xvi THE NERVOUS SYSTEM 225 



A nerve fiber differs from a telegraph or telephone 

 wire in that it carries messages in one direction only, 

 while a wire will carry messages in either direction. 

 A sensory fiber conveys messages to the brain or spinal 

 cord, but never from it ; while a motor fiber carries 

 messages out to muscles and glands, but it cannot 

 carry impressions to the brain. All of the spinal nerves 

 and three pairs of cranial nerves contain both sen- 

 sory and motor fibers and therefore are called mixed 

 nerves. 



156. The Spinal Cord. The spinal cord is a great 

 nerve trunk extending from the base of the skull down 

 through the canal of the vertebrae or backbone. In the 

 adult it is about eighteen inches long and three quarters 

 of an inch in diameter. The spinal cord and brain are 

 carefully protected by a bony covering, but besides this 

 there are three membranes 

 that closely invest these deli- V 

 cate organs and supply them 

 with nutrition. 



The outer part of the spi- 

 nal cord is called ivhite matter. ^r^*- "war 

 It consists of bundles of nerve ^^^^J \~~~*^^ * 

 fibers that serve as ingoing FIQ I2I ._ showing the white 



and OUtCOming pathways for and gray matter of the spinal 



the sensory and motor mes- cord (Pratt), 

 sages carried to and from the brain. White matter 

 always means nerve fibers and never does anything except 

 carry messages. The entire surface of the body except 

 the head and face is supplied with spinal nerves. 



Q 



