170 



ELEMENTS OF BIOLOGY 



a sac connected with the pharynx by a narrow tube (Fig. 114), 

 serves as an accessory respiratory device, enabUng the fish to utiHze 

 air during those seasons when the water habitat is dried up or, in 

 some forms, enabhng the fish to leave its water habitat for a time. 

 This sac is within the ccelom and by many is regarded as the fore- 

 runner of lungs. 



Lungs. In the adult tailless Amphibia, for example the frog, 

 in the reptiles, the birds, and the mammals, true lungs are formed. 

 These are essentially sacs with thin membranous walls that are fur- 



AIR BLADDERS 



PHARYNX 



Fig. 114. — The African dipnoid or lung fish. The body wall has been cut away 

 to show the position of the air sacs which under certain conditions serve as respira- 

 tory organs. 



nished with a network of fine blood vessels. They lie in the anterior 

 portion of the ccelom, which, as in Man, may be set off as separate 

 compartments, the pleural cavities. In all air breathers these internal 

 lungs are correlated with some sort of muscular device for drawing 

 in and expelling the air. In Man this is accomplished by the peri- 

 odic and automatic movements of the muscles of the diaphragm 

 and of the chest and abdomen. The air enters the nares where 

 it is conditioned by moist membranes. By way of the internal nares 

 the air passes through the larynx, a cartilaginous box-like structure 

 containing two membranous folds, the vocal cords. 



