The thoracic cage (compages thoracis, thorax) is ovoid with a narrow upper end and a wider lower end. Both ends are cut slantwise, the upper part from the front upward to the back and the lower part in the opposite direction. In addition, the thorax is somewhat compressed from front to back. The anterior wall, of which the sternum is a component, is shorter than the posterior wall in the formation of which the vertebral column takes part. The thoracic cavity (cavum thoracis) has two apertures: the supe­rior aperture (inlet) (apertura thoracis superior) and the inferior aperture (outlet) (apertura thoracis inferior), which is closed by a muscular partition, the diaphragm. The anterior border of the inferior aperture has an incisure shaped like an angle, the infrasternal angle of the thorax (angulus infrasternalis); the xiphoid process is located at its apex. The vertebral column protrudes into the thoracic cavity on the midline and the above-mentioned wide pulmo­nary sulci (sulci pulmonales) form to the sides of the column between it and the ribs; the posterior margins of the lungs are lodged in them. The spaces between the ribs are called intercostal spaces (spatia intercostalia). Fig. 41. The shape of the thoracic cage of a quadruped (a) and man (6) In mammals the thoracic viscera exert pressure on the inferior wall be­cause of their horizontal posture and the thoracic cage is long and narrow; the ventro-dorsal dimension is larger than the transverse dimension as a re­sult of which the thorax appears compressed on the sides and its ventral wall protrudes like the keel of a boat (keeled, or pigeon chest). In monkeys, in whom the hands and feet are distinguished and who begin acquiring a vertical posture, the thoracic cage is wider and shorter, but the ventro-dorsal dimen­sion still predominates over the transverse dimension (monkey chest). Finally, in man, with the final change to an upright position, the hand is freed from the function of locomotion and becomes the organ of labour, as a result of which the thorax experiences the traction of the upper limb muscles attached to it. The viscera exert pressure not on the ventral wall, which is now the an­terior wall, but on the inferior wall formed by the diaphragm. As a conse­quence the line of weight in a vertical posture of the body is displaced closer to the vertebral column. As a result the thoracic cavity becomes flat and wide so that the transverse dimension predominates over the antero-posterior di­mension. In reflection of this process of phylogenesis, the chest changes in shape during ontogenesis. As the child gradually begins to stand, walk, and use his limbs the whole locomotor system and the viscera grow and develop, the thorax gradually takes the shape characteristic of the human chest with the predominance of the transverse dimension1. The shape and size of the thor also marked by considerable individual variation consequent upon the degree of the development of the muscles and lungs, which in turn is associated with the life-style and occupa­tion of the given person. Since the chest contains such vitally important or­gans as the heart and lungs, these variations are very important in evaluating the physical development of the individual and in diagnosing internal dis­eases. Three chest shapes are usually distinguished: flat chest, barrel chest, and conic chest. Individuals with well-developed muscles and lungs have a wide but short thoracic cage, which acquires a conic shape, i.e. its lower part is wider than the upper part, the ribs slope only slightly, and the infrasternal angle is large. Such a thorax appears to be in a state of inspiration and is therefore called an inspiratory chest. In contrast, in individuals with weak development of the muscles and lungs the thoracic cage is narrow and long and acquires a flat shape with a greatly flattened anteroposterior diameter. The anterior wall is almost vertical, the ribs slope markedly, and the in­frasternal angle is acute. The thorax appears to be in state of expiration and is therefore called expiratory. The barrel chest occupies an intermediate position between the two forms described. The female chest is shorter and narrower in the lower part and more rounded than the male chest. Social factors also influence the shape of the chest. In capitalist coun­tries and colonies, for example, children of the exploited population, who live in poorly lit dwellings and who are undernourished, develop rickets ("English disease"), in which the thorax acquires the shape of a pigeon chest (the ante­roposterior dimension predominates and the sternum protrudes forward ab­normally as it does in pigeons). The thorax of trumpet players and glass-blow­ers is wide and bulging because it is continuously expanded in intensified inspiration. In contrast, in cobblers, who worked sitting in a bent posture and used their chest as a support for the heel when they nailed the soles of the shoe, a depression formed on the anterior thoracic wall and the chest became hollower (foveated chest of cobblers). In children with a long and flat chest due to deficient development of the muscles, the thorax is as if in a state of collapse when they sit improperly at the desk, which has an effect on the activ­ity of the heart and lungs. To avoid disease, children need a certain amount of physical exercise. A person is usually considered fit for service in the armed forces if the circumference of his chest is no less than half his height. The average chest circumference of males between the ages of 20 and 21 is 85.3 cm. Movement of the thoracic cage. Respiratory movement consists in raising and lowering the ribs, together with,which the sternum moves also. During inspiration the posterior ends of the ribs rotate on the axis indicated above (on p. 153). The anterior ends rise at the same time and, because of their forward sloping, simultaneously move away from the spine together with the sternum so that the thorax expands in the anteroposterior dimen­sion. Moreover, due to the oblique direction of the pivotal axis, the ribs are simultaneously moved apart laterally, as the result of which the transverse chest dimension also increases. The costal cartilages, being flexible an tic, play an important role in the respiratory excursions. When the ribs are raised, the angulated curvatures of the cartilages straighten out, and move­ments occur at the joints between them and the sternum, after which the car­tilages themselves are stretched and twisted. When inspiration induced by muscular action is completed, the ribs are lowered and expiration occurs. Three zones can be distinguished in the thorax according to its mobility: an upper zone above the level of the fourth thoracic segment characterized by slight mobility of the ribs and thoracic vertebrae; a middle zone (from the fourth to the eighth thoracic segment) marked by highest mobility during respiration; and a lower zone (below the level of the eighth segment), which in its movements is linked with the lumbar region of the spine and the diaphragm. To understand the respiratory excursions of the chest and their disorders in fractures of the ribs, it is important to bear in mind the interaction of the three zones.