INTERNAL STRUCTURE 119 



off from the rest of the brain, and, as shown in Figure 117, 

 connect with (2) the cerebral (ser'e-bral) hemispheres, 

 which are oval in outline. (3) A short mid-brain region, 

 partly covered by the back part of the cerebral hemi- 

 spheres, connects the front and back part of the brain. 

 (4) Two large optic lobes, the widest part of the brain, 

 are just back of the mid-brain. (5) The cerebellum 

 (se'r-e-bel'lum) of the amphibians is small and easily over- 

 looked (Figure 117). The last region of the brain is the 

 (6) medulla (me-dul'la), which is occupied by a large 

 triangular cavity called the fourth ventricle. 



The work which each of these regions of the brain does 

 is not sharply defined. The olfactory lobes receive the 

 smell stimuli. The cerebral hemispheres control muscular 

 action. When the latter are removed the frog loses all 

 power to initiate any movement and will sit still in a dry, 

 warm room for hours unless disturbed. This he never 

 does when the cerebral region of the brain is uninjured. 

 The mid-brain region is the passageway for all nerve-path- 

 ways that travel to and fro in the brain. The mid-brain 

 and optic lobes explain to the frog the sight stimuli. In 

 the frog, the cerebellum is less important than in man and 

 is poorly developed. The medulla gives off more nerves 

 than any other region of the brain. Here are found the 

 nerves to the face, tongue, ear, heart, and lungs. While 

 there is a great difference between the shape of the parts 

 of the brain of the frog and those of man, yet the work 

 done by each region is of the same kind. 



The brain joins the spinal cord, and there is no external 

 sign to indicate where one begins and the other leaves 

 off. A definite number (ten pairs) of nerves leave the 

 brain proper and are devoted to the special senses of the 

 head and to moving the muscles of the throat and head. 

 The frog has ten other pairs of nerves joined to the spinal 



