From I 

 brain t 



t 



To 

 brain 



Sensory cell 

 body 



Receptor 



Association 

 nerves 



Afferent 



nerve 



path 



Effector 

 (muscle) 



Efferent 

 nerve path 



THE BURNT HAND DRAWS BACK 



When nerve endings in the skin are disturbed, an impulse travels up the afferent, 

 or sensory, nerve cell. The disturbance is discharged to an efferent, or motor, nerve 

 cell. Some is discharged also to an associated cell and transmitted to the brain. 

 The stimulus in the motor nerve cell arouses contraction of muscle. The path from 

 the receptor to the spinal cord to the effector is called a reflex arc 



vegetative. A person without a brain, or with one not working, is also 

 largely vegetative, even if he sometimes uses his striped muscles vigorously. 



The Brain and Reflexes^ It is not diflftcult to show that animals — 

 whether those with brains or those without — depend upon reflexes, or upon 

 the reflex arcs in the nervous system. Let us suppose that a certain part of 

 the sciatic nerve (the main nerve trunk running down the leg) were broken, 

 destroying the continuity of the a^erefit fibers (see illustration, p. 284). One 

 might then walk on carpet tacks or hot iron and not know it unless he 

 happened to be watching his feet. Accordingly, one would not jump to 

 avoid injury. Under these circumstances a person would still be able to 

 move his legs or to jump if he wanted to. On the other hand, if the por- 

 tion carrying e§erent fibers were cut, one would remain just as sensitive as 

 ever to carpet tacks or hot iron or tickling; but he could not move his legs, 

 no matter how much he wanted to. And they certainly would not move 

 of themselves, for the part of the reflex arc connecting the spinal cord with 

 the muscles would be broken. 



A large part of human activity may thus be seen to be mechanical, or 



^See Nos. 4 and 5, p. 299. 

 282 



