin. In it are a number of fine fibrils, which are continuous with the
fibrils of the dental pulp. After the earthy matter has been removed by steeping a tooth in
weak acid, the animal basis remaining may be torn into laminæ which run parallel with
the pulp cavity, across the direction of the tubes. A section of dry dentin often displays a
series of somewhat parallel lines—the incremental lines of Salter. These lines are
composed of imperfectly calcified dentin arranged in layers. In consequence of the
imperfection in the calcifying process, little irregular cavities are left, termed
interglobular spaces (Fig. 1008). Normally a series of these spaces is found toward the
outer surface of the dentin, where they form a layer which is sometimes known as the
granular layer. They have received their name from the fact that they are surrounded by
minute nodules or globules of dentin. Other curved lines may be seen parallel to the
surface. These are the lines of Schreger, and are due to the optical effect of
simultaneous curvature of the dentinal fibers.
Chemical Composition.—According to Berzelius and von Bibra, dentin consists of 28
parts of animal and 72 parts of earthy matter. The animal matter is converted by boiling
into gelatin. The earthy matter consists of phosphate of lime, carbonate of lime, a trace of
fluoride of calcium, phosphate of magnesium, and other salts.
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FIG. 1008– Transverse section
of a portion of the root of a canine tooth. X 300. (See
enlarged image)
The enamel (substantia adamantina) is the hardest and most compact part of the tooth,
and forms a thin crust over the exposed part of the crown, as far as the commencement of
the root. It is thickest on the grinding surface of the crown, until worn away by attrition,
and becomes thinner toward the neck. It consists of minute hexagonal rods or columns
termed enamel fibers or enamel prisms (prismata adamantina). They lie parallel with
one another, resting by one extremity upon the dentin, which presents a number of
minute depressions for their reception; and forming the free surface of the crown by the
other extremity. The columns are directed vertically on the summit of the crown,
horizontally at the sides; they are about 4μ in diameter, and pursue a more or less wavy
course. Each column is a six-sided prism and presents numerous dark transverse
shadings; these shadings are probably due to the manner in which the columns are
developed in successive stages, producing shallow constrictions, as will be subsequently
explained. Another series of lines, having a brown appearance, the parallel striæ or
colored lines of Retzius, is seen on section. According to Ebner, they are produced by
air in the interprismatic spaces; others believe that they are the result of true
pigmentation.
Numerous minute interstices intervene between the enamel fibers near their dentinal
ends, a provision calculated to allow of the permeation of fluids from the dental
canaliculi into the substance of the enamel.
Chemical Composition.—According to von Bibra, enamel consists of 96.5 per cent. of
earthy matter, and 3.5 per cent. of animal matter. The earthy matter consists of phosphate
of lime, with traces of fluoride of calcium, carbonate of lime, phosphate of magnesium,
and other salts. According to Tomes, the enamel contains the merest trace of organic
matter.
The crusta petrosa or cement (substantia ossea) is disposed as a thin layer on the roots
of the teeth, from the termination of the enamel to the apex of each root, where it is
usually very thick. In structure and chemical composition it resembles bone. It contains,
sparingly, the lacunæ and canaliculi which characterize true bone; the lacunæ placed
near the surface receive the canaliculi radiating from the side of the lacunæ toward the
periodontal membrane; and those more deeply placed join with the adjacent dental
canaliculi. In the thicker portions of the crusta petrosa, the lamellæ and Haversian canals
peculiar to bone are also found.
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As age advances, the cement increases in thickness, and gives rise to those bony
growths or exostoses so common in the teeth of the aged; the pulp cavity also becomes
partially filled up by a hard substance, intermediate in structure between dentin and bone
(osteodentin, Owen; secondary dentin, Tomes). It appears to be formed by a slow
conversion of the dental pulp, which shrinks, or even disappears.
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Development of the Teeth (Figs. 1009 to 1012).—In describing the development of the
teeth, the mode of formation of the deciduous teeth must first be considered, and then
that of the permanent series.
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FIG. 1009– Sagittal
section through the first lower deciduous molar of a human embryo 30
mm. long. (Röse.) X 100. L.E.L. Labiodental lamina, here separated from the dental
lamina. Z.L. Placed over the shallow dental furrow, points to the dental lamina, which is
spread out below to form the enamel germ of the future tooth. P.p. Bicuspidate papilla,
capped by the enamel germ. Z.S. Condensed tissue forming dental sac. M.E. Mouth
epithelium (See enlarged image)
FIG. 1010– Similar
section through the canine tooth of an embryo 40 mm. long. (Röse.) X
100. L.F. Labio dental furrow. The other lettering as in Fig. 1009. (See enlarged image)
FIG. 1011– Vertical
section of the mandible of an early human fetus. X 25. (See enlarged
image)
Development of the Deciduous Teeth.—The development of the deciduous teeth
begins about the sixth week of fetal life as a thickening of the epithelium along the line
of the future jaw, the thickening being due to a rapid multiplication of the more deeply
situated epithelial cells. As the cells multiply they extend into the subjacent mesoderm,
and thus form a ridge or strand of cells imbedded in mesoderm. About the seventh week
a longitudinal splitting or cleavage of this strand of cells takes place, and it becomes
divided into two strands; the separation begins in front and extends laterally, the process
occupying four or five weeks. Of the two strands thus formed, the labial forms the
labiodental lamina; while the other, the lingual, is the ridge of cells in connection with
which the teeth, both deciduous and permanent, are developed. Hence it is known as the
dental lamina or common dental germ. It forms a flat band of cells, which grows into
the substance of the embryonic jaw, at first horizontally inward, and then, as the teeth
develop, vertically, i. e., upward in the upper jaw, and downward in the lower jaw. While
still maintaining a horizontal direction it has two edges—an attached edge, continuous
with the epithelium lining the mouth, and a free edge, projecting inward, and imbedded
in the mesodermal tissue of the embryonic jaw. Along its line of attachment to the buccal
epithelium is a shallow groove, the dental furrow.
About the ninth week the dental lamina begins to develop enlargements along its free
border. These are ten in number in each jaw, and each corresponds to a future deciduous
tooth. They consist of masses of epithelial cells; and the cells of the deeper part—that is,
the part farthest from the margin of the jaw—increase rapidly and spread out in all
directions. Each mass thus comes to assume a club shape, connected with the general
epithelial lining of the mouth by a narrow neck, embraced by mesoderm. They are now
known as special dental germs. After a time the lower expanded portion inclines
outward, so as to form an angle with the superficial constricted portion, which is
sometimes known as the neck of the special dental germ. About the tenth week the
mesodermal tissue beneath these special dental germs becomes differentiated into
papillæ; these grow upward, and come in contact with the epithelial cells of the special
dental germs, which become folded over them like a hood or cap. There is, then, at this
stage a papilla (or papillæ) which has already begun to assume somewhat the shape of
the crown of the future tooth, and from which the dentin and pulp of the tooth are
formed, surmounted by a dome or cap of epithelial cells from which the enamel is
derived.
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FIG. 1012– Longitudinal section of the lower part of a growing tooth, showing the
extension of the layer of adamantoblasts beyond the crown to mark off the limit of
formation of the dentin of the root. (Röse.) ad. Adamantoblasts, continuous below with
ep.sch., the epithelial sheath of Hertwig. d. Dentin. en. Enamel. od. Odontoblasts. p.
Pulp. (See enlarged image)
In the meantime, while these changes have been going on, the dental lamina has been
extending backward behind the special dental germ corresponding to the second
deciduous molar tooth, and at about the seventeenth week it presents an enlargement, the
special dental germ, for the first permanent molar, soon followed by the formation of a
papilla in the mesodermal tissue for the same tooth. This is followed, about the sixth
month after birth, by a further extension backward of the dental lamina, with the
formation of another enlargement and its corresponding papilla for the second molar.
And finally the process is repeated for the third molar, its papilla appearing about the
fifth year of life.
After the formation of the special dental germs, the dental lamina undergoes atrophic
changes and becomes cribriform, except on the lingual and lateral aspects of each of the
special germs of the temporary teeth, where it undergoes a local thickening forming the
special dental germ of each of the successional permanent teeth—i. e., the ten anterior
ones in each jaw. Here the same process goes on as has been described in connection
with those of the deciduous teeth: that is, they recede into the substance of the gum
behind the germs of the deciduous teeth. As they recede they become club-shaped, form
expansions at their distal extremities, and finally meet papillæ, which have been formed
in the mesoderm, just in the same manner as was the case in the deciduous teeth. The
apex of each papilla indents the dental germ, which encloses it, and, forming a cap for it,
becomes converted into the enamel, while the papilla forms the dentin and pulp of the
permanent tooth.
The special dental germs consist at first of rounded or polyhedral epithelial cells; after
the formation of the papillæ, these cells undergo a differentiation into three layers. Those
which are in immediate contact with the papilla become elongated, and form a layer of
well-marked columnar epithelium coating the papilla. They are the cells which form the
enamel fibers, and are therefore termed enamel cells or adamantoblasts. The cells of
the outer layer of the special dental germ, which are in contact with the inner surface of
the dental sac, presently to be described, are much shorter, cubical in form, and are
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named the external enamel epithelium. All the intermediate round cells of the dental
germ between these two layers undergo a peculiar change. They become stellate in shape
and develop processes, which unite to form a net-work into which fluid is secreted; this
has the appearance of a jelly, and to it the name of enamel pulp is given. This
transformed special dental germ is now known under the name of enamel organ (Fig.
1011).
While these changes are going on, a sac is formed around each enamel organ from the
surrounding mesodermal tissue. This is known as the dental sac, and is a vascular
membrane of connective tissue. It grows up from below, and thus encloses the whole
tooth germ; as it grows it causes the neck of the enamel organ to atrophy and disappear;
so that all communication between the enamel organ and the superficial epithelium is cut
off. At this stage there are vascular papillæ surmounted by caps of epithelial cells, the
whole being surrounded by by membranous sacs.
Formation of the Enamel.—The enamel is formed exclusively from the enamel cells or
adamantoblasts of the special dental germ, either by direct calcification of the columnar
cells, which become elongated into the hexagonal rods of the enamel; or, as is more
generally believed, as a secretion from the adamantoblasts, within which calcareous
matter is subsequently deposited.
The process begins at the apex of each cusp, at the ends of the enamel cells in contact
with the dental papilla. Here a fine globular deposit takes place, being apparently shed
from the end of the adamantoblasts. It is known by the name of the enamel droplet, and
resembles keratin in its resistance to the action of mineral acids. This droplet then
becomes fibrous and calcifies and forms the first layer of the enamel; a second droplet
now appears and calcifies, and so on; successive droplets of keratin-like material are
shed from the adamantoblasts and form successive layers of enamel, the adamantoblasts
gradually receding as each layer is produced, until at the termination of the process they
have almost disappeared. The intermediate cells of the enamel pulp atrophy and
disappear, so that the newly formed calcified material and the external enamel
epithelium come into apposition. This latter layer, however, soon disappears on the
emergence of the tooth beyond the gum. After its disappearance the crown of the tooth is
still covered by a distinct membrane, which persists for some time. This is known as the
cuticula dentis, or Nasmyth’s membrane, and is believed to be the last-formed layer of
enamel derived from the adamantoblasts, which has not become calcified. It forms a
horny layer, which may be separated from the subjacent calcified mass by the action of
strong acids. It is marked by the hexagonal impressions of the enamel prisms, and, when
stained by nitrate of silver, shows the characteristic appearance of epithelium.
Formation of the Dentin.—While these changes are taking place in the epithelium to
form the enamel, contemporaneous changes occurring in the differentiated mesoderm of
the dental papillæ result in the formation of the dentin. As before stated, the first germs
of the dentin are the papillæ, corresponding in number to the teeth, formed from the soft
mesodermal tissue which bounds the depressions containing the special enamel germs.
The papillæ grow upward into the enamel germs and become covered by them, both
being enclosed in a vascular connective tissue, the dental sac, in the manner above
described. Each papilla then constitutes the formative pulp from which the dentin and
permanent pulp are developed; it consists of rounded cells and is very vascular, and soon
begins to assume the shape of the future tooth. The next step is the appearance of the
odontoblasts, which have a relation to the development of the teeth similar to that of the
osteoblasts to the formation of bone. They are formed from the cells of the periphery of
the papilla—that is to say, from the cells in immediate contact with the adamantoblasts
of the special dental germ. These cells become elongated, one end of the elongated cell
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resting against the epithelium of the special dental germs, the other being tapered and
oftened branched. By the direct transformation of the peripheral ends of these cells; or by
a secretion from them, a layer of uncalcified matrix (prodentin) is formed which caps
the cusp or cusps, if there are more than one, of the papillæ. This matrix becomes
fibrillated, and in it islets of calcification make their appearance, and coalescing give rise
to a continuous layer of calcified material which covers each cusp and constitutes the
first layer of dentin. The odontoblasts, having thus formed the first layer, retire toward
the center of the papilla, and, as they do so, produce successive layers of dentin from
their peripheral extremities—that is to say, they form the dentinal matrix in which
calcification subsequently takes place. As they thus recede from the periphery of the
papilla, they leave behind them filamentous processes of cell protoplasm, provided with
finer side processes; these are surrounded by calcified material, and thus form the dental
canaliculi, and, by their side branches, the anastomosing canaliculi: the processes of
protoplasm contained within them constitute the dentinal fibers (Tomes’ fibers). In this
way the entire thickness of the dentin is developed, each canaliculus being completed
throughout its whole length by a single odontoblast. The central part of the papilla does
not undergo calcification, but persists as the pulp of the tooth. In this process of
formation of dentin it has been shown that an uncalcified matrix is first developed, and
that in this matrix islets of calcification appear which subsequently blend together to
form a cap to each cusp: in like manner successive layers are produced, which ultimately
become blended with each other. In certain places this blending is not complete, portions
of the matrix remaining uncalcified between the successive layers; this gives rise to little
spaces, which are the interglobular spaces alluded to above.
Formation of the Cement.—The root of the tooth begins to be formed shortly before the
crown emerges through the gum, but is not completed until some time afterward. It is
produced by a downgrowth of the epithelium of the dental germ, which extends almost
as far as the situation of the apex of the future root, and determines the form of this
portion of the tooth. This fold of epithelium is known as the epithelial sheath, and on its
papillary surface odontoblasts appear, which in turn form dentin, so that the dentin
formation is identical in the crown and root of the tooth. After the dentin of the root has
been developed, the vascular tissues of the dental sac begin to break through the
epithelial sheath, and spread over the surface of the root as a layer of bone-forming
material. In this osteoblasts make their appearance, and the process of ossification goes
on in identically the same manner as in the ordinary intramembranous ossification of
bone. In this way the cement is formed, and consists of ordinary bone containing
canaliculi and lacunæ.
Formation of the Alveoli.—About the fourteenth week of embryonic life the dental
lamina becomes enclosed in a trough or groove of mesodermal tissue, which at first is
common to all the dental germs, but subsequently becomes divided by bony septa into
loculi, each loculus containing the special dental germ of a deciduous tooth and its
corresponding permanent tooth. After birth each cavity becomes subdivided, so as to
form separate loculi (the future alveoli) for the deciduous tooth and its corresponding
permanent tooth. Although at one time the whole of the growing tooth is contained in the
cavity of the alveolus, the latter never completely encloses it, since there is always an
aperture over the top of the crown filled by soft tissue, by which the dental sac is
connected with the surface of the gum, and which in the permanent teeth is called the
gubernaculum dentis.
Development of the Permanent Teeth.—The permanent teeth as regards their
development may be divided into two sets: (1) those which replace the deciduous teeth,
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and which, like them, are ten in number in each jaw: these are the successional
permanent teeth; and (2) those which have no deciduous predecessors, but are
superadded distal to the temporary dental series. These are three in number on either side
in each jaw, and are termed superadded permanent teeth. They are the three molars of
the permanent set, the molars of the deciduous set being replaced by the premolars of the
permanent set. The development of the successional permanent teeth—the ten anterior
ones in either jaw—has already been indicated. During their development the permanent
teeth, enclosed in their sacs, come to be placed on the lingual side of the deciduous teeth
and more distant from the margin of the future gum, and, as already stated, are separated
from them by bony partitions. As the crown of the permanent tooth grows, absorption of
these bony partitions and of the root of the deciduous tooth takes place, through the
agency of osteoclasts, which appear at this time, and finally nothing but the crown of the
deciduous tooth remains. This is shed or removed, and the permanent tooth takes its
place.
The superadded permanent teeth are developed in the manner already described, by
extensions backward of the posterior part of the dental lamina in each jaw.
Eruption of the Teeth.—When the calcification of the different tissues of the tooth is
sufficiently advanced to enable it to bear the pressure to which it will be afterward
subjected, eruption takes place, the tooth making its way through the gum. The gum is
absorbed by the pressure of the crown of the tooth against it, which is itself pressed up
by the increasing size of the root. At the same time the septa between the dental sacs
ossify, and constitute the alveoli; these firmly embrace the necks of the teeth, and afford
them a solid basis of support.
The eruption of the deciduous teeth commences about the seventh month after birth,
and is completed about the end of the second year, the teeth of the lower jaw preceding
those of the upper.
The following, according to C. S. Tomes, are the most usual times of eruption:
Lower central incisors
6 to 9 months.
Upper incisors
8 to 10 months.
Lower lateral incisors and first molars
15 to 21 months.
Canines
16 to 20 months.
Second molars
20 to 24 months.
There are, however, considerable variations in these times; thus, according to Holt:
At the age of 1 year a child should have 6 teeth.
At the age of 1 1/2 years a child should have 12 teeth.
At the age of 2 year a child should have 16 teeth.
At the age of 2 1/2 years a child should have 20 teeth.
Calcification of the permanent teeth proceeds in the following order in the lower jaw (in
the upper jaw it takes place a little later): the first molar, soon after birth; the central and
lateral incisors, and the canine, about six months after birth; the premolars, at the second
year, or a little later; the second molar, about the end of the second year; the third molar,
about the twelfth year.
The eruption of the permanent teeth takes place at the following periods, the teeth of the
lower jaw preceding those of the upper by short intervals:
First molars
6th year.
Two central incisors
7th year.
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Two lateral incisors
8th year.
First premolars
9th year.
Second premolars
10th year.
Canines
11th to 12th year.
Second molars
12th to 13th year.
Third molars
17th to 25th year.
Toward the sixth year, before the shedding of the deciduous teeth begins, there are
twenty-four teeth in each jaw, viz., the ten deciduous teeth and the crowns of all the
permanent teeth except the third molars.
The Tongue (lingua).—The tongue is the principal organ of the sense of taste, and an
important organ of speech; it also assists in the mastication and deglutition of the food. It
is situated in the floor of the mouth, within the curve of the body of the mandible.
Its Root (radix linguæ base) (Fig. 954) is directed backward, and connected with the
hyoid bone by the Hyoglossi and Genioglossi muscles and the hyoglossal membrane;
with the epiglottis by three folds (glossoepiglottic) of mucous membrane; with the soft
palate by the glossopalatine arches; and with the pharynx by the Constrictores pharyngis
superiores and the mucous membrane.
Its Apex (apex linguæ tip), thin and narrow, is directed forward against the lingual
surfaces of the lower incisor teeth.
Its Inferior Surface (facies inferior linguæ under surface) (Fig. 1013) is connected
with the mandible by the Genioglossi; the mucous membrane is reflected from it to the
lingual surface of the gum and on to the floor of the mouth, where, in the middle line, it
is elevated into a distinct vertical fold, the frenulum linguæ. On either side lateral to the
frenulum is a slight fold of the mucous membrane, the plica fimbriata, the free edge of
which occasionally exhibits a series of fringe-like processes.
The apex of the tongue, part of the inferior surface, the sides, and dorsum are free.
The Dorsum of the Tongue (dorsum linguæ) (Fig. 1014) is convex and marked by a
median sulcus, which divides it into symmetrical halves; this sulcus ends behind, about
2.5 cm. from the root of the organ, in a depression, the foramen cecum, from which a
shallow groove, the sulcus terminalis, runs lateralward and forward on either side to the
margin of the tongue. The part of the dorsum of the tongue in front of this groove,
forming about two-thirds of its surface, looks upward, and is rough and covered with
papillæ; the posterior third looks backward, and is smoother, and contains numerous
muciparous glands and lymph follicles (lingual tonsil). The foramen cecum is the
remains of the upper part of the thyroglossal duct or diverticulum from which the
thyroid gland is developed; the pyramidal lobe of the thyroid gland indicates the position
of the lower part of the duct.
The Papillæ of the Tongue (Fig. 1014) are projections of the corium. They are thickly
distributed over the anterior two-thirds of its dorsum, giving to this surface its
characteristic roughness. The varieties of papillæ met with are the papillæ vallatæ,
papillæ fungiformes, papillæ filiformes, and papillæ simplices.
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FIG. 1013– The mouth
cavity. The apex of the tongue is turned upward, and on the right
side a superficial dissection of its under surface has been made. (See enlarged image)
The papillæ vallatæ (circumvallate papillæ) (Fig. 1015) are of large size, and vary
from eight to twelve in number. They are situated on the dorsum of the tongue
immediately in front of the foramen cecum and sulcus terminalis, forming a row on
either side; the two rows run backward and medialward, and meet in the middle line, like
the limbs of the letter V inverted. Each papilla consists of a projection of mucous
membrane from 1 to 2 mm. wide, attached to the bottom of a circular depression of the
mucous membrane; the margin of the depression is elevated to form a wall (vallum), and
between this and the papilla is a circular sulcus termed the fossa. The papilla is shaped
like a truncated cone, the smaller end being directed downward and attached to the
tongue, the broader part or base projecting a little above the surface of the tongue and
being studded with numerous small secondary papillæ and covered by stratified
squamous epithelium.
The papillæ fungiformes (fungiform papillæ) (Fig. 1017), more numerous than the
preceding, are found chiefly at the sides and apex, but are scattered irregularly and
sparingly over the dorsum. They are easily recognized, among the other papillæ, by their
large size, rounded eminences, and deep red color. They are narrow at their attachment
to the tongue, but broad and rounded at their free extremities, and covered with
secondary papillæ.
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FIG. 1014– The mouth
cavity. The cheeks have been slit transversely and the tongue pulled
forward. (See enlarged image)
FIG. 1015– Circumvallate papilla
in vertical section, showing arrangement of the taste-buds
and nerves. (See enlarged image)
The papillæ filiformes (filiform or conical papilæ) (Fig. 1016) cover the anterior twothirds of the dorsum. They are very minute, filiform in shape, and arranged in lines
parallel with the two rows of the papillæ vallatæ, excepting at the apex of the organ,
where their direction is transverse. Projecting from their apices are numerous
filamentous processes, or secondary papillæ these are of a whitish tint, owing to the
thickness and density of the epithelium of which they are composed, which has here
undergone a peculiar modification, the cells having become cornified and elongated into
dense, imbricated, brush-like processes. They contain also a number of elastic fibers,
which render them firmer and more elastic than the papillæ of mucous membrane
generally. The larger and longer papillæ of this group are sometimes termed papillæ
conicæ.
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FIG. 1016– A
FIG. 1017– Section
filiform papilla. Magnified (See enlarged image)
of a fungiform papilla. Magnified. (See enlarged image)
FIG. 1018– Semidiagrammatic view of
a portion of the mucous membrane of the tongue.
Two fungiform papillæ are shown. On some of the filiform papillæ the epithelial
prolongations stand erect, in one they are spread out, and in three they are folded in. (See
enlarged image)
The papillæ simplices are similar to those of the skin, and cover the whole of the
mucous membrane of the tongue, as well as the larger papillæ. They consist of closely
set microscopic elevations of the corium, each containing a capillary loop, covered by a
layer of epithelium.
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Muscles of the Tongue.—The tongue is divided into lateral halves by a median fibrous
septum which extends throughout its entire length and is fixed below to the hyoid bone.
In either half there are two sets of muscles, extrinsic and intrinsic; the former have their
origins outside the tongue, the latter are contained entirely within it.
The extrinsic muscles (Fig. 1019) are:
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Genioglossus.
Chondroglossus.
Glossopalatinus. 160
FIG. 1019– Extrinsic
Hyoglossus.
Styloglossus.</TD< TR>
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muscles of the tongue. Left side. (See enlarged image)
The Genioglossus (Geniohyoglossus) is a flat triangular muscle close to and parallel
with the median plane, its apex corresponding with its point of origin from the mandible,
its base with its insertion into the tongue and hyoid bone. It arises by a short tendon from
the superior mental spine on the inner surface of the symphysis menti, immediately
above the Geniohyoideus, and from this point spreads out in a fan-like form. The inferior
fibers extend downward, to be attached by a thin aponeurosis to the upper part of the
body of the hyoid bone, a few passing between the Hyoglossus and Chondroglossus to
blend with the Constrictores pharyngis; the middle fibers pass backward, and the
superior ones upward and forward, to enter the whole length of the under surface of the
tongue, from the root to the apex. The muscles of opposite sides are separated at their
insertions by the median fibrous septum of the tongue; in front, they are more or less
blended owing to the decussation of fasciculi in the median plane.
The Hyoglossus, thin and quadrilateral, arises from the side of the body and from the
whole length of the greater cornu of the hyoid bone, and passes almost vertically upward
to enter the side of the tongue, between the Styloglossus and Longitudinalis inferior. The
fibers arising from the body of the hyoid bone overlap those from the greater cornu.
The Chondroglossus is sometimes described as a part of the Hyoglossus, but is
separated from it by fibers of the Genioglossus, which pass to the side of the pharynx. It
is about 2 cm. long, and arises from the medial side and base of the lesser cornu and
contiguous portion of the body of the hyoid bone, and passes directly upward to blend
with the intrinsic muscular fibers of the tongue, between the Hyoglossus and
Genioglossus.
A small slip of muscular fibers is occasionally found, arising from the cartilago triticea
in the lateral hyothyroid ligament and entering the tongue with the hindermost fibers of
the Hyoglossus.
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The Styloglossus, the shortest and smallest of the three styloid muscles, arises from the
anterior and lateral surfaces of the styloid process, near its apex, and from the
stylomandibular ligament. Passing downward and forward between the internal and
external carotid arteries, it divides upon the side of the tongue near its dorsal surface,
blending with the fibers of the Longitudinalis inferior in front of the Hyoglossus; the
other, oblique, overlaps the Hyoglossus and decussates with its fibers.
The intrinsic muscles (Fig. 1020) are:
Longitudinalis superior.
Transversus.
Longitudinalis inferior.
Verticalis.
The Longitudinalis linguæ superior (Superior lingualis) is a thin stratum of oblique
and longitudinal fibers immediately underlying the mucous membrane on the dorsum of
the tongue. It arises from the submucous fibrous layer close to the epiglottis and from
the median fibrous septum, and runs forward to the edges of the tongue.
FIG. 1020– Coronal
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section of tongue, showing intrinsic muscles. (Altered from Krause.)
(See enlarged image)
The Longitudinalis linguæ inferior (Inferior lingualis) is a narrow band situated on
the under surface of the tongue between the Genioglossus and Hyoglossus. It extends
from the root to the apex of the tongue: behind, some of its fibers are connected with the
body of the hyoid bone; in front it blends with the fibers of the Styloglossus.
The Transversus linguæ (Transverse lingualis) consists of fibers which arise from the
median fibrous septum and pass lateralward to be inserted into the submucous fibrous
tissue at the sides of the tongue.
The Verticalis linguæ (Vertical lingualis) is found only at the borders of the forepart of
the tongue. Its fibers extend from the upper to the under surface of the organ.
The median fibrous septum of the tongue is very complete, so that the anastomosis
between the two lingual arteries is not very free.
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Nerves.—The muscles of the tongue described above are supplied by the hypoglossal
nerve.
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Actions.—The movements of the tongue, although numerous and complicated, may be
understood by carefully considering the direction of the fibers of its muscles. The
Genioglossi, by means of their posterior fibers, draw the root of the tongue forward, and
protrude the apex from the mouth. The anterior fibers draw the tongue back into the
mouth. The two muscles acting in their entirety draw the tongue downward, so as to
make its superior surface concave from side to side, forming a channel along which
fluids may pass toward the pharynx, as in sucking. The Hyoglossi depress the tongue,
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and draw down its sides. The Styloglossi draw the tongue upward and backward. The
Glossopalatini draw the root of the tongue upward. The intrinsic muscles are mainly
concerned in altering the shape of the tongue, whereby it becomes shortened, narrowed,
or curved in different directions; thus, the Longitudinalis superior and inferior tend to
shorten the tongue, but the former, in addition, turn the tip and sides upward so as to
render the dorsum concave, while the latter pull the tip downward and render the dorsum
convex. The Transversus narrows and elongates the tongue, and the Verticalis flattens
and broadens it. The complex arrangement of the muscular fibers of the tongue, and the
various directions in which they run, give to this organ the power of assuming the forms
necessary for the enunciation of the different consonantal sounds; and Macalister states
“there is reason to believe that the musculature of the tongue varies in different races
owing to the hereditary practice and habitual use of certain motions required for
enunciating the several vernacular languages.”
Structure of the Tongue.—The tongue is partly invested by mucous membrane and a
submucous fibrous layer.
The mucous membrane (tunica mucosa linguæ) differs in different parts. That
covering the under surface of the organ is thin, smooth, and identical in structure with
that lining the rest of the oral cavity. The mucous membrane of the dorsum of the tongue
behind the foramen cecum and sulcus terminalis is thick and freely movable over the
subjacent parts. It contains a large number of lymphoid follicles, which together
constitute what is sometimes termed the lingual tonsil. Each follicle forms a rounded
eminence, the center of which is perforated by a minute orifice leading into a funnelshaped cavity or recess; around this recess are grouped numerous oval or rounded
nodules of lymphoid tissue, each enveloped by a capsule derived from the submucosa,
while opening into the bottom of the recesses are also seen the ducts of mucous glands.
The mucous membrne on the anterior part of the dorsum of the tongue is thin, intimately
adherent to the muscular tissue, and presents numerous minute surface eminences, the
papillæ of the tongue. It consists of a layer of connective tissue, the corium or mucosa,
covered with epithelium.
The epithelium is of the stratified squamous variety, similar to but much thinner than
that of the skin: and each papilla has a separate investment from root to summit. The
deepest cells may sometimes be detached as a separate layer, corresponding to the rete
mucosum, but they never contain coloring matter.
The corium consists of a dense felt-work of fibrous connective tissue, with numerous
elastic fibers, firmly connected with the fibrous tissue forming the septa between the
muscular bundles of the tongue. It contains the ramifications of the numerous vessels and
nerves from which the papillæ are supplied, large plexuses of lymphatic vessels, and the
glands of the tongue.
Structure of the Papillæ.—The papillæ apparently resemble in structure those of the
cutis, consisting of cone-shaped projections of connective tissue, covered with a thick
layer of stratified squamous epithelium, and containing one or more capillary loops
among which nerves are distributed in great abundance. If the epithelium be removed, it
will be found that they are not simple elevations like the papillæ of the skin, for the
surface of each is studded with minute conical processes which form secondary papillæ.
In the papillæ vallatæ, the nerves are numerous and of large size; in the papillæ
fungiformes they are also numerous, and end in a plexiform net-work, from which brushlike branches proceed; in the papillæ filiformes, their mode of termination is uncertain.
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Glands of the Tongue.—The tongue is provided with mucous and serous glands.
The mucous glands are similar in structure to the labial and buccal glands. They are
found especially at the back part behind the vallate papillæ, but are also present at the
apex and marginal parts. In this connection the anterior lingual glands (Blandin or Nuhn)
require special notice. They are situated on the under surface of the apex of the tongue
(Fig. 1013), one on either side of the frenulum, where they are covered by a fasciculus of
muscular fibers derived from the Styloglossus and Longitudinalis inferior. They are from
12 to 25 mm. long, and about 8 mm. broad, and each opens by three or four ducts on the
under surface of the apex.
The serous glands occur only at the back of the tongue in the neighborhood of the
taste-buds, their ducts opening for the most part into the fossæ of the vallate papillæ.
These glands are racemose, the duct of each branching into several minute ducts, which
end in alveoli, lined by a single layer of more or less columnar epithelium. Their
secretion is of a watery nature, and probably assists in the distribution of the substance to
be tasted over the taste area. (Ebner.)
The septum consists of a vertical layer of fibrous tissue, extending throughout the
entire length of the median plane of the tongue, though not quite reaching the dorsum. It
is thicker behind than in front, and occasionally contains a small fibrocartilage, about 6
mm. in length. It is well displayed by making a vertical section across the organ.
The hyoglossal membrane is a strong fibrous lamina, which connects the under
surface of the root of the tongue to the body of the hyoid bone. This membrane receives,
in front, some of the fibers of the Genioglossi.
Taste-buds, the end-organs of the gustatory sense, are scattered over the mucous
membrane of the mouth and tongue at irregular intervals. They occur especially in the
sides of the vallate papillæ. In the rabbit there is a localized area at the side of the base of
the tongue, the papilla foliata, in which they are especially abundant (Fig. 1021). They
are described under the organs of the senses (page 991).
FIG. 1021– Vertical
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section of papilla foliata of the rabbit, passing across the folia.
(Ranvier.) (See enlarged image)
Vessels and Nerves.—The main artery of the tongue is the lingual branch of the
external carotid, but the external maxillary and ascending pharyngeal also give branches
to it. The veins open into the internal jugular.
The lymphatics of the tongue have been described on page 696.
The sensory nerves of the tongue are: (1) the lingual branch of the mandibular, which
is distributed to the papillæ at the forepart and sides of the tongue, and forms the nerve of
ordinary sensibility for its anterior two-thirds; (2) the chorda tympani branch of the
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facial, which runs in the sheath of the lingual, and is generally regarded as the nerve of
taste for the anterior two-thirds; this nerve is a continuation of the sensory root of the
facial (nervus intermedius); (3) the lingual branch of the glossopharyngeal, which is
distributed to the mucous membrane at the base and sides of the tongue, and to the
papillæ vallatæ, and which supplies both gustatory filaments and fibers of general
sensation to this region; (4) the superior laryngeal, which sends some fine branches to
the root near the epiglottis.
The Salivary Glands (Fig. 1024).—Three large pairs of salivary glands communicate
with the mouth, and pour their secretion into its cavity; they are the parotid,
submaxillary, and sublingual.
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Parotid Gland (glandula parotis).—The parotid gland (Figs. 1022, 1023), the largest of
the three, varies in weight from 14 to 28 gm. It lies upon the side of the face,
immediately below and in front of the external ear. The main portion of the gland is
superficial, somewhat flattened and quadrilateral in form, and is placed between the
ramus of the mandible in front and the mastoid process and Sternocleidomastoideus
behind, overlapping, however, both boundaries. Above, it is broad and reaches nearly to
the zygomatic arch; below, it tapers somewhat to about the level of a line joining the tip
of the mastoid process to the angle of the mandible. The remainder of the gland is
irregularly wedge-shaped, and extends deeply inward toward the pharyngeal wall.
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FIG. 1022– Right
parotid gland. Posterior and deep aspects. (See enlarged image)
The gland is enclosed within a capsule continuous with the deep cervical fascia; the
layer covering the superficial surface is dense and closely adherent to the gland; a
portion of the fascia, attached to the styloid process and the angle of the mandible, is
thickened to form the stylomandibular ligament which intervenes between the parotid
and submaxillary glands.
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FIG. 1023– Right
parotid gland. Deep and anterior aspects. (See enlarged image)
The anterior surface of the gland is moulded on the posterior border of the ramus of
the mandible, clothed by the Pterygoideus internus and Masseter. The inner lip of the
groove dips, for a short distance, between the two Pterygoid muscles, while the outer lip
extends for some distance over the superficial surface of the Masseter; a small portion of
this lip immediately below the zygomatic arch is usually detached, and is named the
accessory part (socia parotidis) of the gland.
The posterior surface is grooved longitudinally and abuts against the external acoustic
meatus, the mastoid process, and the anterior border of the Sternocleidomastoideus.
The superficial surface, slightly lobulated, is covered by the integument, the
superficial fascia containing the facial branches of the great auricular nerve and some
small lymph glands, and the fascia which forms the capsule of the gland.
The deep surface extends inward by means of two processes, one of which lies on the
Digastricus, styloid process, and the styloid group of muscles, and projects under the
mastoid process and Sternocleidomastoideus; the other is situated in front of the styloid
process, and sometimes passes into the posterior part of the mandibular fossa behind the
temporomandibular joint. The deep surface is in contact with the internal and external
carotid arteries, the internal jugular vein, and the vagus and glossopharyngeal nerves.
The gland is separated from the pharyngeal wall by some loose connective tissue.
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Structures within the Gland.—The external carotid artery lies at first on the deep
surface, and then in the substance of the gland. The artery gives off its posterior
auricular branch which emerges from the gland behind; it then divides into its terminal
branches, the internal maxillary and superficial temporal; the former runs forward deep
to the neck of the mandible; the latter runs upward across the zygomatic arch and gives
off its transverse facial branch which emerges from the front of the gland. Superficial to
the arteries are the superficial temporal and internal maxillary veins, uniting to form the
posterior facial vein; in the lower part of the gland this vein splits into anterior and
posterior divisions. The anterior division emerges from the gland and unites with the
anterior facial to form the common facial vein; the posterior unites in the gland with the
posterior auricular to form the external jugular vein. On a still more superficial plane is
the facial nerve, the branches of which emerge from the borders of the gland. Branches
of the great auricular nerve pierce the gland to join the facial, while the
auriculotemporal nerve issues from the upper part of the gland.
The parotid duct (ductus parotideus; Stensen’s duct) is about 7 cm. long. It begins by
numerous branches from the anterior part of the gland, crosses the Masseter, and at the
anterior border of this muscle turns inward nearly at a right angle, passes through the
corpus adiposum of the cheek and pierces the Buccinator; it then runs for a short distance
obliquely forward between the Buccinator and mucous membrane of the mouth, and
opens upon the oral surface of the cheek by a small orifice, opposite the second upper
molar tooth. While crossing the Masseter, it receives the duct of the accessory portion; in
this position it lies between the branches of the facial nerve; the accessory part of the
gland and the transverse facial artery are above it.
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Structure.—The parotid duct is dense, its wall being of considerable thickness; its canal
is about the size of a crow-quill, but at its orifice on the oral surface of the cheek its
lumen is greatly reduced in size. It consists of a thick external fibrous coat which
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contains contractile fibers, and of an internal or mucous coat lined with short columnar
epithelium.
Vessels and Nerves.—The arteries supplying the parotid gland are derived from the
external carotid, and from the branches given off by that vessel in or near its substance.
The veins empty themselves into the external jugular, through some of its tributaries.
The lymphatics end in the superficial and deep cervical lymph glands, passing in their
course through two or three glands, placed on the surface and in the substance of the
parotid. The nerves are derived from the plexus of the sympathetic on the external
carotid artery, the facial, the auriculotemporal, and the great auricular nerves. It is
probable that the branch from the auriculotemporal nerve is derived from the
glossopharyngeal through the otic ganglion. At all events, in some of the lower animals
this has been proved experimentally to be the case.
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Submaxillary Gland (glandula submaxillaris).—The submaxillary gland (Fig. 1024) is
irregular in form and about the size of a walnut. A considerable part of it is situated in
the submaxillary triangle, reaching forward to the anterior belly of the Digastricus and
backward to the stylomandibular ligament, which intervenes between it and the parotid
gland. Above, it extends under cover of the body of the mandible; below, it usually
overlaps the intermediate tendon of the Digastricus and the insertion of the
Stylohyoideus, while from its deep surface a tongue-like deep process extends forward
above the Mylohyoideus muscle.
Its superficial surface consists of an upper and a lower part. The upper part is
directed outward, and lies partly against the submaxillary depression on the inner surface
of the body of the mandible, and partly on the Pterygoideus internus. The lower part is
directed downward and outward, and is covered by the skin, superficial fascia, Platysma,
and deep cervical fascia; it is crossed by the anterior facial vein and by filaments of the
facial nerve; in contact with it, near the mandible, are the submaxillary lymph glands.
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FIG. 1024– Dissection,
showing salivary glands of right side. (See enlarged image)
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The deep surface is in relation with the Mylohyoideus, Hyoglossus, Styloglossus,
Stylohyoideus, and posterior belly of the Digastricus; in contact with it are the
mylohyoid nerve and the mylohyoid and submental vessels.
The external maxillary artery is imbedded in a groove in the posterior border of the
gland.
The deep process of the gland extends forward between the Mylohyoideus below and
externally, and the Hyoglossus and Styloglossus internally; above it is the lingual nerve
and submaxillary ganglion; below it the hypoglossal nerve and its accompanying vein.
The submaxillary duct (ductus submaxillaris; Wharton’s duct) is about 5 cm. long,
and its wall is much thinner than that of the parotid duct. It begins by numerous branches
from the deep surface of the gland, and runs forward between the Mylohyoideus and the
Hyoglossus and Genioglossus, then between the sublingual gland and the Genioglossus,
and opens by a narrow orifice on the summit of a small papilla, at the side of the
frenulum linguæ. On the Hyoglossus it lies between the lingual and hypoglossal nerves,
but at the anterior border of the muscle it is crossed laterally by the lingual nerve; the
terminal branches of the lingual nerve ascend on its medial side.
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Vessels and Nerves.—The arteries supplying the submaxillary gland are branches of
the external maxillary and lingual. Its veins follow the course of the arteries. The nerves
are derived from the submaxillary ganglion, through which it receives filaments from the
chorda tympani of the facial nerve and the lingual branch of the mandibular, sometimes
from the mylohyoid branch of the inferior alveolar, and from the sympathetic.
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Sublingual Gland (glandula sublingualis).—The sublingual gland (Fig. 1024) is the
smallest of the three glands. It is situated beneath the mucous membrane of the floor of
the mouth, at the side of the frenulum linguæ, in contact with the sublingual depression
on the inner surface of the mandible, close to the symphysis. It is narrow, flattened,
shaped somewhat like an almond, and weighs nearly 2 gm. It is in relation, above, with
the mucous membrane; below, with the Mylohyoideus; behind, with the deep part of the
submaxillary gland; laterally, with the mandible; and medially, with the Genioglossus,
from which it is separated by the lingual nerve and the submaxillary duct. Its excretory
ducts are from eight to twenty in number. Of the smaller sublingual ducts (ducts of
Rivinus), some join the submaxillary duct; others open separately into the mouth, on the
elevated crest of mucous membrane (plica sublingualis), caused by the projection of the
gland, on either side of the frenulum linguæ. One or more join to form the larger
sublingual duct (duct of Bartholin), which opens into the submaxillary duct.
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Vessels and Nerves.—The sublingual gland is supplied with blood from the sublingual
and submental arteries. Its nerves are derived from the lingual, the chorda tympani, and
the sympathetic.
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Structure of the Salivary Glands.—The salivary glands are compound racemose
glands, consisting of numerous lobes, which are made up of smaller lobules, connected
together by dense areolar tissue, vessels, and ducts. Each lobule consists of the
ramifications of a single duct, the branches ending in dilated ends or alveoli on which the
capillaries are distributed. The alveoli are enclosed by a basement membrane, which is
continuous with the membrana propria of the duct and consists of a net-work of branched
and flattened nucleated cells.
The alveoli of the salivary glands are of two kinds, which differ in the appearance of
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their secreting cells, in their size, and in the nature of their secretion. (1) The mucous
variety secretes a viscid fluid, which contains mucin; (2) the serous variety secretes a
thinner and more watery fluid. The sublingual gland consists of mucous, the parotid of
serous alveoli. The submaxillary contains both mucous and serous alveoli, the latter,
however, preponderating.
The cells in the mucous alveoli are columnar in shape. In the fresh condition they
contain large granules of mucinogen. In hardened preparations a delicate protoplasmic
net-work is seen, and the cells are clear and transparent. The nucleus is usually situated
near the basement membrane, and is flattened.
In some alveoli are seen peculiar crescentic bodies, lying between the cells and the
membrana propria. They are termed the crescents of Gianuzzi, or the demilunes of
Heidenhain (Fig. 1025), and are composed of polyhedral granular cells, which
Heidenhain regards as young epithelial cells destined to supply the place of those
salivary cells which have undergone disintegration. This view, however, is not accepted
by Klein. Fine canaliculi pass between the mucus-secreting cells to reach the demilunes
and even penetrate the cells forming these structures.
In the serous alveoli the cells almost completely fill the cavity, so that there is hardly
any lumen perceptible; they contain secretory granules imbedded in a closely reticulated
protoplasm (Fig. 1026). The cells are more cubical than those of the mucous type; the
nucleus of each is spherical and placed near the center of the cell, and the granules are
smaller.
Both mucous and serous cells vary in appearance according to whether the gland is in a
resting condition or has been recently active. In the former case the cells are large and
contain many secretory granules; in the latter case they are shrunken and contain few
granules, chiefly collected at the inner ends of the cells. The granules are best seen in
fresh preparations.
The ducts are lined at their origins by epithelium which differs little from the pavement
form. As the ducts enlarge, the epithelial cells change to the columnar type, and the part
of the cell next the basement membrane is finely striated.
The lobules of the salivary glands are richly supplied with bloodvessels which form a
dense net-work in the interalveolar spaces. Fine plexuses of nerves are also found in the
interlobular tissue. The nerve fibrils pierce the basement membrane of the alveoli, and
end in branched varicose filaments between the secreting cells. In the hilus of the
submaxillary gland there is a collection of nerve cells termed Langley’s ganglion.
FIG. 1025– Section
of submaxillary gland of kitten. Duct semidiagrammatic. X 200. (See
enlarged image)
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FIG. 1026– Human
submaxillary gland. (R. Heidenhain.) At the right is a group of mucous
alveoli, at the left a group of serous alveoli. (See enlarged image)
Accessory Glands.—Besides the salivary glands proper, numerous other glands are
found in the mouth. Many of these glands are found at the posterior part of the dorsum of
the tongue behind the vallate papillæ, and also along its margins as far forward as the
apex. Others lie around and in the palatine tonsil between its crypts, and large numbers
are present in the soft palate, the lips, and cheeks. These glands are of the same structure
as the larger salivary glands, and are of the mucous or mixed type.
145
Note 160. The Glossopalatinus (Palatoglossus), although one of the muscles of the tongue,
is more closely associated with the soft palate both in situation and function; it has
consequently been described with the muscles of that structure (p. 1139). [back]