CHAPTER VIII. 



Respiration. Thoracic cavity; pleura. Organs of respiration: lungs, 

 trachea, bronchia. Respiration; influence of respiration on the blood, 

 Lavoisier^- theory, theory of catalytic phenomena; mechanism of respira- 

 tion, respiratory sounds, frequency of respiration; capacity of the lungs; 

 modification of t/ie air in the lungs. Influence of atmospheric pressure 

 on respiration; mountain-sickness. 



Thoracic cavity . 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. 1 1, 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 down- 

 ward, 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 vertebrae; at its base 

 by the false ribs, the costal cartilages, and the xyphoid car- 

 tilage. 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 vertebrae. This last attachment is effected by mus- 

 cular fasciculi, which are called the pillars of the diaphragm. 

 The central aponeurosis of this muscle is in the form of a 



