Respiration. Thoracic cavity; pleura.—Organs of respiration: lungs, trachea, bronchia.—Respiration; influence of respiration on the blood, Lavoisier's theory, theory of catalytic phenomena; mechanism of respira-tion, respiratory sounds, frequency of respiration; capacity of the lungs; modification of the air in the lungs.—Influence of atmospheric pressure on respiration; mountain-sickness.
The thorax or chest, as we have already seen, is formed by the vertebral column, the ribs, and the sternum. The shoulder-blades and collar-bones belong to the arm, which is an appendix of the thorax. The thorax resembles a bony cage (fig. 11, p. 27), the interstices of which are filled with the muscles; the interior of this cage is the thoracic cavity (fig. 21, p. 74). It is the second cavity in point of size in the body; it has the form of a cone, slightly flattened from before backwards, with the base turned downward, and hollowed out in front It is bounded at its apex by the sternum, the clavicles, the first rib on the right and left, and the seventh cervical vertebra; at its circumference by the sternum, the ribs, and the dorsal vertebræ; at its base by the false ribs, the costal cartilages, and the xyphoid cartilage. The diaphragm corresponds to this base (fig. 21, p. 74); this is a muscular partition, the fibres of which radiate from a central aponeurosis; it closes the chest at the bottom, into which it rises like an arch, a little depressed in the centre. The diaphragm is attached to the cartilaginous border of the false ribs, to the xyphoid process, and to the lumbar vertebræ. This last attachment is effected by muscular fasciculi, which are called the pillars of the diaphragm. The central aponeurosis of this muscle is in the form, of a clover leaf; it was considered a nervous centre by the ancients, perhaps because of the pain and the peculiar sensations induced in the epigastrium by strong emotions, or because they confounded the tendinous fibres with the nervous tissue.