100 



INTRODUCTION TO NEUROLOGY 



The conduction of nervous impulses within the central nervous 

 system in some cases takes place through well-defined and insu- 

 lated bundles of fibers, which are termed tracts; but in most 

 cases there is more or less complexity introduced by collateral 

 avenues of discharge to other specific centers, as in the complex 

 forms of reflex systems described in Chapter IV, or by a more 

 diffuse type of irradiation (p. 65). The organization of the 

 central nervous system is such that in general the excitation of 

 any peripheral sensory neuron may be transmitted to very di- 

 verse and remote parts of the brain, each of which may call forth 

 its own characteristic form of response. 



The physiological effects of such a dispersal of an incoming nervous im- 

 pulse within the central nervous system may be very different, depending 



sKin 



Fig. 39. Diagram of an arrangement of neurons adapted for the dis- 

 tribution of a single afferent nervous impulse to several different motor 

 organs. 



on the connections of the pathways which are taken by the neurons of the 

 second order. If these pathways diverge so that the stimulus is distributed 

 among several different effector systems, this would tend to disperse the 

 energy of the afferent impulse and a relatively strong stimulus is necessary 

 to call forth a response. This is the situation in case a painful prick on the 

 skin of the face calls forth reflex movements of, say (1) twitching of the 

 facial muscles; (2) turning the head away, and (3) a movement of the hand 

 to remove the irritant. Here the stimulus arising at a single point in the 

 skin (Fig. 39) is distributed to three widely separated motor centers (M.I, 



