Thorax and sternum

In primitive, air-breathin are fishes, breathing was accomplished by sawllowing movements in which air was gulped into the lungs. Early reptiles improved on this system when they evolved a means to respire via the muscuskeletal machanics of the thoracic skeleton. this was made possible as the thoracic ribs extended ventrally from the vertebrae column to reach the sternum. The sternum formed akind of ventral bony column that fused into a bony bar and anchored the distal ends of the ribs. Further soft tissue specializations led to more sophisticated breathing function.
Thorax
Or chest, is like basket or cage composed of cartilage and bone. It is attached dorsally to the vertbral column. It is also the base ehich upper limbs are attached. The major bones forming the thorax are the sternum and twelve ribs on each side.
Male or Female
The thorax of the female differs from that of the male as follows:
1. Its capacity is less.
2. The sternum is shorter.
3. The upper margin of the sternum is on a level with the lower part of the body of the third thoracic vertebra, whereas in the male it is on a level with the lower part of the body of the second.
4. The upper ribs are more movable, and so allow a greater enlargement of the upper part of the thorax.
Sternum
Or breastbone, function at its upper end to connect the shoulder gridle ( Clavicle and Scapula ) to the thorax.The sternum is an elongated, flattened bone, forming the middle portion of the anterior wall of the thorax. its margins articulate with the cartilages of the first seven pairs of ribs. It consists of three parts, named from above downward, the manubrium, the body or gladiolus, and the xiphoid process. The bone is composed of 3 main parts in adulthood but develops from six segements. This segment joints may all fuse in adulthood, but their location is indicated by the costal notches along each side of the sternum. Its average length in the adult is about 17 cm., and is rather greater in the male than in the female.