ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 447 



heart, the excitation of which slows the rate of beat in this 

 organ. 



All the nerves so far mentioned are nerves which run to 

 the periphery. There are, of course, in addition to these, 

 many nerves which never get beyond the central nervous 

 system, but which run in this for their entire course and 

 serve to connect the various centers within the same, car- 

 rying between these centers impulses similar to those car- 

 ried by peripheral nerves. 



THE GENERAL PHYSIOLOGY OF NERVE CENTERS. 



We are in this discussion not yet concerned with the 

 special functions of the various nerve centers, but merely 

 with those phenomena which apply more or less fully to all 

 nerve cells. In this general way nerve cells or collections 

 of the same into ganglia, are classed as follows: 



1. Automatic Centers. These are centers which do not 

 seem to depend on some specific external impulse to arouse 

 them, but which seem to act more or less independently of 

 all such stimuli. The best illustration is possibly that of 

 the higher centers in the brain which we are wont to desig- 

 nate as the centers concerned in free volition. In addition 

 to these higher psychic centers there are lower automatic 

 ones, such, for instance, as the automatic centers in the 

 heart causing the beat of the same entirely independently, so 

 far as we know, of external stimuli. Such automatic cen- 

 ters may, of course, be aroused, and within certain limits, 

 controlled by outside stimulation, but such outside stimula- 

 tion need not be the invariable occasion for their acting. 



2. Reflex Centers. These are centers found mainly in 

 the spinal cord, but present also in the brain, to which sen- 

 sory impulses are carried, and which as a result of such im- 

 pulses originate motor impulses in harmony with these sen- 

 sations. When working normally these reflex centers are 

 groups of cells which co-ordinate the incoming impulses 

 and the outgoing impulses to produce purposive results. 

 When a person unknowingly touches a hot stove with his 



