RESPIRATION. 125 



tive apparatus. (Fig. V.<7, p. 19.) The lungs lie in contact 

 with it above, (Fig. V. a, 6, p. 19,) and the digestive appa- 

 ratus lies in contact with it below. (Fig. V. e, /, p. 19.) 

 When it is expanded, it rises into the chest, and the lungs are 

 pressed up, and the abdominal organs follow immediately 

 behind. And, on the other hand, when it contracts and 

 lessens the arch to give expansion to the lungs, it must 

 press the abdomen and its contents downward and out- 

 ward. A simple and easy illustration of the operation 

 of the diaphragm in breathing, may be found in the com- 

 mon India-rubber bottle. If we hold this in one hand 

 and press the bottom inward with the finger, the air is 

 forced out through the neck. If, then, we remove the fin- 

 ger, the bottom returns to its natural position, and then the 

 air flows through the neck to fill the increased cavity. 



275. By these two combined actions of the muscles of the 

 ribs and of the diaphragm, the chest is enlarged. The mus- 

 cles on the sides of the chest raise the ribs, and extend their 

 circle forward and outward. The diaphragm draws down 

 its arch from the fourth to below the seventh rib, and thus 

 enlarges the chest; and the lungs having room for expan- 

 sion, the air is pressed into them to fill the vacuum left by 

 the enlarging chest. This is the mechanical part of the pro- 

 cess of inspiration. 



276. After the chest is thus sufficiently expanded, the 

 muscles of the ribs and the diaphragm relax and lose their 

 firmness. Then the action of other muscles, aided by the 

 elasticity of the cartilages, carries the ribs downward ; and, in 

 going down, they lessen the diameter, and consequently the 

 capacity of the chest, by bringing the sides nearer to each 

 other, and the breast-bone nearer to the back-bone. At the 

 same time, -the muscles that cover the abdomen press upon 

 its contents, and force them against the diaphragm. This 

 yields to the pressure, and rises upward and presses upon the 

 lungs, which retreat before it, and the air is expelled. This 

 is the process of expiration. 



277. Fig. XVII. represents an outline of the front view of 



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