96 HUMAN PHYSIOLOGY 



away accordingly. But there is every reason to think 

 that this is a false interpretation of what happened; 

 the movement occurred and the pain was felt after- 

 ward. There was no reasoning process, no entertain- 

 ment of a wish, before the finger was pulled away. 

 Turn to the case of the narrowing of the pupil when 

 one raises the eyes toward the window. Here is an 

 act which cannot be performed at will and one which 

 may not be attended by any well-marked sensation. 

 It is one of the best illustrations of the reflex. 



We are attempting to enforce the teaching that re- 

 flexes do not depend on will or attention. Another way 

 of saying the same thing is that they are the result 

 of structure rather than intelligence. A child might 

 think that a mouse was seized by a trap because the 

 trap desired to make the capture. An older person 

 knows that the trap necessarily snaps upon its victim 

 when a certain spring is pressed. Its structure and 

 not its will is responsible for what takes place. To an 

 extent that is rarely appreciated this is true of the 

 nervous system. It is because its reactions are so timely 

 and serviceable that we find it difficult to look upon 

 them as mechanical. 



We must now consider the simplest combination of 

 elements that can account for a reflex. There must be 

 first of all a receptor on which the causative stimulus 

 can be brought to bear. This may be nothing more 

 than the exposed ending of an afferent fiber, as has 

 been said. There must be, second, a path to the central 

 nervous system. Theoretically a single afferent fiber 

 will suffice. If this fiber is one which enters the cord 

 it will be found to pass in by one of the dorsal roots. 

 Each of these roots has, just where it parts company 

 with the ventral root, an enlargement or ganglion. Study 

 with the microscope has shown that the fibers of the 

 dorsal root run unbroken through the ganglion, but each 

 one in passing connects by a side branch with a nerve 

 cell within its compass. These cells have no dendrites. 



