Dorsal (afferent) 



White matter 



Gray matter 



Ventral (efferent) 



Branching 

 nerve 

 roots 



A cross section of the spinal 

 cord shows a rather distinct 

 gray pattern, somewhat like 

 a butterfly in outline, within a 

 whitish moss. The white part 

 of the cord consists largely of 

 axons, which transmit nervous 

 impulses up and down — to- 

 ward the brain and toward 

 other segments of the body. 

 The gray matter consists 

 largely of cell bodies that act 

 as switching centers, receiv- 

 ing impulses from afferent 

 nerves and shunting them oflF 

 into efferent nerves as re- 

 flexes or transmitting impulses 

 from efferent as well as from 

 afferent nerves, up and down 

 the axis. Ganglia usually con- 

 sist of cell bodies; the nerve 

 strands consist of axons and 

 supporting tissues 



THE NERVE CONNECTIONS OF THE SPINAL CORD 



plan, to think. Although man has neither the largest brain by absolute 

 weight nor the largest in relation to the size of his body, his brain is probably 

 the most efficient for bringing about changes in the world and for making 

 adjustments to changes. 



We have seen that an awareness is associated with some of the reflex 

 actions. We interpret this fact by supposing that the receptor and the effec- 

 tor neurons of the reflex arc are connected also with the brain, by way of 

 the spinal cord (see illustration, p. 282). Impulses to the cerebrum have to 

 do with consciousness. Impulses from the cerebrum control voluntary ac- 

 tion, but the cerebrum cannot control the reflexes, of which we are in most 

 cases not aware. 



Certain portions of the cerebral cortex appear to be connected with specific 

 sensations or movements. The charting of these connections is based upon 

 experimental studies with various mammals, and upon experiences with the 

 diseased or injured brains of human beings (see illustration, p. 283). The 

 matter is not so simple, however, as the diagram suggests, for the function 

 of each region seems to be influenced by all the others. Every conscious 

 desire, as well as every deliberate or purposeful action, seems to depend upon 

 impulses starting from the gray matter in the brain or upon stimuli leading 

 to the gray matter. 



280 



