138 THE MECHANSIM OF LIFE 



Reflexes in the Normal Animal. — Now compare with these 

 spinal reflexes those that may be observed in the normal, intact 

 animals. Such a response is easily observed in the case of a 

 dog which is lying sleeping on his side with his legs stretched 

 out. When the skin over the ribs is rubbed vigorously, the 

 hind-leg of the same side will often make a series of foolish little 

 kicks, like incipient scratching movements. We may even study 

 such reflexes in ourselves. We usually start violently on hearing 

 a loud, unexpected noise, and the muscular actions involved 

 have meaning, inasmuch as they seem to be such as would place 

 the body in a posture of defence to meet some sudden menace. 

 When some object makes an unexpected movement towards the 

 eyes the lids are rapidly closed, and here also the action has 

 purpose — -the protection of the eyes. As a general rule these 

 and similar responses are quite involuntary, and it may even 

 require some considerable effort of the will to arrest or prevent 

 them. But that they can be arrested or prevented is the charac- 

 teristic that distinguishes them from the reflexes which are 

 carried out by the spinal cord alone in the lower vertebrates or 

 in the higher vertebrates deprived of their cerebral hemispheres. 

 Even the sleeping dog will not always give the incipient scratch 

 reflex when his side is rubbed, and by strong attention and " will- 

 power " a man may keep his eyes open when someone flicks 

 some object towards the face, or when he puts his head under- 

 neath water, and one may easily arrest the start that he naturally 

 makes when he hears a loud, unexpected noise. 



This point is very important. The response that may be 

 elicited by a stimulus applied to the animal possessing its integral 

 nervous system is not a " fatal " or " inevitable " one. It may 

 occur, but it may not. It may occur in one of several ways, and 

 it may occur and immediately be arrested. All that means that 

 the strict physical determinism that one sees in the " response " 

 or deviation of a compass needle when a magnet is brought near 

 to it, or the equally determined tropistic and tactic responses of 

 the lower organisms, or the spinal animal, do not occur in the 

 higher vertebrate possessing its entire central nervous system. 

 In the responses that one studies in such cases there is always 

 what we must call indeterminism : they are usually unpredictable. 



But while this is the case, it is no less clear that we can study 

 a series of responses beginning with the purely inorganic reaction 

 of the compass needle and magnet, and passing through the 



