278 THE HUMAN MECHANISM 



be kept up for long periods of time. Evidently there is 

 a mechanism consisting entirely of motor or efferent neurones 

 which by itself, independently of any afferent (that is, reflex) 

 or volitional stimulation, can automatically carry out a large 

 part of the act of locomotion. Locomotion becomes funda- 

 mentally the act of an automatic mechanism and is com- 

 parable to the alternate automatic contractions of inspiration 

 and expiration. 



The parallel between the automatism of respiration and 

 the automatism of locomotion becomes still more striking 

 when we find in both cases that afferent impulses do actually 

 intervene to guide and so make more exact and efficient 

 the fundamental automatic movements. Thus we know that 

 afferent impulses started by the expansion of the lungs 

 during inspiration check the inspiratory effort then in prog- 

 ress and so bring on the next expiration sooner than it would 

 automatically occur. Similarly, the pressure upon the sole of 

 the foot as it touches the ground reflexly guides and proba- 

 bly strengthens the automatic extensor thrust; and many 

 other reflexes through the cord are known to serve similar 

 functions. 



To sum up, then: The action of the legs in locomotion 

 seems to be fundamentally an automatic action, but these 

 automatic movements are guided by afferent impulses which 

 stream in from skin, muscles, and joints as the act pro- 

 gresses. Just as we can volitionally hold the breath, so we 

 can volitionally start or stop walking; or just as we can 

 volitionally change the depth and rhythm of respiration, 

 so we can volitionally change the pace or length of stride; 

 or just as a dash of cold water on the skin or the presence 

 of an irrespirable gas reflexly changes the character of the 

 breathing movements, so unevenness of the path or visual 

 impulses from an object in the way changes the character of 

 the locomotion. In all cases reflex and volitional interference 

 acts on a fundamental automatic nervous mechanism. 



