786 THE CENTRAL NERVOUS SYSTEM 



tion neurons and proceed down the ganglion chain to terminate by 

 synapses in one or other of the segmental ganglia. These association 

 neurons exercise a dominating influence over the activities of the seg- 

 mental ganglia, so that they may determine the response of the animal 

 when its safety is threatened by some approaching enemy. When, for 

 example, the stimulus produced by some sight or sound of an approach- 

 ing enemy is received by the head ganglia, these will transmit impulses 

 down the ganglion chain which so influence the various nerve cells of 

 this chain as to produce in all of them a coordinated action for the pur- 

 pose of removing the animal from danger. Even should some local 

 stimulant be acting on one or more of the segments, the response may be 

 inhibited on account of stimuli meanwhile transmitted by way of asso- 

 ciation neurons from the large head ganglia; in other words, the part 

 controlled by the segmental ganglia becomes subservient to the whole 

 through the dominating control of the head ganglia. 



This illustrates the beginnings of the integration of the nervous system; 

 and as we pass to the study of the higher animals, we shall see that this 

 integration becomes more and more complicated, and that, as it does so, 

 the nerve centers acquire the power of storing away the impressions they 

 receive, which they may afterwards apply to regulate the reflex response. 

 Thus memory and volition come to find their place in the nervous inte- 

 gration of the animal. The afferent stimulus arriving, let us suppose, 

 at nerve cells controlling the movement of the leg, may fail to cause 

 a response of the corresponding muscles because of impulses meanwhile 

 transmitted by association neurons from higher memory centers, for 

 the animal may have learned by experience that such a movement as the 

 local stimulus would in itself -call forth is opposed to its own best in- 

 terests. This experience will have been stored away in memory nerve 

 centers, so that, whenever the local stimulus is repeated, impulses are 

 discharged from the memory centers to the local nerve centers, and 

 the reflex response does not occur, or is much modified in nature. For 

 storing away these memories and for related psychological processes of 

 volition, etc., the anterior portions of the nervous system in higher ani- 

 mals become very highly developed so as to constitute the brain, and 

 the simple chain of ganglia of the invertebrates is replaced by the 

 spinal cord. 



As we ascend the scale of the vertebrates, the brain becomes more 

 and more developed, until in the higher mammalia, such as man, very 

 few reflex actions can occur independently of the higher centers which 

 are located in it. The reflex arc now involves, not one nerve center, 

 but several, and of these the most important are located in the brain. 



There is thus no essential difference in the general nature of integra- 



