THE LATERAL VOLAR MUSCLES 461 



maris longus. Its base divides below into four slips, one for each finger. Each 

 slip gives off superficial fibers to the skin of the palm and fhiger, those to the palm 

 joining the skin at the furrow corresponding to the metacarpophalangeal articula- 

 tions, and those to the fingers passing into the skin at the transverse fold at the 

 bases of the fingers. The deeper part of each slip subdivides into two processes, 

 which are inserted into the fibrous sheaths of the Flexor tendons. From the sides 

 of these processes offsets are attached to the transverse metacarpal ligament. 

 By this arrangement short channels are formed on the front of the heads of the 

 metacarpal bones; through these the Flexor tendons pass. The intervals between 

 the four slips transmit the digital vessels and nerves, and the tendons of the Lum- 

 bricales. At the points of division into the slips mentioned, numerous strong, 

 transverse fasciculi bind the separate processes together. The central part of the 

 palmar aponeurosis is intimately bound to the integument by dense fibroareolar 

 tissue forming the superficial palmar fascia, and gives origin by its medial margin 

 to the Palmaris brevis. It covers the superficial volar arch, the tendons of the 

 Flexor muscles, and the branches of the median and ulnar nerves; and on either 

 side it gives off a septum, which is continuous with the interosseous aponeurosis, 

 and separates the intermediate from the collateral groups of muscles. 



The lateral and medial portions of the palmar aponeurosis are thin, fibrous layers, 

 which cover, on the radial side, the muscles of the ball of the thumb, and, on the 

 ulnar side, the muscles of the little finger; they are continuous with the central 

 portion and with the fascia on the dorsum of the hand. 



The Superficial Transverse Ligament of the Fingers is a thin band of transverse 

 fasciculi (Fig. 425) ; it stretches across the roots of the four fingers, and is closely 

 attached to the skin of the clefts, and medially to the fifth metacarpal bone, 

 forming a sort of rudimentary web. Beneath it the digital vessels and nerves 

 pass to their destinations. 



1. The Lateral Volar Muscles (Figs. 426, 427). 



Abductor pollicis brevis. Flexor pollicis brevis. 



Opponens pollicis. Adductor pollicis (obliquus). 



Adductor pollicis (transversus). 



The Abductor pollicis brevis (Abductor pollicis) is a thin, flat muscle, placed 

 immediately beneath the integument. It arises from the transverse carpal liga- 

 ment, the tuberosity of the navicular, and the ridge of the greater multangular, 

 frequently by two distinct slips. Running lateralward and downward, it is 

 inserted by a thin, flat tendon into the radial side of the base of the first phalanx 

 of the thumb and the capsule of the metacarpophalangeal articulation. 



The Opponens pollicis is a small, triangular muscle, placed beneath the pre- 

 ceding. It arises from the ridge on the greater multangular and from the trans- 

 verse carpal ligament, passes downward and lateralward, and is inserted into the 

 whole length of the metacarpal bone of the thumb on its radial side. 



The Flexor pollicis brevis consists of two portions, lateral and medial. The 

 lateral and more superficial portion arises from the lower border of the transverse 

 carpal ligament and the lower part of the ridge on the greater multangular bone; 

 it passes along the radial side of the tendon of the Flexor pollicis longus, and, 

 becoming tendinous, is inserted into the radial side of the base of the first phalanx 

 of the thumb; in its tendon of insertion there is a sesamoid bone. The medial 

 and deeper portion of the muscle is very small, and arises from the ulnar side of the 

 first metacarpal bone between the Adductor pollicis (obliquus) and the lateral 

 head of the first Interosseous dorsalis, and is inserted into the ulnar side of the base 

 of the first phalanx with the Adductor pollicis (obliquus). The medial part of 

 the Flexor brevis pollicis is sometimes described as the first Interosseous volaris. 



