110 



A COURSE ON ZOOLOGY. 



There are two aortic arches, springing from a dilatation 

 or aortic bulb. 



The skeleton is remarkable by the shortness of the 



vertebral column, 



FIG. 61. which has only 



eight vertebrae. 

 There are neither 

 sternum nor ribs, 

 and consequently 

 there is no tho- 

 racic cage ; the 

 frog is therefore 

 unable in respira- 

 tion to make move- 

 ments of inhala- 

 tion and exhala- 

 tion, such as we 

 have studied in 



the animals already considered; it swallows the air 

 by deglutition, as we would swallow a mouthful of 

 food. 



The brain of the frog has two elongated cerebral 

 hemispheres, two large optic thalami behind the hemi- 

 spheres, and a cerebellum consisting only of a transverse 

 nerve band close to the optic thalami. 



The organs of sense are well developed. The eye has 

 a nictating membrane ; the ear has neither pavilion nor 

 external canal, and its tympanum, like that of the lizard, 

 is on a level with the skin. 



The organization of frogs corresponds to their man- 

 ner of life. Like toads and aquatic salamanders they 

 can live under water as do fishes, and they possess a 

 power entirely absent in fish, to live quite as well in 



SKELETON OF FROG. 



