NERVOUS SYSTEM 
89 
the mid-brain. (5) The cerebellum (ser-e-bel'lum) of the 
amphibians is small and easily overlooked (Figure 74). 
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, which is poorly developed, is less 
important than in man. 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 frog’s brain and those of man, yet the work done 
by each region is of the same kind. 
The brain joins the spinal cord, without any external 
sign to indicate where one begins and the other leaves off. 
A definite number (ten pairs) of nerves leaves the brain proper. 
These 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 cord (Fig¬ 
ure 74). In a long salamander there are twenty or thirty 
pairs of nerves on the spinal cord. 
LABORATORY STUDY 
In connection with the study of the frog, the following additional lab¬ 
oratory work should be done in order that the several organs of man 
which are discussed in Part III may be better understood. Frogs that 
