POSTURAL COORDINATION 917 



other than the proprioceptors of the muscles (called exteroceptors). Fig. 

 227 shows how greatly the reflex contraction of the extensors of the 

 knee of the cat may outlast the stimulus. If the afferent fibers from 

 this muscle are destroyed by cutting the dorsal roots the reflex contrac- 

 tion scarcely outlasts the stimulus. It appears that the reflex contrac- 

 tion is accompanied by a tonic shortening reaction which is maintained 

 after the exciting stimulus has come to an end. Its occurrence is de- 

 pendent on the proprioceptive reflex arc of the muscle, so that if this 

 is interrupted the contraction of the muscle cannot be maintained. The 

 reflex response fuses with the proprioceptive reaction so that it cannot 

 be said where one ends and the other begins. It is obvious that this is 

 a mechanism which greatly facilitates the "smoothness" of muscular 



Fig. 227. Records of the contraction of the isolated extensor muscle (vasto crureus) of the knee 

 of the cat produced by stimulating the popliteal nerve of the opposite leg. Periods of stimulation 

 indicated by the signal below the record. A, illustrates the after-discharge in the normal muscle; 

 B the absence of after-discharge following the cutting of the afferent nerves from the muscle. 

 (From bhemngton.) 



acts, enabling the limbs to remain in the position assumed under the 

 effects of certain stimuli, until other stimuli cause new postures to ap- 

 pear. 



The Posture of the Body as a Whole, When the position of the body 

 changes as the result of voluntary and reflex activity the tone of its 

 entire musculature must be modified so that each part may make a har- 

 monious contribution to the act as a whole. Part of the function of the 

 central nervous system is consequently to correlate the simple proprio- 

 ceptive reflexes such as we have described, not only with one another, but 

 with the voluntary and reflex acts which are initiated through the ex- 

 teroceptors. The afferent impulses which give rise to these harmonious 

 changes in the posture of the body as a whole arise in part from the pro- 



