THE RESPIRATORY SYSTEM 341 



mouth itself. It may be remembered that in Petromyzon 

 the nostril is single and confluent with the opening of the 

 hypophysial sac. The same is true of Myxine, but here the 

 hypophysial sac opens into the alimentary canal. This con- 

 nexion between nose and gut is, however, quite different from 

 that of the other forms just mentioned, and was independently 

 acquired. 



The amphibia when adult breathe air into their lungs, but 

 the mechanism for doing so is similar to that which the fish 

 use for breathing with their gills. The floor of the mouth is 

 lowered and air is taken into the mouth cavity. The mouth 

 and nostrils are then closed, and the floor of the mouth raised, 

 which forces the air down the throat and larynx into the lungs. 



The method of respiration in the amniotes is more efficient. 

 The volume of the lungs is increased by the expansion of the 

 thoracic box, and this is accomplished by movements of the 

 ribs (assisted in the mammals by movements of the diaphragm). 

 The muscles concerned in these movements are somatic and 

 innervated by ventral nerve-roots of the neck and thorax. 

 The tortoises, whose ribs are, of course, fixed to the carapace 

 which surrounds them, replenish the air in their lungs by 

 movements of the neck, arms, and legs. 



The lungs of Polypterus, Dipnoi, and amphibia are more 

 or less hollow sacs. In reptiles the internal surface of the 

 lungs are increased by foldings of the walls, with the result 

 that the lungs can no longer be described as simple hollow sacs. 

 In birds and mammals, this process has been carried still 

 further, and the lungs are spongy masses of tissue penetrated 

 by innumerable small air-spaces. In mammals, the internal 

 surface-area of the lungs may be thirty times that of the 

 external surface of the body. 



The lungs of the chamaeleon are of interest in that they are 

 produced into a number of blind diverticula or air-sacs. 

 These air-sacs reach their highest degree of development in 

 the birds, in which they may occupy a large volume. Air is 

 led into the air-sacs from the bronchi passing straight through 

 the lungs, and it then passes back into the lungs where it 

 oxygenates the blood, and out again through the trachea. The 



