The skeleton of the thorax, or chest (fig. 308), is an osseocartilaginous cage which contains and protects the principal organs of respiration and circulation. It is conical in shape, narrow above and broad below, flattened from before backwards, and longer behind than in front. It is reniform on horizontal section on account of the forward projection of the vertebral bodies.
The inlet of the thorax is reniform in shape; its anteroposterior diameter is about 5 cm., its transverse about 10cm. It slopes downwards and forwards, and is bounded by the first thoracic vertebra behind, the superior border of the manubrium sterni in front, and the first rib on each side. The outlet is bounded by the twelfth thoracic vertebra behind, by the eleventh and twelfth ribs at the sides, and in front by the cartilages of the tenth, ninth, eighth, and seventh ribs, which ascend on each side and form an angle, termed the infrasternal (subcostal) angle, into the apex of which the xiphoid process projects. The outlet is wider transversely than from before backwards, and slopes obliquely downwards and backwards ; it is closed by the diaphragm, which forms the floor of the thorax. The thorax of the female differs from that of the male as follows ; 1. Its capacity is less. 2. The sternum is shorter, and its upper margin 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. 3. The upper ribs are more movable and so allow a greater expansion of the upper part of the thorax.
Applied anatomy.- Fracture of the sternum is by no means common, owing no doubt to the elasticity of the ribs and their cartilages which support it like so many springs. The ribs are frequently broken, though from their connections and shape they are, able to withstand great force, yielding under pressure and recovering themselves like a spring. The middle ribs are the most liable to fracture. The first and to a less extent the second, being protected by the clavicle, are rarely fractured: and the eleventh and twelfth on account of their loose and floating condition enjoy a like immunity. The fracture generally occurs from indirect violence from forcible compression of the chest walls: and the bone then gives way at its weakest part, i.e. just in front of the angle. But the ribs may also be broken by direct violence, in which case the bone is driven inwards at the point of impact. Fracture of the ribs is frequently complicated with some injury to the viscera contained within the thorax or upper part of the abdominal cavity; this is most likely to occur in fractures from direct violence.
Cervical ribs derived from the seventh cervical vertebra are of not infrequent occurrence, and are important clinically because they may give rise to nervous or vascular symptoms. The cervical rib may be a mere epiphysis articulating only with the transverse process of the vertebra, but more commonly it consists of a definite head, neck and tubercle, with or without a shaft. It extends laterally, or forwards and laterally into the posterior triangle of the neck, where it may terminate in a free end or may join the first thoracic rib, the first costal cartilage, or the sternum. It varies much in shape, size, direction, and mobility. If it reaches far enough forwards, its relations are similar to those of the first thoracic rib ; part of the brachial plexus and the subclavian artery and vein cross over it, and are apt to suffer compression in so doing. Pressure on the plexus affects the eighth cervical and first thoracic nerves, causing paralysis of the muscles they supply, and neuralgic pains, trophic changes and paresthesisa in the area of skin to which they are distributed: no oculopupillary changes are to be found.
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