12 REFLEX ACTION 



lids causing them to contract. This process takes place in 

 a small fraction of a second after stimulation, and occurs 

 not only without the action of the will, but even in spite 

 of efforts to prevent it. 



Another reflex act, the so-called tendon reflex, may readily 

 be observed by striking the large tendon which is inserted 

 into the patella just below the knee. If the leg is hanging 

 loosely a light blow on the tendon will cause the foot to be 

 suddenly extended. Here the impulses set up by the blow 

 proceed to the spinal cord, and are returned along motor 

 nerves to the extensor muscles of the leg causing them to 

 contract. If the nervous centers of the spinal cord are im- 

 paired, as occurs in certain diseased conditions, this reflex 

 can no longer be evoked. 



In a frog these spinal reflexes may readily take place if the 

 spinal cord is cut across a short distance hi front of the center 

 which supplies nerves to the parts concerned. A stimulus 

 supplied to the hind leg of such a frog causes the leg to be 

 withdrawn. If in a frog whose brain has been destroyed the 

 side of the body is stimulated by acid there may be produced, 

 besides the twitching of the muscles near the stimulated part, 

 a movement forward of the hind foot of the same side which 

 often succeeds in wiping away the acid. If the stimulus is 

 applied near the middle of the body both hind feet may 

 be brought forward to remove it. And if the stimulus be 

 applied to one side and the leg of the same side be held, the 

 opposite leg may be brought into play, especially if the 

 stimulus is a strong one, and may succeed in removing the 

 offending substance. The accuracy of these movements, 

 especially in the crossed reflex, is exaggerated in most 

 accounts, but sufficient exactness is frequently attained to 

 effect the removal of the irritating material. 



Reflex action may occur in various degrees of complexity. 



