v:i.] THE COMMO.\ FROG. 103 



Moreover, there is another toad {RJiinophryiius) 

 which is even more exceptional in its order than these 

 two ; in that its tongue is not free behind, but, like 

 that of ordinary vertebrates, in front (Fig. 13). 



The fact is, that the large tongue-bone of these 

 animals serves, with the muscles attached to it, as 

 much to facilitate respiration as nutrition. 



It has already been said that the frog has no ribs 

 by the elevation and depression of which it m.ay 

 alternately fill and empty its lungs. Neither does it 

 possess that transverse muscular partition, the dia- 

 phragm, or midrif, which in man's class is the main 

 agent in carrying on that function. 



The lungs of the frog are inflated as fellows : — The 

 mouth is filled with air through the nostrils and kept 

 shut while the internal openings of the nostrils are 

 stopped by the tongue, and the entrance to the gullet 

 is closed. Then, by the contraction of the muscles 

 attached to it, the os-hyoides is elevated ; and every 

 other exit from the mouth being closed, except that 

 leading to the larynx, air is thus driven down the 

 glottis into the lungs. 



Thus, for pulmonary respiration it is necessary to 

 the frog to keep the mouth shut ; and in this way, 

 but for the action of the skin, the animal might be 

 choked by keeping its mouth open. 



It has been already stated that the typical segmen- 

 tation of the limbs is wanting in all fishes, but present 

 in all Batrachians that have limbs at all. Similarly, 

 in all Batrachians that have limbs at all the muscles 

 of those limbs have essentially and fundamentally the 

 same arrangement as in higher animals. In the higher 



