142 THE MECHANISM OF LIFE 



A nervous inhibition is the arrest, entire or partial, of some 

 activity in progress, and such effects are very common and very 

 important. The rhythmic activity of the heart, for instance, is 

 intrinsic that is, it apparently goes on as the result of some 

 periodic stimulus originating in its own substance. But this 

 rhythmic activity is subject to nervous control, the heart-beat 

 being regulated by two sets of nerves, one coming from the 

 sympathetic system, and the other from the tenth (pneumo- 

 gastric or vagus) nerve. The stimulation of the former 

 accelerates, and that of the latter inhibits, the rate of the heart, 

 so that, by suitable stimulation of the vagus, the beat may be 

 slowed down or may even be arrested altogether. 



Now nervous " shock " that is, the great prostration arising 

 from an injury, or the effects of a surgical operation is to be 

 regarded as a series of inhibitions. In some way or other im- 

 pulses passing down from the higher brain centres are blocked 

 (as when the spinal cord is severed), or are not initiated (probably 

 as the result of afferent stimulation in the case of a severe injury), 

 and the stopping of these impulses is the main cause of the 

 prostration and other effects called " shock." In the decerebrate 

 mammal, then, impulses that normally issued from the cortex 

 cerebri cease, and the general activities of the animal are affected. 

 The restlessness referred to above may be traced to the cessation 

 of inhibitory impulses which in normal life arrest or modify 

 aimless, random movements. The abnormally large consump- 

 tion of food may also be traced to the cessation of cortical 

 impulses regulating the metabolism of the tissues and economis- 

 ing energy, and the condition that anger and dislike could be 

 elicited, but not pleasure and affection, may also be traceable to 

 the loss of inhibitions. 



For one may (taking a general survey of organic behaviour) 

 conclude that the feral animal has normally a " bad time." It 

 struggles for its existence both with inorganic nature and with 

 its organic enemies. It must " eat or be eaten." " Softness of 

 heart," dalliance, pity for others, and the like, are feelings that 

 are wanting, or, if present, are likely to be detrimental or fatal, 

 while their opposites make for self-preservation. What we call 

 loosely the altruistic motives must be regarded as the product 

 of the herd instinct, and they are to be interpreted as inhibitions 

 or checks upon the natural, predatory, and highly individualistic 

 modes of beha/iour. They are opposed to most tendencies that 



