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 in 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withdrawn.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. 
