452 



RESPIRATION 



walls of the lungs, eventually invade the membranous partitions. 

 As a result of this extension, a much larger sheet of blood is brought 

 into direct diffusion contact with the air in the alveoli. 



The mechanism of respiration in these animals is very simple. The 

 floor of the mouth is depressed by muscular activity so that the pres- 

 sure within this cavity falls below that of the outside air. A certain 

 quantity of air then enters through the nostrils until an equalization 

 of pressure has been effected. The nostrils are then closed and the 

 glottis opened. The subsequent elevation of the floor of the mouth 

 now forces the air into the lungs. Here it remains for a time until a 

 part of it is allowed to escape through the opened glottis and nostrils 



FIG. 231. HUMAN RESPIRATORY APPARATUS SHOWING THE BRANCHING OF THE BRONCHI 

 IN THE INTERIOR OF THE LUNGS. (Duval.) 



in consequence of the passive recoil of the parts previously put under 

 elastic tension. This mechanism again illustrates the action of a 

 force-pump. 



The Mammalian Lung. The lung of the mammal exhibits several 

 of the characteristics of the reptilian lung. Beginning at the pharyn- 

 geal cavity, the trachea with its modified upper portion, known as the 

 larynx, passes backward for a distance of about 12 cm. and divides 

 into two main branches, the bronchi. The latter subdivide again and 

 again until small terminals, or bronchioles, are obtained which in- 

 dividually connect with irregular spaces, known as infundibula. These 

 in turn are made up of a number of minute cellular spaces, or alveoli. 



