468 
THE NERVOUS SYSTEM OF MAN 
fibers. Each nerve looks like a small white thread and is 
covered with a thick, fatty sheath (medullary sheath). In 
the living animal, this fatty sheath is white and the nerve 
fibers so covered are found to occupy a certain part of the 
spinal cord and brain. Thus, we get the name white sub¬ 
stance. Other of the nerve fibers and cell bodies are not 
covered with a sheath and so have a gray appearance. Thus 
we have the term gray substance in connection with the 
nervous system. 
The Central Nervous System. — In Figure 412 the nerves 
of the legs, arms, and trunk are all seen to be united to a 
central body, the spinal 
cord. There are nerves 
in the upper part of the 
neck and in the head re¬ 
gion that unite with the 
large mass at the upper 
end of the spinal cord, the 
brain. The brain and 
i 
Figure 413. — Nerve Cells. 
Stained by the silver process which spinal cord are known as 
blackens all the parts. This is an excel¬ 
lent stain to show the branching processes. 
the central nervous sys¬ 
tem. The brain is di¬ 
vided into the following parts : cerebrum, the most anterior; 
the mid-brain, to which the optic nerves join; the cere¬ 
bellum ; and the medulla oblongata, to which the nerves of 
hearing, tasting, and the facial nerves belong. 
The cerebrum is the most important of the several regions 
of the brain. It regulates and controls all our nervous activ¬ 
ities. The cerebellum gives tone and vigor to the contrac¬ 
tion of the muscles and helps us to know when we are prop¬ 
erly balanced and is hence known as the equilibration center. 
The remaining parts of the brain give off and receive nerves 
and transmit nerve stimuli to the cerebrum and cerebellum. 
362. Growth of the Nervous System. — The nervous 
system of man, like all other parts of the body, has a definite 
