THE SKELETAL SYSTEM
The skeletal system includes the bones or
cartilaginous structures of the skeleton, joint
capsules, and other tissues that connect and
stabilize the system such as joint cartilages and
ligaments. The functions of the skeletal system
include: (a) support of the body and its parts; (b)
protection of various body structures; (c)
leverage for the functioning of the muscular
system; (d) storage of minerals and energy; and
(e) blood cell production.
In higher vertebrates, bones can be classified
based on their shape and location. Long bones
are, just as the name implies, longer than they are
wide. Short bones are boxy in appearance. Flat
bones are plate-like and relatively thin. There
are also bones that do not fall into these
categories; these are known as irregular bones.
Sesamoid bones are round and flat and develop
within tendons.
Each bone has markings that distinguish it
from other bones. These markings serve functions such as muscle attachment or allow passage
of blood vessels and nerves; they can also be
used to identify the individual bones of the
skeletal system. Below is a list of the various
types of bone markings.
Condyle: a large, rounded articular
prominence
Epicondyle: a prominence above a condyle
Facet: a smooth, flat surface
Foramen: an opening through which blood
vessels, nerves, or ligaments pass.
Fossa: a depression in or on a bone
Groove or Sulcus: a furrow or elongated
depression that accommodates a soft
structure such as a blood vessel, nerve, or
tendon.
Head: a rounded articular projection supported on a constricted portion, the neck,
of a bone
Meatus: a tube-like passageway running
within a bone
Sinus: an air filled cavity within a bone
Spine: a sharp, slender process
Trochanter: a large projection found only on
the femur
Tuberosity: a large, rounded, usually
roughened process
Tubercle: a small rounded process
The skeleton is divided into two major divisions: the axial skeleton and the appendicular
skeleton. The axial skeleton consists of bones
that form the central axis of the body. It consists
of the skull, vertebral column, hyoid bone and
rib cage. The appendicular skeleton contains the
bones of the pectoral (or shoulder) girdle, upper
extremities, the pelvic girdle and lower
extremities.
SHARK SKELETON
I. Chondrocranium: portion of skull consisting
of a single large mass of cartilage which encloses brain; olfactory and otic capsules are
fused with chondrocranium
A. Rostrum: scoop-like anterior projection.
1. rostral carina: keel-shaped structure on
mid-ventral surface.
B. Olfactory Capsule: rounded swellings on
lateral aspects of rostrum base.
C. Orbits: located on either side of chondrocranium posterior to olfactory capsule; perforated by a number of foramina.
1. Ant- and Infraorbital Shelves: ridges
that demarcate anterior and medial boundaries of orbit.
D. Basal Plate: expanded posterior part of
chondrocranium extending back from
infraorbital shelf.
E. Epiphyseal Foramen: located mid-dorsally; epiphysis passes through here.
F. Otic Capsules: irregularly shaped masses
protecting inner ear; fused posteriorly with
chondrocranium
G. Postotic Process: ring-shaped process at
each posterolateral corner of ventral
chondrocranium.
H. Endolymphatic Fossa: large deep
depression in median line in otic capsule
region.
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J. Occipital Condyles: processes located
ventrolaterally on either side of foramen
magnum; articulate with first vertebra.
II. Splanchnocranium: visceral skeleton of
skull; consists of seven cartilaginous visceral
arches.
A. Mandibular Arch: first, most anterior and
largest of visceral arches; modified into
two teeth-bearing parts.
1. Palatopterygoquadrate Cartilages:
fused in anterior midline to form upper
jaw; bears orbital process, which projects into orbit and quadrate process,
projecting from lateral end.
2. Meckel’s Cartilages: fused at midline
to form lower jaw.
B. Hyoid Arch: second visceral arch consisting of five cartilages; laterally articulates
with chondrocranium.
C. Visceral Arches Three through Seven: gill
or branchial arches; has cartilaginous
spine-like processes: gill rakers (anterior)
and gill rays (posterior).
III. Vertebral Column: two distinct types of
vertebrae
A. Trunk Vertebrae: each has biconcave
centrum; projecting from this is a dorsal
neural arch from which projects a neural
spine; between adjacent spines are intercalary plates; these last three forms neural
canal which houses spinal cord; spinal
nerve emerge through foramina of intercalary plates; transverse processes project
from ventrolateral border of centrum.
B. Caudal Vertebrae: in addition to the
above structures these vertebrae have
hemal arches that form a hemal canal
through which the caudal artery and vein
pass.
IV. Pectoral Girdle and Fins: anterior fins supported by U-shaped cartilage just posterior to
splanchnocranium.
A. Coracoid Bar: ventral part of girdle.
B. Scapular Cartilage: extends dorsally from
each side of coracoid bar.
1. Glenoid Surface: lateral end of scapular cartilage that articulates with fin
skeleton.
C. Basal Cartilages: three cartilaginous
structures that articulate with glenoid
surface.
D. Radial Cartilages: small cartilages,
arranged in rows that articulate with
basals.
E. Ceratotrichia: distal cartilages that
support fin.
CAT SKELETON
I. Cranium
A. Frontal Bone: paired anterior bones of
cranium forming forehead and parts of
orbits.
B. Parietal Bone: paired bones posterior to
frontal bones; constitute the greater portion
of skull roof.
C. Temporal Bones: paired bones inferior to
parietal bones; constitutes greater portion
of cranial sides.
1. Squamosal Segment: large flat region
which gives rise to zygomatic process.
2. Mandibular Fossa: oval depression just
inferior to base of zygomatic process;
articulates with mandible.
3. External Auditory Meatus: canal-like
opening just posterior to mandibular
fossa; serves as a passageway for sound
waves to reach “eardrum”.
4. Mastoid Process: somewhat triangular,
projecting anteroventrally adjacent to
tympanic bulla.
5. Tympanic Bulla: rounded bony eminence ventral to squamousal.
D. Occipital Bone: single posterior bone
of cranium.
1. Supraoccipital Portion: located immediately posterior to parietals; forms lambdoidal ridge.
2. Basioccipital Portion: forms posterior
floor of cranium.
3. Foramen Magnum: single large opening on floor of skull which allows
passage of spinal cord.
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4. Occipital Condyles: oval processes on
either side of foramen magnum; articulates with vertebral column.
E. Presphenoid Bone: single bone lying in
mid-line; forms roof of pharynx.
F. Basisphenoind Bone: unpaired midline
bone posterior to presphenoid.
G. Alisphenoid Bone: wing-like bone
located on ventral surface of skull.
H. Ethmoid Bone: single bone in anterior
part of cranial floor between orbits.
1. Mesethmoid: medial vertical plate;
forms part of nasal septum.
2. Ethmoturbinals: thin scroll-like projections on either side of nasal septum; aid
in warming and humidifying incoming
air.
II. Facial Bones
A. Nasal Bones: paired long pointed bones
posterior to each extenal nares.
B. Premaxillary Bones: paired bones forming lower boundary of external nares.
C. Maxillary Bones: paired bones forming
upper jaw and hard palate.
D. Malar Bones: paired bones that form
prominences of cheeks with postorbital
and zygomatic processes.
E. Palatine Bones: paired bones posterior to
maxillary bones that forms part of hard
palate.
F. Mandible: the lower jaw consisting of
two halves; bears lower teeth.
1. Body: curved horizontal portion.
2. Ramus: perpendicular portion at posterior end of body.
3. Coronoid Process: large anterior
projection of ramus.
4. Condyloid Process: posterior branch of
ramus; articulates with mandibular
fossa.
III. Vertebral Column: composed of a number
of different bones known as vertebrae; divided into five sections, each having a specific
number of vertebrae: 7 cervical, 13 thoracic,
7 lumbar, 3 fused sacral, and 23 caudal; all
have common characteristics but vertebrae of
each section have unique characteristics.
A. Common Characteristics
1. Body (or Centrum): thick disc-shaped
anterior portion.
2. Vertebral Foramen: opening through
which the spinal cord passes; formed by
pedicles and laminae.
3. Intervertebral Foramen: opening formed by two articulating vertebrae allowing passage of single spinal nerve.
4. Transverse Processes: paired processes
on either side of a vertebra.
5. Spinous Process: single process projecting posteriorly from the vertebra.
IV. Thorax
A. Sternum: attaches ribs through costal
Cartilages; consists of eight sternebrae;
divided into three parts.
1. Manubrium: first sternebra.
2. Body: composed of six ternebra.
3. Xiphoid process: last sternabra.
B. Ribs: thirteen pairs of flat bones that
make up thoracic wall.
V. The Pectoral Girdle
A. Clavicle: a long slender bone.
B. Scapula: a large triangular flat bone.
1. Spine: sharp ridge on lateral surface.
2. Acromion: short blunt process projecting from ventral end of spine.
3. Glenoid Fossa: concavity on ventral
apex; articulates with humeral head.
VI. Arm
A. Humerus: the single bone of the arm.
1. Head: rounded proximal end that articulates with glenoid fossa.
2. Capitulum: rounded knob that articulates with radius.
3. Trochlea: pulley-like surface that
articulates with ulna.
VII. Forearm
A. Ulna: medial bone of forearm.
1. Trochlear Notch: large curved area on
anterior aspect of proximal end; articulates with humeral trochlea.
2. Radial Notch: curve lateral area
allowing articulation of radial head.
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B. Radius: lateral bone of forearm.
1. Head: disc-shaped proximal end.
VIII. Carpus and “Hand”
A. Carpals: the seven bones of the wrist
(know them as a group).
B. Metacarpals: five bones between carpus
and digits.
C. Phalanges: bones of the digits; each digit
has a proximal and distal phalanx and each
finger also has a middle phalanx.
IX. Pelvic Girdle
A. Innominate Bone: paired bones that are
formed by fusion of three separate bones
during development.
1. Ilium: long anterior portion; articulates
with sacrum.
2. Ischium: posterior portion.
3. Pubis: flat, curved area; left articulates
with right.
4. Acetabulum: deep depression which receives head of femur to form hip joint.
5.. Obturator Foramen: large round
opening formed by ischium and pubis.
The two innominate bones, with sacrum of
spine, form pelvis, a bowl-like structure.
Superior opening is pelvic inlet; inferior opening is pelvic outlet.
VI. Thigh
A. Femur: single bone of thigh.
1. Head: rounded proximal end that
articulates with acetabulum of pelvis.
2. Medial and Lateral Condyles: large
rounded surfaces at distal end that
articulate with tibia.
3. Medial and Lateral Epicondyles: rough
projections on either side of distal end
just proximal to respective condyles.
B. Patella: sesamoid bone on anterior side of
Knee.
2. Tibial Tuberosity: roughened area on
ventral surface serving as a point of
attachment for patellar ligament.
B. Fibula: smaller, lateral bone of leg.
VIII. Tarsus and Foot
A. Tarsals: seven bones of ankle area of
foot (know them as a group).
B. Metatarsals: five bones of mid-foot.
C. Phalanges: bones of toes; each toe has a
proximal, middle and distal (there are only
four toes).
The HUMAN KNEE
I. Bones
A. Femur: at proximal side.
B. Tibia: at mediodistal side.
C. Fibula: at laterodistal side.
D. Patella: within patellar ligament.
II. Ligaments
A. Quadriceps Ligament: insertion of
quadriceps femoris muscle of thigh; ends
at patella.
B. Patellar Ligament: continuation of quadriceps ligament; begins at patella; attaches to
tibial tuberosity.
C. Tibial and Fibular Collateral Ligaments:
proximal ends attached to respective femoral epicondyles; distal end attached to
respective bones.
D. Anterior and Posterior Cruciate Ligaments: proximal end of both attached to
posterior femur in intercondyler space;
posterior cruciate at attached distally on
posterior tibia; anterior cruciate attached
distally anteriorly between tibial condyles.
III. Articular Pads: made of fibrocartilage
A. Medial Meniscus: C-shaped ring.
B. Lateral Meniscus: complete ring.
VII. Leg
A. Tibia: larger, medial bone of leg.
1. Medial and Lateral Condyles: expanded area at proximal end which articulates with respective condyles of femur
to form knee joint.
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