OF RESPIRATION. 



119 



Fig. 90. 



the vertebral column, and enclosing the heart (h) between 

 them, (Fig. 90, Z Z.) The lungs communicate with the atmos- 

 phere by means of a tube composed of cartilaginous rings 

 which arises from the back part of the mouth, and divides 

 below, first into a branch for each organ, and then into in- 

 numerable branches penetrating their 

 whole mass, and finally terminating in 

 minute sacs. This tube is the trachea 

 or windpipe, (w?,) and its branches are 

 the bronchi. In the higher air-breath- 

 ing animals the lungs and heart occupy 

 an apartment by themselves, the chest, 

 which is separated from the other con- 

 tents of the lower arch of the vertebral 

 column, (161,) by a fleshy partition, 

 called the diaphragm, passing across 

 the cavity of the body, and arching up into the chest. The 

 only access to this apartment from without is by the glottis, 

 (Fig. 22, 0,) through the trachea. 



248. The mechanism of respiration by lungs may be com- 

 pared to the action of a bellows. The cavity of the chest is 

 enlarged by raising the ribs, the arches of which naturally 

 slope somewhat downward, but more especially by the con- 

 traction of the diaphragm, whereby its intrusion into the 

 chest is diminished. This enlargment causes the air to rush 

 in through the trachea, distending the lung so as to fill the 

 additional space. When the diaphragm is again relaxed, 

 and the ribs are allowed to subside, the cavity is again dimin- 

 ished, and the air expelled. These movements are termed 

 inspiration or inhalation, and expiration. The spongy pul- 

 monary substance being thus distended by air, the blood sent 

 from the heart is brought into such contact with it as to allow 

 the requisite interchange to take place, (235.) 



249. The respiration of animals breathing in water is ac- 



