THE CENTRAL NERVOUS SYSTEM. 6l 



muscles of the thigh from their lower attachments and connecting them 

 with recording levers. It is then found that the application of firm 

 pressure to the foot produces simultaneously contraction of the extensor 

 and inhibition of the flexor muscles. 



This process, which holds good for the action of antagonistic muscles 

 in general, is spoken of as reciprocal inneyvation. ; and its importance 

 for the efficiency of a reflex action is demonstrated by the effects 

 which are observed when it no longer takes place. The scratch reflex, 

 for example, in the dog consists of rhythmic movements of flexion and 

 extension of the leg, each flexion recurring about four times a second ; 

 in the normal animal the movements are directed to the removal of 

 an irritant. The same reflex can be evoked in a spinal dog by the 

 application of a weak electrical stimulus to the skin of the "scratch 

 area," which occupies the thoracic region ; with each flexion of the 

 thigh the flexor muscles contract and the extensors are inhibited ; with 

 each extension of the thigh the converse takes place. After the injec- 

 tion of strychnine into the animal, the application of the stimulus 

 produces contraction of both flexor and extensor muscles, and, since the 

 extensors are the more powerful, the limb becomes rigidly extended and 

 the reflex can no longer be carried out. 



Since the muscles in the body are limited in number, whereas the 

 impulses which may reach the spinal cord or brain are almost infinite 

 in variety, it is clear that the same muscle must at times be used in 

 response to different kinds of stimuli. For example, the flexor muscles 

 of the leg contract during the scratch reflex, and also in response to a 

 painful stimulus ; in each case the impulse travels down the efferent 

 nerve to the muscle. The motor side of the reflex arc is, therefore, to 

 some extent identical for both the scratch reflex and the response 

 to the painful stimulus ; it is therefore spoken of as a final common 

 path. 



Further, we find, as might be expected, that the final common path 

 can only be traversed by one set of impulses at the same time. If, for 

 example, the scratch reflex is evoked in a spinal animal, and while it is 

 taking place a strong nociceptive stimulus is applied to the sole of 

 the foot, the scratch reflex immediately stops and is replaced by 

 flexion of the leg (fig. 14). Conversely, if the flexion reflex is in pro- 

 gress, the application of the scratch stimulus, if sufficiently strong, 

 may inhibit the flexion reflex and produce the scratch reflex. 



The two reflexes cannot co-exist; one or the other must prevail, 

 and the one which prevails (prepotent reflex) is usually that which is 

 most important for the well-being of the body. The fact, |hat_tfflfl> 

 opposed reflexes such as those just mentioned p.a.nnnt. ' 



