300 THE NERVOUS SYSTEM 



437. Functions of the Spinal Cord. A nerve center is a 

 group of nerve cells which join together in some particular 

 form of nervous action. The gray matter of the spinal 

 cord is a series of nerve centers, while the white matter 

 is arranged in bundles of nerve fibers whose function is, 

 like that of the nerves themselves, the transmission of 

 impulses. 



Two sets of functions belong to the spinal cord. It is 

 the channel by which volitions of the brain are conveyed 

 to many of the muscles, and the channel by which many 

 sensory impulses reach the brain ; that is, by means of 

 the cord voluntary action takes place, and by means of it 

 sensory impulses are transmitted. 



Another set of functions belonging to both the spinal 

 cord and the brain is the production of reflex action. Some 

 writers upon physiology maintain that all nervous action 

 is, in the final analysis, reflex, and due, more or less 

 remotely, to external stimulus, what is called mind not 

 being able to originate any nervous impulses whatever. 

 Others support the opposite view, that all nervous action 

 is voluntary in the beginning, and becomes reflex by prac- 

 tice, either on the part of the individual, or on that of 

 previous generations of ancestors, by whom the tendency 

 to act in certain ways has been transmitted to their de- 

 scendants. This is not the place for a discussion of such 

 a question, and we may accept the usual distinction 

 between voluntary and reflex action, the latter being 

 invariably the result of an impression received from 

 without. 



438. Reflex Action. By reference to Figs. 48 and 52, 

 on pp. 72 and 90, the course of a nervous impulse in 

 reflex action may be traced. A nerve is stimulated at 

 the surface, the impulse is conveyed by the afferent 



