310 



THE RISE OF ANIMAL LIFE 



TEETH 



•PHAGOS 



EUSTACHIAN TUBE 

 OPENING 



VOCAL 

 SAC 



OPENING 



Fig. 13-33. Oral cavity of the frog, showing the openings of various tubes that enter it. 



poorly developed in amphibians. Animals 

 that move in three dimensions, such as fish 

 and birds, have proportionately larger cere- 

 bellums. 



The most conspicuous objects of the en- 

 tire brain are the optic lobes (outgrowths 

 of the mesencephalon — 3) in which nerves 

 from the eyes terminate. These lobes seem 

 to function in inhibition of spinal cord re- 

 flexes, rather than as the centers for sight. 

 The small projection just anterior to the 

 optic lobes ( diencephalon — 2 ) is the epiph- 

 ysis, an organ of doubtful function. On 

 the ventral side of this same region is a 

 tube-like stalk which terminates in an 

 enlargement, the pituitary (hypophysis) 

 ( Fig. 13-32 ) , a very important gland of in- 

 ternal secretion about which more will 

 be learned later. 



The anterior part of the brain (telen- 



cephalon — 1) is only poorly developed in 

 the frog. It is composed of a pair of lobes 

 which are partly divided transversely. The 

 two anterior parts are the olfactory lobes 

 to which the olfactory nerves are attached. 

 The posterior parts of these lobes make up 

 the cerebral hemispheres, the functions of 

 which are not clear. In fact, when this por- 

 tion is removed the animal responds, in a 

 near-normal fashion when various stimuli 

 are applied (Fig. 16-15). This is one of the 

 greatest differences between the frog and 

 man, for in man many important sensations 

 occur in the conspicuous cerebral hemi- 

 spheres. 



The frog has only ten cranial nerves, but 

 reptiles, birds, and mammals possess well- 

 developed eleventh and twelfth cranial 

 nerves. There is some evidence that primi- 

 tive amphibians, too, had these additional 



