Skeletal System

Chapter- 4

Skeletal System 

Introduction

• Skeleton constitutes the bony framework of the body.

• The skeletal system consists of about 206 bones to make a strong, movable living framework for the body. It supports and protects softer, delicate tissues and organs and they form joints for the movement of the body. The bones making up the skeleton are of various types e.g. long bones, short bones, flat bones, irregular bones etc.

• The bones perform following important functions:

(1) They form the supporting framework of the body;

(2) They form boundaries for the cranial, thoracic and pelvic cavities;

(3) They give protection to delicate organs;

(4) They form joints which are essential for the movement of the body.

The Axial Skeleton

The bones of the axial skeleton constitute the central bony core of the body.

THE SKUM

• It rests upon the upper end of the vertebral column and its bony structure is divided into two parts viz., cranium and face.

1. The Frontal Bone:

• The frontal bone is a large flat bone which forms the forehead and also the upper part of the orbital cavities. It develops in two parts which gradually fuse into one bone. It contains two cavities called the frontal sinuses which lie one over each orbit. They contain air which enters by a small opening leading from nasal cavities. These sinuses give lightness to the bone and resonance to the voice, acting as sounding chambers.

2. The Parietal Bones:

• The parietal bones are two flat bones forming sides and roof of the skull. They articulate with each other and with frontal, occipital and temporal bones. The inner surface is concave and is grooved by the brain and blood vessels.

3. The Occipital Bone:

• The occipital bone forms back of the head and part of the base of the skull. It forms immovable joints with parietal, temporal and sphenoid bones. On the outer surface, there is a roughened area called occipital protuberance. In this bone, there is a large opening known as the foramen magnum, for the passage of spinal cord.

4. The Temporal Bones:

• The temporal bones lie on each side of the head (Fig. 4.2). Each temporal bone is divided into four parts.

The Vertebral Column

• It consists of twenty-four separate, movable, irregular bones called vertebrae which are divided into three groups viz.; cervical (seven); thoracic (twelve) and lumbar (five). (Fig. 4.6). In addition, the sacrum consisting of five fused bones and the coccyx consisting of four fused bones which also form a part of the vertebral column. When viewed from the side, the vertebral column shows four anteroposterior curves. They are: cervical curve, in the neck which is convex forwards; thoracic curve, convex backwards; lumbar curve, convex forwards and the pelvic curve convex backwards. Posteriorly convex curves, thoracic and pelvic, are called primary curves and anteriorly convex curves are called secondary curves.

• The Cervical Vertebrae: These are the smallest separate vertebrae with relatively large openings; they run down the neck forming a slightly forward curve. They have two special features: (a) Each transverse process carries an opening through which a vertebral artery passes upwards to the brain. (b) The spinous process is forked or bifid giving attachment to muscles and ligaments.

The Thoracic Cage

• The bones of the thoracic cage (Fig. 4.14) are as follows: 1 sternum; 12 pairs of ribs and 12 thoracic vertebrae.

• The Sternum: It is a flat bone in the middle of the chest. It is shaped like a dragger and is described in three parts.

• The manubrium is the uppermost part and presents two articular facets on its lateral borders for articulation with the clavicles.

• The body or middle portion is called gladiolus. It presents facets on the lateral borders for the attachment of the ribs.

• The Xiphoid process is the tip of the bone which gives attachment to the diaphragm and muscles of the anterior abdominal wall. The process is also called Xiphisternum. The ribs join the sternum by strips of cartilage.

• Each rib is a flat curved bone having a head, neck, tubercle, angle, sternal end, anterior and posterior surface and a superior and inferior border.

The Appendicular Skeleton

• It consists of shoulder girdles with upper limbs and pelvic girdle with lower limbs. Each shoulder girdle consists of one clavicle and one scapula. On each side of the shoulder girdle following bones are attached: one humerus, one radius, one ulna, eight carpal bones; five metacarpal bones and fourteen phalanges.

Tarsals:

Tarsal or ankle bones are seven in number and form the posterior part of the foot. The bones are one talus; one calcaneus; one navicular; three cuneiform and one cuboid.

The Joints

• A joint is the site at which any two or more bones come together. The joints are classified as fibrous, cartilaginous and synovial. In fibrous or fixed joint, there is no movement between the bones concerned. There is a fibrous tissue between their ends e.g., the joints between the bones of the skull and the joints between the teeth and the maxilla and mandible. In cartilaginous joints, there is a pad of white fibrocartilage between the ends of the bones. Movement is possible because of the compression of the pad of cartilage, e.g. symphysis pubis and joints between the bodies of the vertebrae.

• Synovial joints are characterized by the presence of synovial membrane. A considerable amount of movement is possible. Limitations on movement is mainly due to the shape of the bony surface which forms the joint. They are subdivided according to the movements possible.

(1) Ball and Socket joint: A hemispherical head fits into a cup-shaped socket e.g., shoulder and hip.

(2) Hinge Joints: These joints allow movement in one direction only, e.g. elbow, knee and ankle.