Skeletal System

SKELETAL SYSTEM IN HUMAN

The skeletal system is divided into two main parts.

(i) Axial Skeleton. It lies along the principal axis of the body. It includes skull, vertebral column, ribs, sternum and ribs.

(ii) Appendicular Skeleton. It is made up of the girdles and limb bones.

 

Hyoid

It serves as a point of attachment for some of the muscles of the tongue and floor of the mouth but does not articulate with any other bone.

Ear Ossicle

Ear ossicles are three in number

VERTEBRAE

Vertebral column protects the spinal cord, supports the head and serves as the point of attachment for the ribs and musculature of the back.

Cervical vertebrae

(i) 1, 2, 7 cervical vertebrae are atypical and 3 to 6 are typical.

(ii) The transverse processes are perforated by foramina (foramen transversarium) for the passage of the vertebral arteries except in VII Vertebrae.

(iii) The first cervical vertebra or atlas supports the head and consists of a complete ring of bone. On its upper surface it presents kidney-shaped facets for articulation with the condyles of the occipital bone, forming a condyloid joint, the atlanto-occipital joint, at which the nodding movements of the head take place. The atlas articulates with the second cervical vertebra.

(iv) The second cervical vertebra or axis is the pivot on which the atlas turns in the rotary movements of the head. From the body of the axis a process of bone rises which is called the odontoid peg which articulates with the back of the anterior arch of the atlas and is held in position by the transverse ligament of the atlas. The lateral masses of the atlas articulate with corresponding facets on the axis placed on each side of the odontoid peg. The atlas moves round the odontoid peg of the axis, forming a pivot joint at which the head rotates.

(v) The seventh cervical vertebra is the first vertebra with an undivided spinous process. This process has a tubercle at its tip. It forms a distinct projection in the neck and can be seen at the lower part of the back of the neck. Because of this characteristic the bone is called the vertebra prominens.

Thoracic Vertebrae

(i) Thoracic 2-8 are typical 1, 9, 10, 11, 12 are atypical.

(ii) These are larger than the cervical and they increase in size as they extend downwards.

(iii) The body is heart-shaped, with facets on each side for attachment of the ribs.

(iv) The neural arch is relatively small, the spinous process is long and is directed downwards.

(v) The transverse processes which help to support the ribs are thick, strong and carry articular facets for the ribs.

Lumbar Vertebrae

(i) These are the largest vertebrae as compared with the bodies of the other vertebrae and is kidney-shaped.

(ii) The spinous process is broad and hatchet-shaped.

(iii) The transverse processes are long and slender.

(iv) The fifth lumbar vertebra articulates with the sacrum at the lumbo-sacral joint.

Sacrum

(i) It is a triangular bone situated at the lower part of the vertebral column, wedged in between the two innominate bones and forming the back of the pelvic cavity.

(ii) The base of the sacrum lies above and articulates with the fifth lumbar vertebra, forming a typical intervertebral joint.

(iii) The junction between the fifth lumbar vertebra and the sacrum forms the sacro-vertebral angle.

(iv) At the extremities of transverse ridges, on each side, sacral foramina are present for the passage of nerves. The apex of the sacrum articulates with the coccyx. At the sides the sacrum articulates with the innominate bones, forming the right and left sacro-iliac joint.

Coccyx

It is composed of four or five rudimentary vertebrae, fused to form one bone. It articulates above with the sacrum.