ACTION OF MUSCLES OF FOREARM AND HAND. 225 



the fingers is either completed or is oi^posed by any resistance, as wnen the 

 over-extended hand is pressed against a sui-face in pushing, or in the support of 

 the body. 



Uj-tcnsion of the ovrist, in a similar manner, is accomplished not only by the 

 three muscles specially devoted to that fimctiou, but also by the extensors of the 

 fingers. 



To ensure the efficient action of the long extensor and flexor muscles of the 

 fingers it is necessary that there should be simultaneous action of the flexors and 

 extensors of the wrist resijectively ; for the wrist-joint must be fixed backwards 

 by its extensors in order that the long flexors of the fingers may act. and the 

 wTist must be fixed forwards by its flexors in order that the long extensors may 

 act upon the fingers. 



TliQ flexor comiimnis digitorum .suMimis and the flexor 2>J'(lfiiH(hts bend respec- 

 tively the second and the thu-d phalanges of the fingers, while the extensor com- 

 ww«w extends the first phalanx. The foiu- ZwwZi/vV'rt //".?, on the other hand, and 

 the seven hiterossel muscles have a double action, in consequence of their insertion 

 into the lateral expansions of the extensor tendons, and of some of the interossei 

 directly into the first phalanges. This action consists fii'st, in the flexion of the 

 fingers at tlie metacariio-phalangeal articrdations, and second, in extension of the 

 second and third phalanges. The lumbricales and interossei, therefore, are antao-o- 

 nists to both the long flexors and to the long extensor. This partial and combined 

 action of the long and short muscles upon the fingers, of which the movements 

 made in forming the hair-stroke in writing may be taken as an example, has 

 been well known for a considerable time, especially as regards the lumbricales 

 but it has recently been confirmed and elucidated as regards the interossei by the 

 electro-physiological experiments and pathological observations of Duchenne 

 whose tateresting work as quotod below,* may be advantageously consulted on 

 this and other muscular movements. 



With respect to the interossei, it is further to be observed that, besides beino* 

 flexors of the first and extensors of the second and third phalanges in the manner 

 previously stated, they severally exercise an abdiicting or adducting action on 

 certain fingers, or direct them away from or towards the middle line of the hand, 

 according to the jilaces of their resijective insertions : and thus the four dorsal 

 interossei are abductors of the index, middle and ring fingers, and the three 

 palmar interossei are adductors of the index, fourth and fifth fingers respectively. 



"Wlien the thuml) is opposed to the other fingers, the metacarpal bone is drawn 

 forwards, chiefly by the opponens, the adductor, and the flexor hrevis muscles. 

 The precise action of the abductor is to draw the thumb forwards. The extensor 

 ossls metacarpl is entirely an abductor, as indicated by the name given to it by 

 Albinus. The extensor prhni internodil likewise is an abductor as well as an 

 extensor of the fii-st phalanx. The extensor seeiindi Internodil extends both 

 phalanges and also draws the metacarpal bone backwards. The adductor and the 

 inner fibres of the flexor b7-ecls adduct the thmnb. A combination of the actions 

 of the different muscles in succession produces circumduction. 



The little finger is withdrawn from the others by its abductor, as the ring- 

 finger is withdrawn from the middle finger by the fourth dorsal interosseous 

 muscle ; and the abdvctor acting with the long flexors, likewise assists the flexor 

 brerls in keeping the first phalanx firmly down in grasping. 



"While the j)al!naris lonrjus has the effect of tightening the palmar fascia, the 

 jndmarls brevis assists in deepening the hollow of the hand by raising the inte- 

 gument on the inner side, so as to increase the hypothenar prominence. 



* Dr. D. B. Dncheune, " Physiologie des Mouvemenls, &,c.," Paris, 1S67. 



VOL. I. 



