THE SKELETAL
SYSTEM
Agriscience 332
Animal Science
#8646-C
TEKS: (c)(2)(A)
Introduction
The skeletal system is all the bony
tissues in an animal’s body.
Animal’s that have an internal
skeleton, or endoskeleton, include
humans and domestic animals.
The skeletons are similar in most
species, but may vary in lengths
and sizes of bones.
Functions of the skeleton include:
• Giving the body shape and form,
• Protecting vital organs,
• Allowing for body movement,
• Storing minerals, and
• Serving as a site for formation of
blood cells.
Anatomy of Bones and
Bone Tissue
The outer portion of the bone is hard,
dense bone and forms the cortex.
The inner portion of the bone is
spongy, porous bone that forms a
network called the medulla cavity.
The medulla cavity has a membrane
lining called the endosteum.
Bone marrow is a thick, red mass
of cells inside the medulla cavity,
which makes essential blood cells.
Blood cells created in the bone
marrow include the following:
• Leukocytes – fight infection,
• Erythrocytes – carry oxygen, and
• Platelets – help the blood to clot.
As the bone ages, the red bone
marrow gradually changes into
yellow fatty marrow.
Parts of the Bone
Epiphysis – refers to either end or
extremity of a long bone.
Proximal epiphysis – end closest
to the main body of the animal.
Distal epiphysis – end farthest
from the main body of the animal.
Diaphysis – the long bone shaft
between the two joint ends.
Epiphysial cartilage – layer of
cartilage between the joint ends
and the shaft that allows the bone
to increase in length.
Periosteum – fibrous membrane
that covers the exterior of the bone,
excluding the joint ends.
Articular cartilage – thin layer of
cartilage that covers each joint end.
Total Bone Mass
Of the total bone mass,
26% is mineral matter;
the other chemical
compositions are
20% protein, 4% fat,
and 50% water.
The outer layer of a bone is
composed of mineral deposits,
which makes the bone hard and
inflexible.
Calcium phosphate makes up
almost 85% of the mineral matter
and the remaining 15% is calcium
carbonate and magnesium
phosphate.
A bone is the body’s primary mineral
reservoir, which is constantly being
depleted or replenished.
One-third of the bone’s total weight
is comprised of living tissues that
contain replicating cells, blood
vessels, lymphatic vessels, and
nerves.
Because a bone is made up of living
organic matter, composed of fibrous
tissue and cells, it is vulnerable to
disease.
A bone can repair itself if injured and
reacts to changes caused by stress.
Classification of Bones
Bones are classified based on
function and shape.
Classifications include long bones,
short bones, flat bones, sesamoid
bones, pneumatic bones, and
irregular bones.
Long bones – bones found in
limbs that serve as supporting
columns and levers for the
skeleton, assisting in body
support, locomotion, and eating.
A long bone is an elongated, round
shaft with two ends.
Examples of long bones include
the femur and the humerus.
Short bones – short bones are
cube-shaped bones that contain
a spongy substance filled with
marrow spaces surrounded by a
thin layer of compact bone.
Short bones function to reduce
friction and change the direction
of tendons in the joint of a limb.
Examples of short bones can be
found in the knee and hock.
Flat bones – relatively thin, long,
and wide bones that contain two
plates of compact bone surrounded
by spongy bone.
Flat bones function to protect vital
organs, such as the brain, heart,
lungs, and pelvic viscera, and serve
as areas of muscle attachment.
Examples of flat bones are the ribs,
sternum, and scapula.
Sesamoid bones – flat and round
bones that are located along the
course of tendons.
Sesamoid bones reduce friction
and change the direction of
tendons or the angle of muscle
pull.
The kneecap, or patella, is an
example of a sesamoid bone.
Pneumatic bones – bones that
contain air spaces (sinuses) which
are in contact with the atmosphere.
Frontal and maxillary bones are
examples of pneumatic bones that
can be found in the face.
Irregular bones – bones that
protect and support the central
nervous system and are points
of some muscle attachments.
The bones in the vertebral column
and some unpaired bones of the
skull are examples of irregular
bones.
The surfaces of bones have
projections and depressions
that differ in size and shape,
depending on the function.
Articular projections and depressions,
located in the joint, are covered with
articular cartilage.
Non-articular projections and
depressions, serve as points of
attachment for tendons and
ligaments.
Physiology of Bones
Bones grow in the region of the
Section of Bone (Red)
epiphysial cartilage,
from Growth Plate Area
which is located
between the
epiphysis (end)
and diaphysis (shaft).
This bone growth
is an increase in both
diameter and length.
Epiphysial Cartilage
Photo by Rob Flynn courtesy of USDA Agricultural Research Service.
The periosteum produces new
boney tissue that increases the
diameter of the bone.
The periosteum, which is the outside
covering of the bone, is also involved
with repairing bone fractures.
As the bone increases in diameter,
the bone marrow cavity increases.
This is accomplished by the removal
or re-absorption of portions of the
inner bone.
As an animal matures, bone growth
stops.
Ossification occurs; that is, the
epiphysial cartilage becomes
calcified, bony material.
Although bone continues to be
reabsorbed and replaced, there is
no net bone growth.
Osteogenesis is the process of
bone formation.
Osteoblasts, which are the parent
cells of connective tissue,
accomplish this process by
multiplying and secreting an
enzyme called phosphatase.
Phosphatase causes some of
the cells to mature and secrete
calcium salts for ossification.
Osteocytes (mature bone cells)
are surrounded by calcified
osteoid material.
This osteoid material appears as
small cavities, which are connected
by tiny canals that transmit tissue
fluid, the life support for osteocytes.
Some of the new cells stay in the
periosteum, where they reproduce
and are stored until needed.
Re-absorption of bone will occur
due to the following reasons:
• bone growth or repair of fractures,
• aging,
• hormonal imbalances,
• inflamation,
• pressure, and
• certain bone diseases.
Osteoclasts, cells that secrete
phosphatase which dissolves
bone tissue, and increasing blood
supply are responsible for bone
re-absorption.
Bone growth is affected by
hormones, vitamins, and other
nutrients.
Nutritional deficiencies can make
bones fragile and distorted.
Because of their rigidity, especially
in older animals, bones can break
easily.
Bone tissue can also repair itself.
When a fracture occurs, it becomes
filled with blood and connective
tissue cells.
The bone-forming cells, osteoblasts,
replicate rapidly, forming a fibrin clot
or callus, which becomes calcified
into true bone tissue with a marrow
cavity.
Osteoclasts then reabsorb excess
bone tissue from the callus.
Common Types of Bone Fractures
Simple Fracture – a broken bone
that does not puncture the skin.
Compound Fracture – a broken
bone that results in the bone
protruding through the skin, making
infections possible.
Greenstick Fracture – one side of
the bone is fractured and the other
side is bent.
Epiphysial Fracture – a break in
the bone that occurs at the juncture
of the epiphysis (end) and diaphysis
(shaft).
Greenstick and epiphysial fractures
occur in young animals only.
Complete Fracture – the bone is
broken completely across.
Comminuted Fracture – the bone
is broken into fragments due to
crushing or splintering.
When a broken bone occurs, both
ends of the fracture should be
held together and immobilized by
a splint or bone pin.
Anatomy of Bone Joints
There are three main types of
joints or articulations that join
bones together:
• Immovable joints,
• Slightly movable joints, and
• Freely movable joints.
Immoveable Joints – joints that are
filled with fibrous tissue early in life
and ossify as the animal matures,
making them immobile.
Example: skull
Slightly Movable Joints – joints
that allow limited movement
forward, backward, and sideways.
These joints have flattened discs of
cartilage and are sometimes called
gliding joints.
Example: joints of vertebral column
(backbone) and those adjacent to
the pelvic bone.
Freely Movable Joints – also called
synovial joints, allow friction-free
movement.
These joints consist of the following
• Articular surfaces – bone surfaces
are shaped to operate smoothly
with the bones to which they
connect;
• Articular cartilage – cartilage that
covers the articular surface and
absorbs concussions;
• Joint capsule – capsule that
contains synovial fluid, which
lubricates the joint and allows
for the friction-free movement; and
• Ligaments – connective tissue
bands that connect bone to bone.
Types of freely movable joints:
• Hinge joints – moves in two
directions, flexion and extension.
Example: knee joint.
• Plane joints – move in slight
gliding motions between flat
surfaces.
Example: carpals or small cubeshaped bones in knee joint.
• Pivot joint – allows rotary motion.
Example: in the neck between
the axis and atlas vertebrae; and
• Ball-and-Socket Joint – allows
movement in almost all directions
and is characterized by a
spherical head on one bone
fitting into a cup-shaped socket of
the other bone.
Example: hip joint
Parts of the Skeleton
There are three general areas of the
vertebrate’s skeletal system:
• Axial skeleton,
• Pectoral limb, and
• Pelvic limb.
Anatomy of the Axial Skeleton
The axial skeleton consists of the
bones that are either on the midline
of the back or are attached to the
bones of the midline, including the
skull, vertebral column, ribs, and
sternum.
Skull – protects the brain, houses
many of the sense organs, and
contains the beginnings of the
digestive and respiratory systems.
It is composed of the cranial part
and the facial part.
Many cranial and facial bones occur
in pairs, one on each side of the
head and are connected at joints
called sutures.
Major bones in the cranium:
• Occipital bones – situated at the
back and lower part of the
cranium;
• Parietal bones – form the sides
and roof of the cranium;
• Frontal bones – serve as the origin
of horns in horned animals.
• Ethmoid bone – contains openings
for olfactory nerves that are
responsible for the sense of
smell; and
• Sphenoid bone – supports the
brain and pituitary gland.
Facial parts of the skull:
• Orbital section – contains eye sockets
that house and protect the eyes;
• Nasal section – two small, oblong
bones that form the “bridge” of the
nose.
• Oral section – “mouth” bones that
support the teeth and provide
muscle attachment for chewing
and swallowing.
Vertebral Column – the vertebral
column, or backbone, is divided
into 5 anatomical regions including
the cervical, thoracic, lumbar,
sacral, and coccygeal vertebrae.
The number of vertebrae in each
region varies among the different
species.
Cervical vertebrae – the cervical
vertebrae are in the neck area and
allow for movement of the head.
Atlas – first cervical vertebrae that
forms a hinge joint with the
occipital bone of the skull and
allows the head to move up
and down.
Axis – second cervical vertebrae
that forms a pivotal joint with
the atlas and allows the head
to turn from side to side.
Thoracic Vertebrae – the thoracic
vertebrae are in the chest area and
are the attachment sites of the ribs.
Lumbar vertebrae – the lumbar
vertebrae are in the loin area and
are slightly more mobile than the
thoracic vertebrae.
The large, flat projections of the
lumbar vertebrae, that extend to
either side of the midline, are the
long bones seen in a T-bone cut.
Sacral vertebrae – the sacral
vertebrae are in the pelvic region
and are usually fused to form a
single wedge-shaped bone, sacrum,
to which the pelvic limb is attached.
Coccygeal Vertebrae – the
coccygeal vertebrae are in the tail
area and vary greatly in number
depending on the species.
Number of Vertebrae in Selected
Animal Species
Cattle Swine Sheep Horse Humans Poultry
Cervical
7
Thoracic
13
Lumbar
6
5-7
Sacral
5
4
Coccygeal
18-20
7
7
7
7
18
12
6-7
6
5
4
5
5
(fused)
(fused)
13-17 13-14
20-23 16-18
4
(fused)
14
6
Ribs – thin, flat, curved bones that
protect the heart, lungs, stomach,
spleen, and kidneys.
Ribs also assist in respiration by
lifting up and allowing the lungs to
expand during inhalation and by
moving down and squeezing air
out during exhalation.
The number of rib pairs usually
corresponds to the number of
thoracic vertebrae, but sometimes
extra ribs can occur in front of or
behind the thoracic vertebrae.
True ribs (sternal ribs) – ribs that are
attached to the sternum by cartilage.
False ribs – ribs that do not connect
directly to the sternum, but may
connect to the last sternal rib by
cartilage.
Floating ribs – ribs that have no
connection to other ribs in the
sternum area.
Sternum – several small bones
(sternebrae) that fuse together as
animal ages to form the floor of the
thoracic cavity.
The sternum, as previously
mentioned, is the site of attachment
for the sternal (true) ribs.
Number of Sternebrae
Swine and sheep ……………. 6
Cattle …………………………. 7
Horses and dogs ……………. 8
Anatomy of the Pectoral Limbs
The front legs of four-legged animals
(quadrupeds) are the pectoral limbs.
The bones included in the pectoral
limb are the scapula, humerus,
radius and ulna, carpus, metacarpus,
and phalanges.
The pectoral limbs are connected to
the axial skeleton (body) by muscles
and connective tissues.
The joints formed by the scapula and
humerus are ball-and-socket joints,
but function as hinge joints.
The remaining joints in the pectoral
limb also function as hinge joints.
Scapula (shoulder blade) – a
triangular-shaped flat bone that
attaches to the humerus.
Humerus (arm) – a long bone
that extends toward the front of
the animal, forming the point of
the shoulder.
The articular angle, the angle
formed by the scapula and
humerus, is important to the
soundness of the front legs.
In poultry, the wishbone or
coracoid, connects to the scapula.
Radius – the larger, well-developed
bone in the forearm, located on the
inside of the foreleg.
The radius connects to the humerus
forming the elbow joint.
Ulna – the smaller of the two bones
in the forearm.
Together, the radius and ulna make
up the bones in the forearm.
In horses, the radius and ulna are
fused, allowing no movement
between the bones.
In cattle, sheep, and swine, the
ulna is more developed and limited
movement occurs between the
radius and ulna.
Carpus (knee) – a complex region
of small, cube-shaped bones (sliding
joints) that function together as a
hinge joint.
Metacarpal (cannon) bones – bones
that form the lower part of the foreleg.
The number of metacarpal bones
vary with the species.
Phalanges (digits) – the bones that
form the toes (fingers in humans) on
the pectoral limb.
The number of digits vary with the
species: horses (1), cattle and
sheep (2), swine (4), and dogs (5).
Dewclaws – second and fifth digits
in swine (more developed), cattle
and sheep (non-functional).
Each digit (toe) is made up of
three phalanges (small bones).
The first phalanx is the long
pastern bone, the second phalanx
is the short pastern bone, and the
third phalanx is the coffin bone.
Anatomy of the Pelvic Limbs
The hind legs of quadrupeds (fourlegged animals) are the pelvic limbs.
The pelvic limbs are connected to the
axial skeleton by the pelvic girdle.
The femur, tibia, fibula, tarsals,
metatarsals, and phalanges are the
bones that form the hind leg.
Pelvic girdle – three bones (ilium,
ischium, and pubis) that are fused
together to form an irregular bone
called the os coxae or pelvis.
The pelvis is connected to the
sacrum at the sacroiliac joint.
Ilium bones – front, dorsal bones in
the pelvis; in cattle, the front points
of the ilium bones are called the
hook bones.
Ischium bones – rear, dorsal
bones in the pelvis; in cattle, the
rear points of the ischium bones are
called the pin bones.
Pubis – ventral bones in the pelvis
that form the floor of the pelvic girdle.
Femur – a long bone that extends
from the hip joint to the stifle joint and
is the site of several hip and thigh
muscle attachments.
Patella (kneecap in humans) – the
largest sesamoid bone in quadrupeds.
Tibia – the larger, thicker of the
two long bones in the hind leg,
the tibia is located on the inside
of the hind leg and extends from
the stifle joint to the hock joint.
Fibula – the thinner bone in the
hind leg, the fibula extends from
the upper end of the tibia to lengths
that vary depending on the species.
Tarsus (hock) – two rows of tarsal
bones in the hind leg that correspond
to the ankle in humans and are
similar to the carpus in the front leg.
Fibular tarsal – bone that forms
the point of the hock and serves
as a lever for muscles that extend
the hock.
Metatarsus (rear cannon bone) –
bones that are similar to the
metacarpals of the foreleg, but are
slightly longer.
Phalanges – bones that make up
the digits of the hind leg, which are
similar to those in the front leg.
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Instructional Materials Service
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2007