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SKELETAL SYSTEM
What are Bones?
Bones are calcified connective tissue forming the major portion of the skeleton of
most vertebrates. There are about 206 bones in your body. They contain more
calcium than any other organ.
Bones begin to develop before birth. When the skeleton first forms, it is made of
flexible cartilage, but within a few weeks it begins the process of ossification.
Ossification is a process where cartilage is replaced by hard deposits of calcium
phosphate and stretchy collagen. It takes about 20 years for ossification to be
completed.
The study of bones that collectively make up the skeleton or frame work of the body
is called osteology. The figure below indicates the skeleton of various domestic
animals.
The skeleton of a living animal is made up living structures of bones. The bones have
blood vessels, lymphatic vessels and nerves. They are subject to disease, repairs
themselves and adjust to changes during stress.
The Structure and Functions of Bones
1) The functions of the skeleton- generally - as opposed to the functions of
particular bones.
2) Types of Bones- with examples.
3) The structure of Bone- with diagram and definitions.
1. Functions of The Skeleton
The bones provide the following functions:
 Protection: protection of some vital organs from the external damages is
one of the important functions of bones. The central nervous system (CNS)
is protected by the skull and vertebral column; the heart and lungs by rib
cage; and pelvis protects the internal parts of urogenital system.
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 Rigidity and form to the body: animals without a skeleton of some type
have little or no regular form. The skeleton gives a basis for the external
structure and appearance of most animals as we know them.
 Act as lever: in the vertebrates, locomotion, defense, offense, grasping, and
other activities of this type depend largely upon the action of muscles that
are attach to the levers. Almost without exception, these levers are made of
bone and are integral parts of skeleton.
 Storage of minerals: the entire skeleton serves as a dynamic storage area
for minerals, particularly calcium and phosphorous. These minerals are
deposited and withdrawn as needed in the on-going homeokinetic process.
 Site for blood formation: blood formation is not strictly a function of bone
proper, but of the marrow found within the marrow cavity of long bones
and within the spongy substance of all young bones.
2. Classification of bones
Any bone may be classified into one of the following groups:
Long bones: "Long bones" have greater length than width and consist of a shaft. They are
usually somewhat curved for strength. And also
are relatively cylindrical in shape with
two extremities called epiphyses (see Figure). There is metaphysis between each
epiphysis and the diaphysis. A long bone grows in length only at the epiphyseal
cartilage which is located within the metaphysis.
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Figure 5:A- longitudinal section of the humerus of a young dog. B- longitudinal section of the
humerus of a mature dog.
Function of long bones: chiefly as levers and aid in support, locomotion and
prehension. The best examples of long bones are pectoral limb, humerus, radius, ulna,
metacarpals, phalanges; pelvic limb, femur, fibula, tibia, metatarsals and phalanges.
Short bones: are somewhat cuboid in shape i.e approximately equal in all dimensions.
There is no marrow cavity. They are found in complex joints such as the carpus
(knee) and tarsus (hock). Example of short bones: Patella.
Function: - for variety of movement
- absorption of shock
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Flat bones: are relatively thin and expanded in two dimensions. They consist of two
plats of compact substance, lamina externa and lamina interna, separated by diploe.
Example of flat bone: frontal base of skull bone, scapula and pelvic bones
Functions: - protects vital organs such as brain, the heart and lungs.
- many provide large areas for muscle attachment.
Sesamoid bones: they are developed along the course of tendons. Example: Patella
(knee cap) is the largest sesamoid in the body.
Functions: - reduces friction a change the course of tendons.
- may change the angle of the pull of muscles and this give a greater
mechanical advantage.
Phunumatic bones: they contain air spaces or sinuses that communicate with the
exterior. Example: long bones of bird, frontal bones and maxillary bones of the skull.
Irregular bones: are unpaired bones located on the median plane and include the
vertebrae and some of the unpaired bones of the skull.
Functions: - protection, support and muscle attachment.
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Figure 1 Skeleton of the cow. (Source: Banerjee, 1991)
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Figure 2 Skeleton of the chicken. (Source: Banerjee, 1991)
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Figure 3 Skeleton of the horse. (Source: Banerjee, 1991)
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Figure 4: Skeleton of the goat. (Source: Frandson & Spurgeon, 1992)
In general, the whole of the skeleton system can be broadly divided into two groups:
1. Axial skeleton system
2. Appendicular skeleton system
1) Axial skeleton system: includes the bones of skull, vertebral column, sternum and
ribs. The table below indicates the bones of the axial skeleton by regions.
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Table: Bones of the Axial skeleton system. (Source: Spurgeon, 1992)
Skull
Vertebrae
Cranial bones
-occipital
- parietal
- interparietal
- temporal
- frontal
- ethmoid
- sphenoid
cervical
thoracic
lumbar
sacral
caudal
Facial bones
- pterygoid
- lacrimal
- nasal
- palatine
- conchae (turbinates)
- maxilla
- incisive (premaxilla)
- zygomatic (malar)
Vomer
Mandible
Hyoid
Ribs
ture-join sternum
cartilages
by
costal
false- not directly connected with
sternum
floating- last 1 or 2 pair connected
only with vertebrae
Sternum
manubrium
body
xiphoid process
Skull: forms the basis of the head. It consists of cranial bones which surrounds the
brain and facial bones which exhibits observable variation among the species.
Function: - protection of brain
- supports many sense organs
- forms passage for the beginning of digestive and respiratory system
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Figure: Head of the horse (Source:
Spurgeon, 1992).
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Cranial bones
Diploe
Facial Bones
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Figure: Showing the skull of the horse (Source: Spurgeon, 1992).
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Vertebral column: composed of median, unpaired and irregular bones. The following
indicates the part of vertebral column and letters are used to designate the respective
regions.
 Cervical vertebrae (C) - neck region
 Thoracic or dorsal (T) - chest region
 Lumbar (L) - loin region
 Sacral (S) - in region of pelvis- fused vertebrae
 Fused Lumbar and Sacral (LS)- in fowl
 Caudal or Coccygeal (Cd) - located in tail
Vertebral formula: for a given species consist of the letter symbol for each region
followed by the number of vertebrae in that region in the given species. The
following shows the vertebral formula of common farm animals.
Cow: C7 T13 L6 S5 cd18-20
Sheep: C7 T13 L6-7 S4 cd16-18
Pig: C7 T14-15 L6-7 S4 cd20-23
Horse: C7 T18 L6 S5 cd15-20
Chicken: C14 T7 LS14 cd6
Sternum and Ribs: forms the floor of the bony thoracic and gives attachment to the
costal cartilages of the sternal (true) ribs as well as forming a place of origin for the
pectoral muscles. The sternum consists of segments called sternebrae which tend to
fuse together as age advances. The number of sternebrae varies with species as
follows:
Pig: 6; Sheep: 6; Cow: 7; Goat: 7; Horse: 8
Sometimes the last one or two pair of ribs have no connection with other ribs at the
ventral end. Such ribs are called floating ribs. The spaces between the ribs are called
intercostal spaces.
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T
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S
L
C
Cd
Skull
Sternum
Ribs
2) Appendicular skeleton system: is made up of the bones of the limbs. Table below
compares the bones of the front (pectoral) limb to that of the hind (pelvic) limb by
region.
Table: Comparison of Pectoral and Pelvic bones (Source: Spurgeon, 1992).
Pectoral limb
Pectoral girdle (shoulder girdle)
Scapula
Clavicle
Coracoid
Humerus-arm
Radius-forearm
Ulna- forearm
CarpusMetacarpus- cannon
Phalanges- digits
Pelvic limb
Pelvic girdle (os coxae)-pelvis
Ilium
Ishium
Pubis
Femur- thigh
Patella
Tibia- leg
FibulaTarsus- hock
Metatarsus- cannon
Phalanges- digits
Pectoral limbs:
 Scapula (shoulder blade)- in all animals, it is rather flat, triangular bone.
 Humerus (arm bone)- is a typical long bone that varies only in minor
details from one animal to another.
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 Radius- is the larger of the two forearm bones, and the ulna is the smaller
mammals but not in birds. The radius is well developed in all species.
 Ulna- varies in its degree of development from species to species. In horse
the proximal portion of the shaft of the ulna is well developed but fused to
the radius. The cow, sheep, goat and pig each have a complete ulna, but
with restricted or no movement between the ulna and radius. The cat and
dog have considerably more movement between these complete bones, but
not nearly as much as man.
 Carpus- in all animals is a complete region that includes two rows of small
bones. Those in the proximal row are called radial, intermediate and ulnar.
Those in the distal row are numbered as 1,2,3, and 4.
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Figure: Forelimb (Pectoral) skeletons of domestic animals. A = horse; B = cow; C = pig; D = dog
(Source: Spurgeon, 1992)
Pelvic limb consists of
 Pelvic girdle (os coxae) - consists of three bones- ilium, Ischium,
pubis.
 Ilium-largest, triangular shape, with apex at acetabulum.
 Ischium
 Pubis-smallest of the three and forms the cranial part of the floor of
pelvis.
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 Femur(thigh)-extends from the hip joint to the stifle (the joint
corresponding to the human knee)
 Patella-distal end got two condyles for articulation with tibia.
 Tibia (leg)-larger and located medially. Where as the fibula is located
laterally.
 Tarsus
 Metatarsus.
 Phalanges.
Bones for the Veterinary Student - a summary.
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The skeleton provides a frame of reference for anatomically and clinically
significant features of the animal.
Many of the conditions you will treat in your patients, especially horses, and
to a lesser extent dogs, will be conditions of skeletal or joint pathology.
Since the developing skeleton is molded by the forces acting on it an
understanding of its growth, biomechanics and normal anatomy is absolutely
essential to the management of performance animals.
Since the skeleton is constantly in a state of remodeling even the "everlasting
bones" will be affected by chronic disease.
References
1) Notes for Diploma (Dr. Penjor and Nidup Karma).
2) NetPetMagazine presents an Online Gross Anatomy Lecture
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QUESTIONS:
Skeleton of chicken: Level it.
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