186 UNCONSCIOUS NERVOUS OPERATIONS 



are raised to a more nearly horizontal position, and the 

 cavity they inclose is enlarged toward the front. At the 

 same moment the ribs are rotated slightly at their articula- 

 tion with the vertebrae, and by their curvature they enlarge 

 the chest at the sides. Many muscles are involved in this 

 complex action. The external intercostal muscles (the 

 elevators of the ribs) and the muscles in the walls of the 

 abdomen are principally concerned, but certain muscles 

 of the head, neck, and back also assist, and in forced 

 breathing many others are brought into use. In ordinary, 

 quiet expiration, the elasticity of the chest walls, the 

 cartilages, and the lungs is sufficient to expel the air 

 inhaled without special muscular contraction. But in 

 special, extraordinary efforts of expiration, as in cough- 

 ing, sneezing, singing, etc., active muscular effort is 

 required. For this purpose the muscles of the abdominal 

 walls are chiefly brought into action, and by them the 

 diaphragm is forced upward, the cavity of the thorax is 

 made smaller, and the air is driven rapidly out of the 

 lungs. When those muscles again relax, the chest returns 

 to its normal size. 



265. Effect of Respiration upon Air in the Lungs. The 

 lungs are always full of air, about two hundred cubic 

 inches remaining in the lungs of an adult man after 

 expiration. He draws in and breathes out in ordinary 

 breathing from twenty to thirty cubic inches, and in 

 forced respiration may take in and expel one hundred 

 cubic inches more. By what is known as the diffusion of 

 gases the inspired air is equally mixed throughout with 

 that remaining in the air cells and tubes of the lungs, and 

 there is no accumulation anywhere of air which has been 

 deprived of oxygen. A similar interchange takes place 

 through the walls of the capillaries between the gases in 



