240 THE MUSCLES OF THE UPPER LIMB. 



The abductor minimi digiti is found occasionally divided into two or even three slips ; in 

 other cases it is united with the flexor brevis. An accessory head is not unf requently present, 

 arising from the tendon of the flexor carpi ulnaris, the annular ligament, the fascia of the 

 forearm, or the tendon of the palmaris longus. In some cases the additional head arises a con- 

 siderable distance above the wrist from the intermuscular fascia, under either the flexor carpi 

 ulnaris or flexor carpi radialis, and passing downwards covers the ulnar artery to a greater or 

 less extent, ending in the abductor or occasionally in the flexor minimi digiti. A portion of 

 the abductor minimi digiti is occasionally inserted into the fifth metacarpal bone ; or a separate 

 slip may pass from the pisiform bone to this insertion, constituting a pixi-metacarpeus muscle. 

 The pisi-vncinatus is a small muscle, first described by Calori and found by Gruber to be present 

 in 2-5 per cent, of hands, between the pisiform bone and the hook of the unciform ; and the 

 name pisi-annularis may be given to a similar slip inserted into the anterior annular ligament. 



The opponentes muscles are subject to varieties chiefly affecting their extent, and the degree 

 of their union with or separation from the neighbouring muscles. 



The interosseous muscles present some variations, but not of any great magnitude. They are 

 occasionally double in one or more of the spaces. The arrangement which usually exists in 

 the foot, and which is peculiar to man, has also been observed to occur in the hand. 



Nerves of the muscles of the hand. The median nerve supplies the abductor pollicis, 

 the opponens, and the outer head of the flexor brevis by a branch (6 c) arising immediately 

 below the annular ligament, and the first two lumbricales muscles by twigs from the third and 

 fourth digital nerves respectively. The ulnar nerve, besides giving a twig to the palmaris 

 brevis, furnishes by its deep palmar division (8 c) branches to the muscles of the little finger, 

 the two internal lumbricales, all the interosseous muscles, the adductors, and the inner head of 

 the flexor brevis pollicis. 



Actions of the muscles of the forearm and hand. The muscles of the forearm may 

 be distinguished according to their actions as pronators and supinators, flexors and extensors 

 of the wrist, and long flexors and extensors of the fingers ; those of the hand are flexors and 

 extensors, adductors, abductors and opposers of the fingers, the terms adduction and abduction 

 being here used with reference to the middle line of the hand. 



Pronation is mainly effected by the pronator teres and pronator quadratus ; but the flexor 

 carpi radialis also contributes slightly to this movement. The pronator teres is fitted to flex 

 the elbow when pronation has been completed, or when it is prevented by antagonistic muscles, 

 and in this action it receives assistance from the other muscles arising from the internal 

 oondyle. 



Supination is effected principally by the supinator brevis, together with the biceps, the 

 supinator longus having but little influence upon this action. The latter muscle is essentially 

 a flexor of the elbow, acting most efficiently when the hand is placed midway between pro- 

 nation and supination, and tending to bring the hand into that position when it is either fully 

 pronated or supinated. The radial extensors of the wrist assist also in flexing the elbow ; the 

 remaining muscles arising from the external condyle aid in extending that joint. 



Flexion of the wrist is produced by the radial and ulnar flexors of the carpus, and is aided 

 by the flexors of the fingers when the action of those muscles on the fingers is either com- 

 pleted or is opposed by any resistance. 



Extension of the wrist, in a similar manner, is accomplished not only by the three muscles 

 specially devoted to that function, but also by the extensors of the fingers. The lateral move- 

 ments of the wrist are produced by the same muscles, acting in different combinations, 

 abduction by the radial flexor and extensors, assisted by the extensors of the thumb, and ad- 

 duction by the ulnar flexor and extensor. The flexor carpi radialis and extensor carpi ulnaris 

 act specially on the radio-carpal joint, and the flexor carpi ulnaris and extensores carpi radiales 

 on the transverse carpal joint (of. p. 176). 



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 

 respectively ; for the wrist-joint must be fixed backwards by its extensors in order that the 

 long flexors of the fingers may act, and the wrist must be fixed forwards by its flexors in order 

 that the long extensors may act upon the fingers. 



The flexor sublimis digitorum and the flexor profundus bend respectively the second and 

 the third phalanges of the fingers, while the extensor communis extends chiefly the first phalanx. 

 The four lumbricales, on the other hand, and the seven interosseous muscles have a double 

 action, in consequence of their insertion, complete or partial, into the expansions of the exten- 

 sor tendons. This action consists firstly, in the flexion of the fingers at the metacarpo-phalan- 

 geal articulations, and secondly, in extension of the second and third phalanges. The lumbricales 

 and interosseous muscles, therefore, are antagonists 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 

 been confirmed and elucidated as regards the interosseous muscles by the electro-physiological 



