Use.—The simplest way of regarding their action, which is rather complex, is to refer it towards the axis of the hand or a central line drawn through the third metacarpal bone and the middle finger, in which case it is easily perceived that the palmar interossei are adduc tors towards the axis of the hand.
II. The only intrinsic muscles on the dorsal aspect of the hand are the dorsal interossei, interassei externi digitorrun moms. Their com mon points are, that they appear both on the dorsal and palmar aspects of the band ; they are bicipital; arising from the opposed surfaces of two metacarpal bones, both heads termina ting in a common tendon, which is attached to the sides of the first phalanges and extensor tendons that are not supplied by the palmar in terossei. They are four in number ; the first, or adductor indices, alone merits a particular description. It is the largest ; arising from the superior half of the external border of the first metacarpal bone, and externally from all the external surface of the second metacarpal bone; these origins are separated by a fibrous arch, through which the radial artery passes ; they are large and fleshy, and soon unite, forming a triangular flattened muscle, which is inserted into the external side of the first phalanx. The insertion of the other muscles are, the two mid dle into either side of the first phalanx ; they are called the prior and posterior medii; and the last, or posterior annularis, into the inter nal side of the riug-finger.
Relations.—Posteriorly they correspond to the extensor tendons and skin ; anteriorly they appear beside the palmar interossei, from which they are separated by a strong septum derived from the deep palmar aponeurosis. Their other relations are the same as the palmar interossei. The first, the abductor indicis, corresponds an teriorly to the adductor pollicis and part of the flexor brevis, which it crosses at right angles ; its inferior and external margin is subcuta neous.
Use.—They are all abductors of the fingers from the axis of the hand, and by reason of their insertion into the extensor tendons, act best when the hand is extended. The same may be said of the palmar.
Before we enter on the general uses of this complex muscular apparatus, it would be well to remark that the proper muscles of the thumb and little finger appear to be nothing more than exaggerated and multiplied lumbricales and in terossei. We may, in this light, view the short
flexor of the thumb as the first lumbricalis, its abductor and opponens as a dorsal interosseus, while its adductor would represent a palmar interosseous muscle; again, as regards the little finger, its abductor and short flexor together personate a dorsal interosseus, while its adduc tor would be but an internal or palmar inter osseous. Their principal use is, by acting on the carpo-metacarpal articulations of the thumb and little finger, which enjoy freer motion than the intermediate ones, especially that of the thumb, to oppose these extreme points of the hand to each other, more or less increasing its concavity, and thereby giving a firmer grasp, inasmuch as they adapt the cavity of the palm to the volume of the body grasped. The great use of this opposable faculty of the thumb (which action is the peculiar characte ristic of the hand as distinguishing it from the foot) may be shewn by firmly clenching the fist, when the thumb, by its combined powers of opposition and flexion, is made to overlap the fore and middle, and in some the third fingers, pressing them firmly against the palm, while, at the same time, the thenar eminence is thrown forwards and inwards, meeting them in the palm, and by abutting against counteracts their tendency to fly open when a blow is struck, acting at the same time as a cushion to deaden the violence of the shock. We here see, also, the flexion of the fingers modified by the radial interossei and lumbricales, which, by their action, throw the fingers radiad, so as to bring the three outer ones to abut against the thenar eminence ; the little finger is pro tected, in like manner, by the hypothenar, which is thrown forwards and outwards. The converse modification of the flexion of the fingers by means of the ulnar interossei may be seen in the action of the left hand of a fiddler, where the fingers are flexed and pointed ulnad to run up the scale.