DISCUSSION 187 



flexor digit, sublimis plantaris 



flexors •{ flexor digit, profundus flexor digit, longus 



flexor fibularis 



f extensor digiti 5 extensor hallucis 



I extensor digit, communis extensor digit, longus 



extensors ^ 



I extensor indicis peroneus digiti 4 



I peroneus digiti 5 



Most muscles, at least of the more proximal segments, can 

 by virtue of their position do two things. They can either 

 flex or extend, and at the same time adduct or abduct or 

 rotate; but they can act in but one direction as can a rubber 

 band stretched between two points. When the origins are 

 not immovably fixed they can, however, move both bones to 

 which they are attached. Of the above class are the short, 

 direct, flexor and extensor muscles. The long muscles of 

 the limbs, however, can act in at least two ways. Each 

 muscle of this sort is controlled by two associates, one of 

 origin and one of insertion, and an antagonist of origin and 

 one of insertion. By the aid of these four, short muscles, 

 the long one can act in two ways. When two of the short 

 ones are flexed, the long one may be made to flex one seg- 

 ment, while when the two alternate short ones act, the long 

 one extends the adjoining segment. Theoretically a long 

 muscle, if a flexor of the upper segment, acts as an extensor 

 of the one next distad, and vice versa. In actual practice 

 the modifications of its position may be such as largely to 

 inhibit one movement. Thus, certain long muscles of the 

 lower arm may be able to cause no appreciable movements 

 of that segment, and in Neotoma it is doubtful if the long 

 semitendinosus, which flexes the shank, has an appreciable 

 effect upon the thigh. On the other hand, the biceps brachii 

 in man may flex and supinate the forearm, and extend and 

 abduct the femur. It is true that the abduction is but 

 slight, but it can really be said to have four actions, two 

 of them supplementary to the others. 



As the pelvis is immovably fixed while the scapula is not, 



