OF RESPIRATION. 



93 



Fig. 90. 



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

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

 sphere 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 innumerable branches penetrating 

 their whole mass, and finally termina- 

 ting in minute sacs. This tube is the 

 trachea onvindpipe (w),and its branches 

 are the bronchi. In the higher air- 

 breathing animals the lungs and heart 

 occupy an apartment by themselves, 

 the chest, which is separated from the 

 other contents of the lower arch (161), 

 by a fleshy partition, called the dia- 

 phragm, passing across the cavity of the 

 body, and arching into the chest. The only access to this 

 apartment is by the glottis (Fig. 22, o) through the trachea. 



248. The mechanism of respiration by lungs may be 

 compared 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 contraction of the diaphragm, whereby its intrusion into 

 the chest is diminished. This enlargement causes the air to 

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

 fill the additional space. When the diaphragm is again re- 

 laxed, and the ribs are allowed to subside, the cavity is 

 again diminished, and the air expelled. These movements 

 are termed inspiration or inhalation, and expiration. The 

 spongy pulmonary 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- 



