RESPIRATION 



185 



It is the pressure of the atmosphere which causes the air 

 to enter the lungs in what is called inspiration, and the 

 pressure upon the lungs by the walls of the thorax which 

 forces it out in expiration. These two acts together con- 

 stitute respiration. 



262. The Respiratory Mechanism. In respiration the size 

 of the thorax is increased in two ways: (1) by the de- 

 pression of the diaphragm, which forms the floor of the 

 cavity and separates it from the cavity of the abdomen; 

 (2) by the elevation of the forward ends of the ribs. 



263. The Diaphragm is a sheet of muscle and tendon, 

 convex on its upper side, and attached by bands of striped 

 muscle to the lower ribs at 



the side, to the sternum and 

 to the cartilages of the ribs 

 which join it in front, and 

 at the back by very strong 

 bands to the lumbar verte- 

 brae. Its center is a thin ex- 

 panse of tendon. When the 

 muscles about the circumfer- 

 ence contract, the arch is flat- 

 tened upon the contents of the 

 abdomen, which yield to its 

 pressure, and the thorax is en- 

 larged downward (Fig. 103). 



264. Action of the Bibs and 

 Muscles of the Chest. The 



ribs are attached to the spinal column at an angle smaller 

 than a right angle (Fig. 33, p. 45), and slope downward 

 toward the breastbone, the lower ribs sloping more than 

 the upper. When, by the contraction of the chest mus- 

 cles, the sternum is drawn upward and outward, the ribs 



\ 



Fig. 103. Diagram illustrating 

 the varying position of the dia- 

 phragm during respiration. 



