POSTURAL COORDINATION 915 



ening reaction. By this mechanism the tension which muscles exert on 

 their insertions is kept approximately constant, in spite of the changes 

 in their length which are occasioned by alterations in the position of 

 the body (Sherrington 27 ). 



The plastic tone of a skeletal muscle is effected not only by afferent 

 impulses arising in its own proprioceptors, but by impulses arising from 

 other muscles as well. When the lengthening reaction of the extensors of 

 one limb is produced, a shortening reaction occurs in the extensors of the 

 opposite limb. Conversely the stimulus which sets up a shortening re- 

 action of one limb causes a lengthening reaction of the opposite exten- 

 sors. The crossed lengthening and shortening reactions may be seen 

 to be appropriate to the normal mode of using the two legs, for under 

 usual circumstances when one limb is flexed the opposite leg becomes 

 extended to support the weight of the body. The arrangement is conse- 

 quently one which ensures a certain degree of harmony in the tonic ad- 

 justments of muscles of parts which are used synchronously. A similar 

 adjustment of the tonic condition of the muscles must also be made 

 when the position of the body is changed through voluntary or reflex 

 movement. When such movements are produced tone changes of two 

 types occur: (1) The tone of the muscles which oppose the movement is 

 inhibited; (2) the muscles which produce the movement enter into a tonic 

 contraction which maintains the limb in the new position after the ex- 

 citation has come to an end. 



The first phenomenon is seen when the leg of a spinal or decere- 

 brate mammal is thrown into flexion by stimulating a sensory nerve, or 

 the pain receptors of the foot. The tone of the extensors, which in 

 the decerebrate cat is strong enough to support the animals weight, 

 might be expected to offer some resistance to the bending of the joints. 

 It may be shown, however, that at the moment the flexors contract, 

 the tone of the extensors vanishes, so that they do not oppose the flexion. 

 This is demonstrated by separating the flexor muscles from their insertion 

 at the knee and attaching them to a weighted lever, by which changes 

 in their length may be recorded. Since they are freed from their 

 insertion, any increase in their length cannot be due to stretching by 

 the flexors, but must be due to a loss in tone. When flexion is pro- 

 duced, it is found that the extensor muscle lengthens and the lever falls, 

 in synchrony with contraction of the flexor (Fig. 226). This is an exam- 

 ple of the phenomenon of reciprocal inhibition of antagonistic muscles, 

 which ensures their cooperative action. 



The maintenance of the new position of a limb after the stimulus which 

 caused the change has subsided is called the after-discharge of the re- 

 flex. It is due in certain cases at least to a tonic shortening reaction 

 which is set up during the response to a stimulus applied to receptors 



