THE HUMAN ORGANISM 



279 



can easily see the eardrum back of the eye; there is nothing 

 corresponding to the external ears of mammals. If you watch a 

 living frog for a little while, you will notice something peculiar 



Fig. 130. Homologies in the skeletons of vertebrates 



In spite of the different appearance and different functions of the limbs of backboned 



animals the supporting structures have the same general plan. W, the wrist bones : 



a, man; b, lion; c, vulture; d, bat; e, whale; /, halibut 



Fig. 131. Homologies in the bones of the hind leg 



Walking, crawling, swimming, loping — all the various modes of locomotion found 



among backboned animals — are carried on by organs having the same fundamental 



structure. A, ankle bones: a, man; b, lion; c, wolf; d, duck; e, crocodile; /, seal 



in the way he "winks" (see Fig. 128). Fishes have no eyelids at 

 all, and in snakes the eyelids are always closed but transparent. 

 The nostrils in the frog correspond to ours and lead into 

 the mouth cavity; they can be closed. In all vertebrates the 

 breathing organs are connected with the mouth, but in none 

 of the invertebrates. The tongue is a muscular outgrowth 



