Amphibia. 187 



little above the surrounding air or water. Its temperature 

 varies with the degree of its activity. In the winter when 

 it is buried in the mud its temperature sinks nearly to the 

 freezing point, for the lower layers of water have a tem- 

 perature of about 40 F. In other words, the frog's tem- 

 perature is variable, instead of constant as in the case of 

 mammals. 



The Nervous System of the Frog. The widest part of 

 the brain consists of the two optic lobes. In front of these 

 are the two cerebral hemispheres. Barely separated from 

 the anterior ends of the cerebral hemispheres by a slight 

 groove are the olfactory lobes ; they seem to be a part of 

 the cerebral hemispheres. Back of the optic lobes, sepa- 

 rated by a depression, is the cerebellum, a narrow trans- 

 verse band. Beyond the cerebellum is the spinal bulb (see 

 also Fig. 167). There are ten pairs of cranial nerves. 

 There are also ten pairs of spinal nerves. 



The Senses and Sense Organs of the Frog. The promi- 

 nence of the frog's eyes has already been noticed. The 

 upper lid is thick and drops with the eye when it is with- 

 drawn. The thin under lid can be drawn more completely 

 over the eye and is often used when the upper lid remains 

 stationary. Back of the eye the eardrum is conspicuous. 

 From the inner surface of the tympanum extends the bony 

 rod called the " columella " to the inner ear, to which it 

 transmits the vibrations received by the tympanum. The 

 sense of taste is apparently well developed. All parts of 

 the skin seem to possess the sense of touch. The nostrils 

 open directly into the front of the mouth. Of the sense of 

 smell less is known than of the other senses. 



Development of- the Frog. The eggs are formed in the 

 ovaries. When the eggs are mature the ovaries become 

 much folded and plaited and the egg-masses occupy a large 



