XKKVors SYSTEM 385 



projections is greatly elongated and forms a nerve fiber 

 which may be only a fraction of an inch in length or it 

 may be two or three feet. Near the end, a fiber may 

 branch abundantly, sometimes having very complex 

 endings as in the eye or in the ear. Along its course 

 the fiber is protected by two coverings. Xerve cells 

 are found only in nerve centers. 



Nerves. Bundles of nerve fibers, together with blood 

 vessels and connective tissue, form nerves. A nerve 

 is like a telegraph cable. It consists of many fibers, 

 each insulated from the others. Fibers separate from 

 the main group and form a branch of the nerve, each 

 having a particular use. Some carry messages only to 

 the brain and spinal cord; these are known as sensory 

 m.n'es. Others carry messages from the brain and the 

 spinal cord to the organs; these are motor nerves. 



The brain. Most of the organs of our body are in 

 some way protected. None, however, is any better 

 protected than the brain. The human brain is an 

 exceedingly delicate mechanism and would be subject 

 to frequent injury if not properly protected. It is pro- 

 tected by the hair, the loose, tough scalp, and by the bones 

 of the cranium. The arched instep and the curved spinal 

 column give some " spring " to the body and hence tend 

 to keep the brain from being jarred. Finally, the brain 

 itself is enclosed in two membranes. 



The brain is the largest part of the nervous system. 

 It weighs about three pounds and consists of a mass 

 of nerv' c< i lls and many connecting fibers. It maj lx j 



