Hard external or internal structures of the animal body constitute the skeleton. The skeleton which is external is known is exoskeleton, e.g., nails, hairs, etc. The endoskeleton consists of those hard part which are present inside the body of the animal.
On the basis of the position of the skeletal structures in the body the endoskeleton is divisible into two parts.
1. Axial Skeleton:
It is present on the median longitudinal axis of the body. It consist of skull, vertebral column, sternum and ribs.
2. Appendicular Skeleton:
It is situated at the lateral sides which actually extend outwards from the principles axis. It consits of pectoral and pelvic girdles and bones of arms and legs.
Skeleton of head is called skull. The skull consits of the following parts:
(i) Bones of the Cranium: The cranium is formed by 8 bones: 1 frontal bone, 2 parietal bones, 2 temporal bones, 1 occipital bone, 1 sphenoid bone and 1 ethmoid bone.
(ii) Ear Ossicles: There are present 6 ear ossicles (2 malleus, 2 incus and 2 stapes).
(iii) Hyoid Bone: The hyoid bone is situated just above the larynx. Although it is not a bone of the skull proper, it is customarily considered with that portion of the skeleton.
(iv) Bones of the Face: There are 14 bones which form the skeleton of the face, 2 zygomatic bones, 2 maxilla, 2 nasal bones, 2 lacrimal bones, 1 vomer, 2 palatine bones, 2 inferior nasal conchae or turbinated bones and 1 mandible.
Thus the skull consists of 29 bones.
At the posterior end of the cranium there are two rounded protuberances, the occipital condoles, that articulate with the atlas (1st vertebra).
The vertebral column is about 71 cm long. It is curved lying in the mid dorsal line of the neck and trunk. It is made up of 33 vertebrae. However, it consists of 26 bones because five sacral vertebrae are fused to form one sacrum and four coccygeal vertebrae are fused to form one coccyx. The components of the vertebral column are called vertebrae (sing, vertebra). The vertebrae are grouped into five groups:
Curvatures (Bends): Human vertebral column shows four curvatures- (i) Cervical curvature-in the neck region, convex anteriorly; (ii) Thoracic curvature-in the thorax, concave anteriorly; (iii) Lumbar curvature-in the abdomen, convex anteriorly; (iv) Sacral curvature-in the pelvic region, concave anteriorly.
It is about 15 cm long and consists of three parts-the manubrium is the uppermost part, the body is the middle portion and the xiphoid process is the tip of the bone. The true ribs (7 pairs) are attached to the sternum.
Ribs: There are 12 pairs of ribs which form the bony lateral walls of the thoracic cage.
1. The first seven pairs are called true ribs because their anterior ends are attached directly to the sternum by means of small plac.es of hyaline cartilage, the costal cartilages.
2. The eighth, ninth and tenth pairs of the ribs are called false ribs. They articulate by cartilage with costal cartilage of the seventh rib and thus are attached indirectly to the sternum.
3. The last two pairs of ribs are called floating ribs because their anterior ends are not attached to either the sternum or the certilage of another rib. The floating ribs protect the kidneys.
A typical (generalized) rib consits of two parts: vertebral part and sternal part. The vertebral part is long, bony and articulates with sternum or sternal part of its upper rib.
Each pectoral girdle consits of 1 clavicle and 1 scapula. At the point where the superior and lateral borders of the scapula meet there is the lateral angle which presents a shallow particular surface termed the glenoid cavity into which the head of the humerus is articulated.
Each arm consists of 1 humerus, 1 radius, 1 ulna, 8 carpal bones, 5 metacarpal bones, 5 digits (14 phalanges). Phalangeal formula: 2, 3, 3, 3, 3.
The pelvis or pelvic girdle is formed by two in nominate bones (hip bones). The sacrum and coccyx also take part in the formation of the pelvis. Each in nominate bone consits of three separate bones, the ilium, the ischium and the pubis. On its outer surface it has deep depression called the acetabulum which, with the almost spherical head of the femur, forms the hip joint. Above and behind the acetabulum there is large notch in the bone called the great sciatic notch and in front and below there is the obturator foramen occupied by a membrane.