FUNCTIONAL PARTS OF THE NERVOUS SYSTEM 83 



combined message goes in only one direction, perhaps to one region for 

 the performance perhaps of a single end. How all these things are 

 done in particular we do not know, but we see well-nigh universal 

 interconnection, and we observe continually the results in living animals 

 of these processes of adapted collection and distribution by the long and 

 complicated spinal cord. This viatilityof the nerve-centers is one of the 

 functional marvels of biology. 



REFLEXION, OR REFLEX ACTION. Reflexes are actions of the body 

 carried on by the spinal cord (and perhaps by the cerebellum) without 

 the immediate volition or even knowledge of the individual. In a typical 

 reflex action, as the name implies, there is a turning-backward of the 

 nerve-current. The impulse originates in some sense-organ, passes 

 through some nerve-cell or nerve-cells, and goes out of the central 

 nervous system to the muscle or gland, or possibly to exert trophic 

 action on some portion of the tissue. The influence which passes from 

 the sense-organ to the central nerve-organ is called the afferent or 

 centripetal impulse. Its passage through the gray matter is called the 

 central process. It should not be thought that the afferent sense-organ 

 must be on the periphery of the body or that the efferent instrument 

 (muscle or gland usually), must be far away from the reflecting center, 

 because the impulse might and often does originate very close to or 

 even within the spinal column and pass to a muscle cell or gland 

 alveolus close by. The terms afferent and efferent were derived from 

 typical instances of reflex action, such as occurs when a touch on the 

 foot, for example, causes it to be withdrawn. The central process, 

 properly speaking, does not have to be in the spinal cord, but may 

 occur in one of the numerous gray nuclei elsewhere in the body. 

 Matters are so complicated in nerve-action that seldom are processes 

 limited to that which their names denote. 



In general terms, the afferent or centripetal impulse goes into the spinal 

 cord through the posterior roots. Here it is associated, mainly in the gray 

 alseof the cord, with many other impulses, perhaps passing up and down 

 in connected neurones. It then passes out, properly adapted to its purpose, 

 in the anterior spinal roots on its way to the muscle or the gland. Mat- 

 ters in fact, however, are very much more complicated than such a 

 mere formula implies, especially as regards the association which 

 abounds in the gray matter. The greatest mystery concerned with the 

 reflex movements of the body is as to how the necessary distribution 

 and selection of paths (viatility) is made. A single unexpected touch, 

 for example, on the back of the neck causes the exactly adapted and co- 

 ordinated contraction of scores of complicated muscles of the trunk, neck, 

 and legs. The centers in the cord, consisting of knots of neurones, are 

 somehow able to distribute an impulse among the neurones so as to 

 make the succeeding act exactly the one out of possible thousands which 

 is most useful to the animal. This is the great mystery about all neuro- 

 muscular actions, and when the problem is complicated by trying to 

 imagine how the influences from the brain above are connected with those 

 proper to the spinal cord, the problem is obviously a most complicated 



