HOW THE ANIMAL LIVES 225 



411. In the main, the air is changed in 

 breathing as follows : 



Carbon 

 Oxygen Nitrogen dioxid 



Inspired, or breathed-in air contains . . . 20.81 79.15 .04 



Expired, or breathed-out air contains . . . 16.033 79.557 4.38 



In every 100 parts, air loses, by being breathed, 

 about 4 parts of oxygen, and gains about 4 

 parts of carbon dioxid. 



412. In breathing, the air is also charged 

 with water vapor and with small quantities of 

 ammonia and marsh gas. It also receives a 

 volatile organic matter, which may be fo3tid, 

 and when condensed in water soon develops a 

 putrid odor. 



413. In the breathing process, the blood and 

 the air are brought into the closest possible 

 contact. One -celled animals breathe through 

 the entire surface, fishes through gills waved in 

 the water, from which they abstract oxygen, 

 frogs through the walls of a simple air -sac, 

 in which the blood-vessels circulate. In warm- 

 blooded animals, this sac or lung is divided 

 throughout into myriads of minute air- sacs or 

 cells, varying from 7^0 to TO of an inch in 

 diameter. The walls are so thin that the blood 

 flowing through their capillary vessels is con- 

 stantly exposed, on two sides, to the air with 

 which they are filled. The membrane consti- 

 tuting the walls of these sacs is so exceedingly 



