THE STRUCTURAL AND FUNCTIONAL SYSTEMS 63 



When the respiratory mechanism is examined as it appears in a 

 longitudinal section (Fig. 23), the mouth cavity is seen to connect 

 with the outside by means of the nostrils, which can be opened 

 and closed by valve-like action. The glottis is capable of a sim- 

 ilar opening and closing in relation to the incoming and out- 

 going air. The jaws are always pressed close together except dur- 

 ing the taking of food, and the same is true of the walls of the 

 esophagus. Hence, for purposes of breathing, the mouth is a 

 closed cavity opening only by nostrils and glottis. If the reader 

 will now compare the mouth of the frog with a pump guarded at 

 its outer end by a valve, comparable with the nostrils, and the 

 lungs with a tire guarded by its valve, the glottis, he can, by a 

 proper manipulation of such a mechanism, imitate the "pumping 

 up" and sudden "emptying" by which the air enters and leaves 

 the lungs in this animal. 



A further structural difference between the frog and the more 

 familiar birds and mammals is the absence of a windpipe or 

 trachea. The glottis of the frog obviously corresponds to the 

 glottis of man (cf. Figs. 23 and 24). The cavity containing the 

 vocal cords would naturally be called the larynx in the two cases. 



If we now compare the respiratory organs of the frog with the 

 structures foimd in vertebrates below the Amphibia, the fact that 

 fishes breathe by means of gills renders impossible an exact com- 

 paiison of the adult stages in such water-breathing and air-bieath- 

 ing types. However, the fishes possess a structure called the air 

 bladder (Fig. 25), which lies dorsal to the esophagus and ventral 

 to the backbone in the famiUar bony fishes (Teleostomi), and 

 which is probably homologous with the lungs of air-breathing 

 vertebrates. In such fishes the air bladder has a hydrostatic 

 function; but in one small group, the Dipnoi, or lung fishes 

 (Fig. 26), it has actually become a lung and functions in respira- 

 tion. 



The Nervous System. — When considered in its anatomical 

 aspects, the nervous system of a vertebrate animal may be divided 

 into two parts : the central nervous system, including the brain and 

 spinal cord (Fig. 37); and the peripheral nervous system, consist- 

 ing of the nerves by which the central system is connected, on the 

 one hand, with the sense organs and, on the other, with the mus- 

 cles and glands. From its functional aspect this division is less 

 important, because the system works as a ivhole, and because its 



