• Birds lack cheeks, lips and teeth. Rather, the functions of prehension and
mastication are performed by the beak.
• The beak is divided into upper and lower parts. It consists of keratin overlying a
borne framework (incisor bones for the upper jaw and the mandible for the lower
jaw).
There are significant differences in the embryonic development of the oral cavity
that result in there being a permanent connection between the oral and nasal
cavities in birds. This connection is called the palatine (or choanal) cleft. Lying
above the palatine cleft is the orbital fossa which is part of the nasal cavities. The
choanal duct terminate in the choanna at the apical end of this fossa
During respiration, the tongue is applied firmly to the roof of the mouth so only
the caudal part of the choannal cleft is available for air movement. During
swallowing, the orbital fossa and the glottis are closed, leaving the esophageal
opening as the only pathway open for the passage of food
• The Tongue:
1. The shape is very variable between species and is related to the normal diet of
the bird.
2. The body of the tongue usually contains a bone (os entoglossum) which
articulates with the basihyoid bone.
3. The avian tongue differs significantly from the mammalian tongue in that there
is no internal muscle system. The muscles controlling the movements of the
tongue are extralingual.
• The maxillary gland lies against the incisor bones. The ducts of the maxillary
gland open in the apical region of the palate.
• Palatine glands are located in the lateral areas of the palate
• Pterygoidean and the tubariae gland s are located in the mucosal of the roof of
the pharynx. The ducts of the pterygiodean glands open just lateral to the
infundibular cleft. The ducts of the tubariae glands open in the funnel of the
auditory tubes.
• The cranial mandibular gland lies in the apical part of the floor of the mouth (in
the angle at the point of the lower beak).
• Caudal madibular glands have multiple ducts that open in the caudolateral area
of the mouth.
• Cranial lingual glands are found on either side of the tongue.
• Caudal lingual glands are located at the base of the tongue and extend caudally
as far as the larynx.
• Gland of the angularis oris is located in the mucosal at the angle of the mouth.
It is common to refer to the oropharyngeal cavity in the birds because there is no
soft palate. However, the term is not correct. The rostral extent of the pharynxy is
delineated by the beginning of the the infundibular cleft.
• Infundibular cleft
1. The boundary between the mouth and the pharynx
2. Not part of the orbital fossa. Separated by a narrow piece of mucosa
3. Opens into the tubopharyngeal space or infundibulum tubarum. This is the
opening of the auditory tubes.
4. There is a considerable amount of lymphoid tissue surrounding the auditory
tubes and in the region of the infundibular cleft. Collectively called the tubal
tonsils.
Pharynxy in birds has essentially the same functions as the pharynxy in
mammals i.e. it is the cross-over point between the respiratory and alimentary
tracts.
The position of the esophagus in the neck is variable, depending on species. It is
very distensible, particularly in water birds.
The esophagus is divided into cervical and thoracic
parts
• Cervical part
1. In most (but not all species), starts dorsal to the larynx and then courses to the
right side of the neck
2. Covered only by skin and lies next to the jugular vein
3. Before entering the body cavity, it widens into a crop
• Size shape and position of the crop varies with species
• Main function is food storage
• Mucosa contains mucous glands
• On the dorsal surface of the crop is a groove (crop channel) that when closed,
diverts food from the cervical esophagus directly into the thoracic esophagus,
thus by-passing the crop.
• In pigeons and some other birds, the crop produces a substance called crop
milk which is regurgitated and used to feed chicks
a.Consists of desquamated surface epithelial cells that have undergone fatty
change.
• Thoracic part
1. Associated with the carotid A, juglar veins and the trachea
2. Pushed between the syrinx and the ventral surface of the lung, base of the
heart and dorsal surface of the liver.
The stomach of most birds is divided into 2 basic parts: the proventriculus =
glandular part of the stomach and the gizzard = muscular stomach.
Proventriculus
It lies in the lower left quadrant of the body cavity. Its long axis is directed from
craniodorsal, medially, to caudoventral. It lies in a depression on the dorsal
surface of the liver.
Usually there is no distinct point of separation between the thoracic esophagus
and the proventriculus.
It is related to the left abdominal air sac and the left caudal thoracic air sac.
Contained within the same visceral peritoneal sac as the spleen. The dorsal
peritoneal fold envelopes the proventriculus and spleen (the dorsal mesentery)
and then continues as the ventral mesentery and attaches to the liver.
The mucosal surface consists of a series of longitudional folds.
• It contains many compound tubular glands (visible at gross level): called deep
propria glands
a. The glands are radially grouped around a central collecting duct that opens on
a raised papilla on the mucous membrane. These papillae are visible and can be
mistaken for parasites.
b. Near the openings of the deep propria glands are a series of glandular crypts
called the superficial propria glands.
c. Secrete acid and pepsin
Gizzard (muscular stomach)
The gizzard functions as a masticatory organ.
• Mucosa is covered by a thick, hard and abrasive covering (keratinoid layer) that
is produced by the tubular glands of themucosa.
a. Note, the lining is not simple keratin. It consists of desquamated surface
epithelium, proteinaceous rodlets and a cementing mucin. A better term for the
material is koilin.
• It often contains small pebbles, stones or grit which are supposed to facilitate
the grinding of food.
It is shaped like a biconvex lens and almost completely fills the lower left
quadrant.
• It is related to the liver, spleen, various parts of the intestine, the left abdominal
air sac, ovary and the ventral body wall.
• Is attached to the dorsal mesentery
• Is attached to the sternum and various parts of the intestine by a serous
membrane that is continuous with the falciform ligament of the liver.
The gizzard is divided into a body and cranial and caudal diverticula and is very
muscular.
• The 4 main muscles are:
a. Musculus intermedius cran. that runs from the cranial diverticulum to the
dorsal part of the body.
b. Musculus intermedius caud. that runs from the caudal diverticulum to the
ventral part of the body.
c. Lateral muscles: originate from the aponeuroses on the medial and lateral
surfaces and enclose the dorsal and ventral area of the body of the gizzard on
the right side.
d. Muscles are smooth muscles
The pyloris is located near the cranial diverticulum on the right side of the
structure. The junction of the gizzard and duodenum is often described as being
valve like.
In some birds whose food does not require much grinding (e.g. herons), the
pyloris and the esophagus are close together, thus food can bypass the gizzard
and move directly from the proventriculus into the duodenum.
The intestine can be divided into 2 main parts: small and large.
Small Intestine
Usually divided into a number of parts. The small intestine consists of 3 primary
loops. There are 2 systems of nomenclature for these loops:
Duodenal loop = duodenum
Umbilical loop = jejunum
Supraduodenal loop = terminal jejunum and ileum
• Duodenal loop
Forms a characteristic U shape with a descending left ventral limb that lies
against the right lobe of the liver. This runs caudally on the ventral abdominal
wall (overlayed by the umbilical loop) and turns to the left around the caudal
contour of the gizzard and turns into the ascending right ventral limb. This is
related to the dorsal surface of the liver, right testis or the ovary, and curves
towards the cranial pole of the right kidney where it becomes the umbilical loop.
1. The junction between the ascending right ventral limb of the duodenum and
the umbilical loop is usually regarded as the point where the gut bends from left
to right just caudal to the cranial mesenteric A.
2. The 2 limbs are connected by the ligamentum pancreaticoduodenale that
contains the pancreas.
3. The dorsal mesentery gives off a double layer of serosa that attaches to the
ligamentum pancreaticoduodenale. The duodenum is also connected to the
gizzard by a serosal ligament.
4. The bile and pancreatic ducts always empty into the right ventral limb.
• The system of bile/pancreatic ducts depends on the species.
• The bile and pancreatic ducts run in the ligament connecting the porta of the
liver to the ascending right ventral limb of the duodenum (hepatoduodenal
ligament).
• Umbilical loop (jejunum)
1. Is the longest segment of the small intestine.
2. It fills most of the right caudal quadrant of the body.
3. Is related to the stomach, spleen, right liver lobe and the ovary in females.
4. Is supported by a mesojejunum
5. The exact arrangement and length of the umbilical loop varies with species.
• Some species have simple parallel loops arranged along the long axis of the
body (duck and goose)
• In domestic fowl, there are 10-11 loops of varying size that form a circle around
the mesojejunum.
• In the pigeon, there are 3-4 concentric spiral loops (centripetal coils) that form a
cone of gut. Within this cone, there are another 2-3 centrifugal coils. The
centripetal and centrifugal coils are wrapped around the mesojejunum.
6. Contains Meckel's diverticulum
• Remnant of the yolk duct and yolk sac.
• Contains lymphoid tissue in the adult.
• Supraduodenal loop (distal jejunum and ileum)
1. Runs from right to left above the duodenum and related to the gizzard.
2. Ileum is differentiated from the jejunum by the presence of ileocaecal fold(s).
• Number of folds depends on the number of cecae: either none, one or two,
depending on species.
Large Intestine
The large intestine consists of the cecae and the colon.
• Cecae
1. As in mammals, they are blind sacs. Tend to be large in herbivorous species
and small in the carnivores.
2. 2 cecae are present in most of the domestic birds.
• Large in the chicken
• Small in the pigeon and other passerines
• Absent in the budgerigar
3. Some birds (e.g. the herons) only have one cecum.
4. Connected to the ileum by the ileocecal ligament.
5. Position within the body depends on size and species.
6. In some species (fowl, duck, geese) there is an area near the cecal orifice that
contains a large amount of lymphoid tissue: the cecal tonsils.
• Colon
1. Direct continuation of the ileum
2. Supported by a short mesocolon
3. Runs in the midline below the vertebral column and ends in the coprodeum.
The cloaca is the terminal part of the urinary, reproductive and gastrointestinal
systems.
It is divided into three compartments by a series of contractile ring folds
• Coprodeum
1. It receives and temporarily holds the faeces.
• Urodeum
1. It receives the ureters, and in the male, the seminal duct. In females, it
receives the opening of the oviduct.
• Proctodeum
1. It houses the copulatory organ
2. Busa of Fabricii opens on the dorsal wall of the proctodeum.
3. It opens to the outside by the vent or anus.
• Divided into a large right lobe and a smaller left lobe by a cleft (cranial and
caudal incissura).
• The parietal (ventral) surface is molded to the contour of the body wall. The
visceral (dorsal) surface is molded around the heart, proventriculus, gizzard,
spleen, gall bladder and cranial end of the duodenal loop.
• The liver is contained within a peritoneal sac: the hepatoperitoneal sacs.
• Most species have a gall bladder.