Question

According to position, endoskeleton is divided into how many parts? Explain each part.

Answer 1

On the basis of position, endoskeleton is divided into two main parts:
(i) Axial skeleton: This consists of skull, vertebral column, sternum and ribs. Human skull has 22 bones of which 8 are present in the cranium and 14 on the face. Brain is present in the cavity called cranium. In the occipital region is an opening called foramen magnum through which spinal cord passes. Cranium has 8 bones which covers the brain from all sides.
(1) Facial bones: These bones form the face and protect the sensory organs present in the head. It has 14 bones.
(2) Vertebral Column:
It makes the axis of the body and provides support like a pillar. Vertebral column is divided into following five parts:
(a) Cervical Region. It has 7 cervical vertebrae. First vertebra is called atlas, second is axis and rest are called typical cervical vertebrae.
(b) Thoracic Region. It has 12 thoracic vertebrae. Neural spine of these vertebrae is long and pointed, which provdes surface for muscle and attachment.
(c) Lumber Region. It has 5 lumber vertebrae.
(d) Sacral Region. It has 5 sacral vertebrae which are permanently fused to each other. These are situated on both halves of pelvic and form a joint with pelvic girdle.
(e) Caudal Region. It has 4 caudal vertebrae. These are also fused to form a triangular structure which is called coccyx.
(3) Sternum and Ribs
Sternum is the bone at the mid-ventral line of the thorax to which most of the ribs are attached at their ventral ends. Ribs are formed by vertebral and sterna part. First seven pairs of ribs are called true ribs. These are joined on one side with sternum and on second side with thorax.
(ii) Appendicular skeleton: It consists of limbs, shoulder or pectoral girdle and hip or pelvic girdles and bones of limbs.
(a) Bones of the fore arm: Humerus, radius, Ulna, Carpals, Metacarpals, finger bones, etc.
(b) Bones of hind limb: Femur, Tibia, Fibula, Tarsal, Metatarsal, etc.
(c) Pectoral Girdle: In mammals, two halves of pectoral girdle are situated at equal distance from the ribs on the dorsal plane. Each of it is called shoulder bones. Each half of it made up of a triangular flat bone called scapula. From its outer side a projection arises called acromion and has a cavity called glenoid cavity.
(d) Pelvic Girdle: It consists of two similar halves. In each, three bones are found, ilium, ischium and pubis. Pubis bones of both halves are joined and are called pubic sysmphysis. Ilium and ischium form acetabulum cavity in which head of femur articulates by forming ball and socket joint. A hollow space is found between pubis and ischium, which is called obdurate foramen, which provides passage for obturatry artery, vein and nerve.