SUMMARY OF STRUCTURES 251 



in the other. The nuiscular striietures producinjj; these movements must 

 therefore be integrated in a manner to produce this harmony of motor effect. 

 Indeed, certain other effectors are also implicated in this intricate activity 

 which manifests itself in these simultaneous movements of eye and hand. To 

 this group belong the muscles of the trunk which sustain the body in delinite 

 positions to support the movements of the head upon the neck. The muscles 

 of the hindlimbs should also be included in this category. The animal, espe- 

 cially in the erect posture, reqiiires the cooperation of the hindlimbs in 

 support of the body thus to provide a stable basis for the eye and head move- 

 ments which accompany the movements of the hand. 



Synergic Units. One of the requisites for the precise control of such an 

 extensive grouping of muscles in the l^ody is the muscular coordination in 

 each of the groups cooperating to produce the result. Such coordination is 

 dependent upon the proper intermuscular relation of all thespecialized groups 

 forming the agonist and antagonist muscles. Within each group there exists, 

 under normal conditions, a definite relationship in regard to muscular tension. 

 When one muscle, it may be a flexor, contracts to produce a flexor movement, 

 its antagonistic extensor likewise contracts to such an extent as to impose a 

 check or guiding effect upon the muscle producing the movement. Each group 

 of muscles because it acts in this manner to maintain this specialized inter- 

 muscular relation, has been called a synergic miit. The entire body muscula- 

 ture is composed of synergic units which maintain this definite intermuscular 

 relation when in action. 



To secure such coordmate movement, one entire division of the cen- 

 tral nervous system is set apart, namely, the cerebellum. Under the control 

 of this organ the synergic units of the body are maintained in their proper 

 relations to each other. By the same means proper intermuscular relation 

 between the synergic units is established. The function which regulates 

 coordinated action in the muscles of the body is known as syiiergia. In the 





