RESPIRATION. 



225 



Lunjrs 



386. In the lower vertebrata provided with lungs they 

 form a single or- 

 gan ; but in the 

 higher classes 

 they are in pairs, 

 placed in the 

 cavity formed 

 by the ribs, one 

 on each side of 

 the vertebral co- 

 lumn, and en- 

 closing the heart 

 between them 

 (fig. 235). The 

 lungs communi- 

 cate with the 

 atmosphere by 

 means of a tube, 

 composedof car- 

 tilaginous rings, 

 arising at the 

 back part of 

 the mouth, and 

 dividing below, 



first into a branch for each organ, and then into innumerable 

 branches penetrating their whole mass, and finally terminating 

 in minute cells. This tube is the trachea (t), and its branches 

 are the bronchi. In the higher air-breathing animals the lungs 

 and heart occupy an apartment by themselves, the chest (fig. 

 124), which is separated from the other contents of the lower 

 arch of the vertebral column by a fleshy partition, called the 

 diaphragm (fig. 180), passing across the cavity of the body, 

 and arching into the chest. The only access to this apartment 

 from without is by the glottis through the trachea (fig. 235, t). 



386*. 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 con- 

 traction of the diaphragm, whereby its intrusion into the chest 

 is diminished. This enlargement causes the air to rush in 

 through the trachea, distending the lungs so as to fill the ad- 



Q 



Fig. 2o5. LUNGS, HEART, and principal blood- 

 vessels of MAN. 



a r, right auricle : v r, right ventricle ; v I, left 

 ventricle ; a, aorta ; v c, vena cava ; a c, carotid 

 arteries ; vj, jugular veins ; a s, subclavian ar- 

 tery ; v s, subclavian veins ; t, trachea. 



