196 



ELEMENTS OF MAMMALIAN ANATOMY 



sac-like evaginations named air sacs (Figs. 94 and 95). The 

 walls of these air sacs are very thin, somewhat like the perito- 

 neum. Over them course numerous thin-walled capillaries, so 

 that the carbon dio.xid of the blood passes out into the air as 



Fig. 96. — Photograph of Human Heart and Lungs from Ventral Aspect. 



a, Aorta; /, trachea. 



the oxygen passes into the blood. Expiration is the process of 

 forcing the air out of the lungs which have been filled by an 

 inspiration chiefly effected by depressing the diaphragm and 

 elevating the ribs. The two processes together are spoken of 

 as a respiration. 



