226 HUMAN PHYSIOLOGY CHAP. 



The central portion of the spinal cord consists of 

 nerve cell bodies and is known as gray matter. In the 

 cord we find the white matter on the outside and the 

 gray on the inside. The nerve cells or gray matter do 

 not carry messages ; they have very different functions 

 to perform. A collection of nerve cells that has some 

 particular work to do is called a nerve center. The 

 cells of the gray matter of the spinal cord are grouped 

 into a great many nerve centers which perform important 

 offices. These centers control most of our involuntary 

 movements, and do a large amount of work that can be 



rp/na/mrve 



FlG. 122. A cross-section of the spinal cord, showing a spinal nerve and its 

 two roots (Huxley). 



done without the aid of the brain. Certain sensory 

 impressions carried to the cord do not reach the brain 

 at all, but are reflected or turned back to muscles by 

 such centers located in the cord ; they act independently 

 of the brain. These are reflex acts. 



Get a piece of spinal cord from the meat market and examine it 

 when fresh. It may then be hardened by using a mixture of two 

 thirds alcohol and one third 2 per cent solution of formalin. When 

 it is thus preserved, slices may be easily cut for more careful study 

 with a magnifying glass. 



Centers of the cord control the nutrition and growth 

 of the cells of the body by telling them when and how 

 much to eat, and by regulating their blood supply. 



