THE NEKVOUS SYSTEM 277 



of consciousness. What is the nature of this mechanism? 

 In answer to this question we can only make a suggestion 

 without pretending to give a final explanation. 



In the first place, walking obviously involves alternate 

 steps, or forward thrusts of the body by the two legs ; that 

 is, while one leg is pushing the body forward by straighten- 

 ing at hip, knee, and ankle joints, the other leg is bending 

 at these joints, the flexion of each leg at the hip joint bring- 

 ing it forward in preparation for its next forward thrust. 

 In each leg, then, we have an alternation of "forward swing" 

 (flexion at hip, knee, and ankle) and of " extensor thrust " 

 (extension at hip, knee, and ankle). In the same leg the 

 flexors of the hip, knee, and ankle obviously contract at 

 approximately the same time, and the extensors at the three 

 joints similarly act together ; furthermore, the extensor action 

 in one leg is simultaneous with the flexor action in the 

 opposite leg. These actions may be represented in diagram 

 as follows: 



Now it has been shown that in an animal made unconscious 

 by ether anesthesia, and in which no afferent impulses may 

 enter the cord or brain, because of the depth of anesthesia 

 or even because of cutting the dorsal nerve roots, similar 

 movements of the hind legs spontaneously arise and may 



1 Flexion of the toes in each leg occurs simultaneously with extension at 

 the other three joints, and vice versa. With most people, owing to the use 

 of improperly shaped shoes, the toes are little used in walking. See Chapter 

 XXIV on the Hygiene of the Feet. 



