394 MUSCLES AND FASCIAE. 



carpi, and Extensor Primi Internodii Pollicis. 2. Behind the styloid process, 

 for the tendons of the Extensor Carpi Radialis Longior and Brevior. 3. Oppo- 

 site the outer side of the posterior surface of the radius, for the tendon of the 

 Extensor Secundi Internodii Pollicis. 4. To the inner side of the latter, for 

 the tendons of the Extensor Communis Digitorum, and Extensor Indicis. 5. 

 For the Extensor Minimi Digiti, opposite the interval between the radius arid 

 ulna. 6. For the tendon of the Extensor Carpi Ulnaris, grooving the back 

 of the ulna. The synovial membranes lining these sheaths are usually very 

 extensive, reaching from above the annular ligament, down upon the tendons 

 almost to their insertion. 



The palmar fascia forms a common sheath which invests the muscles of the 

 hand. It consists of a central and two lateral portions. 



The central portion occupies the middle of the palm, is triangular in shape, 

 of great strength and thickness, and binds down the tendons in this situation. 

 It is narrow above, being attached to the lower margin of the annular ligament, 

 and receives the expanded tendon of the Palmaris Longus muscle. Below, it is 

 broad and expanded, and opposite the heads of the metacarpal bones divides 

 into four slips, for the four fingers. Each slip subdivides into two processes, 

 which inclose the tendons of the flexor muscles, and are attached to the sides 

 of the first phalanx, and to the glenoid ligament ; by this arrangement, four 

 arches are formed, under which the flexor tendons pass. The intervals left in 

 the fascia, between the four fibrous slips, transmit the digital vessels and 

 nerves, and the tendons of the Lumbricales. At the point of the division of the 

 palmar fascia into the slips above mentioned, numerous strong transverse fibres 

 bind the separate processes together. The palmar fascia is intimately adherent 

 to the integument by numerous fibrous bands, and gives origin by its inner 

 margin to the Palmaris Brevis ; it covers the superficial palmar arch, the ten- 

 dons of the flexor muscles, and the branches of the median and ulnar nerves; 

 and on each side it gives off a vertical septum, which is continuous with the 

 interosseous aponeurosis, and separates the lateral from the middle palmar 

 group of muscles. 



The lateral portions of the palmar fascia 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 dorsal fascia, and 

 in the palm with the middle portion of the palmar fascia. 



MUSCLES OF THE HAND. 



The Muscles of the Hand are subdivided into three groups: 1. Those of the 

 thumb which occupy the radial side. 2. Those of the little finger which 

 occupy the ulnar side. 8. Those in the middle of the palm and between the 

 interosseous spaces. 



RADIAL REGIOX. (Fig. 244.) 

 Muscles of the Thumb. 



Abductor .Pollicis. 



Opponens .Pollicis (Flexor Ossis Metacarpi). 



Flexor Brevis Pollicis. 



Adductor Pollicis. 



The Abductor Pollicis is a thin, flat muscle, placed immediately beneath the 

 integument. It arises from the ridge of the os trapezium and annular liga- 

 ment; and passing outwards and downwards, is inserted by a thin, flat tendon 

 into the radial side of the base of the first phalanx of the thumb. 



Relations. By its superficial surface, with the palmar fascia. By its deep 

 surface, with the Opponens Pollicis, from which it is separated by a thin apo- 

 neurosis. Its inner border is separated from the Flexor Brevis Pollicis by a 

 narrow cellular interval. 



