DR. HILL, FROM REFLEX ACTION TO VOLITION. i I 



the grey matter have done much to throw light upon the 

 route they follow; but we do not yet know whether we 

 ought to speak of the conversion of a sensory into a motor 

 impulse, as its passage through the lower network under the 

 direction of nerve currents which originate in the higher; or 

 whether the impulse when it reaehes the lower grey matter 

 takes in some cases a direct cross path, while in others it 

 makes its transit through a longer loop. One thing is quite 

 certain, namely, that the routes which are most frequently 

 used are the most open, and therefore the most easily 

 traversed. 



Of all the impulses reflected through the grey matter, the 

 most frequent are those upon which the position of the body 

 depends. As we stand upright the body sways ceaselessly 

 backwards and forwards. Swaying backwards, muscles on 

 the front of the thigh are stretched. The stretching gives 

 rise to an impulse which travels up to the spinal cord and 

 down again to the muscles leading to their contraction. As 

 our weight falls forwards the muscles of the calf are 

 stretched, contract, and restore the body to the erect 

 position. These adaptive movements arc made unconsciously 

 many times in every minute. The reflex route is always 

 open. Afferent and efferent nerves are, as it were, constantly 

 "switched on" through the grey matter. The flexor and 

 extensor muscles which play over every joint, bear this see- 

 sawing relation to one another. But peculiar interest attaches 

 to the contraction of the extensor muscles of the thigh, for 

 the reflex action by which this is brought about can 1 >e very 

 easily demonstrated in almost all healthy people, and can be 

 used as a very sensitive test of the condition of the spinal 

 cord when this is diseased. A person sits in an ordinary 

 upright chair, with one leg crossed over flu; other, the 

 muscles lax, and the foot hanging free. If then the tendon 

 below the knee-cap is gently tapped, the slight sudden 

 stretching of the museles of the thigh so produced, leads to 

 their contraction and the sudden jerking forward of the foot. 

 This "knee-jerk," as it is termed, occurs with such rapidity, 

 that physiologists have had difficulty in understanding how- 

 it could really be a reflex action. The jerk follows the tap in 

 ■03 second. If, therefore, we deduct the time taken by the, 

 impulse in ascending to the spinal cord and descending to 

 the muscle, we find that the passage of the grey matter 

 occupies no more than -01 to -015 second. This is only a type 

 of a large number of similar adaptive movements, but I dwell 



