386 MUSCLES AND FASCIAE. 



into the lateral margins of the second phalanges, about their centre. Opposite 

 the base of the first phalanges, each tendon divides, so as to leave a fissured 

 interval, between which passes one of the tendons of the Flexor Profundus, and 

 the tendons of both the Flexors then enter an osseo-aponeurotic canal, formed 

 by a strong fibrous band, which arches across them, and is attached on each 

 side to the margins of the phalanges. The two portions into which the tendon 

 of the Flexor Sublimis divides, so as to admit of the passage of the deep Flexor, 

 expand somewhat, and form a grooved channel, into which the accompanying 

 deep flexor tendon is received ; the two divisions then unite, and finally sub- 

 divide a second time to be inserted into the fore part and sides of the second 

 phalanges (Fig. 244). The tendons, whilst contained in the fibro-osseous canals 

 are connected to the phalanges by slender tendinous filaments, called vincula 

 accessoria tendinum. A synovial sheath invests the tendons as they pass be- 

 neath the annular ligament ; a prolongation from which surrounds each tendon 

 as it passes along the phalanges. 



Relations. In the forearm, by its anterior surface, with the deep fascia and all 

 the preceding superficial muscles ; by its posterior surface, with the Flexor Pro- 

 fundus Digitorum, Flexor Longus Pollicis, the ulnar vessels and nerve, and the 

 median nerve. In the hand, its tendons are in relation, in front, with the palmar 

 fascia, superficial palmar arch, and the branches of the median nerve ; behind, 

 with the tendons of the deep Flexor and the Lumbricales. 



ANTEEIOB BRACHIAL EEGION. 

 Deep Layer. 



Flexor Profundus Digitorum. Flexor Longus Pollicis. 



Pronator Quadratus. 



Dissection. Divide each of the superficial muscles at its centre, and turn either end aside ; 

 the deep layer of muscles, together with the median nerve and ulnar vessels, will then be 

 exposed. 



The Flexor Profundus Digitorum (Perforans) (Fig. 240) is situated on the 

 ulnar side of the forearm, immediately beneath the superficial Flexors. It 

 arises from the upper two-thirds of the anterior and inner surfaces of the shaft 

 of the ulna, embracing the insertion of the Brachialis Anticus above, and 

 extending, below, to within a short distance of the Pronator Quadratus. It 

 also arises from a depression on the inner side of the coronoid process, by 

 an aponeurosis from the upper two-thirds of the posterior border of the 

 ulna, and from the ulnar half of the interosseous membrane. The fibres 

 form a fleshy belly of considerable size which divides into four tendons ; these 

 pass under the annular ligament beneath the tendons of the Flexor Sublimis. 

 Opposite the first phalanges, the tendons pass between the two slips of the 

 tendons of the Flexor Sublimis, and are finally inserted into the bases of the 

 last phalanges. The tendon of the index finger is distinct ; the rest are con- 

 nected together by cellular tissue and tendinous slips, as far as the palm of the 

 hand. 



Four small muscles, the Lumbricales, are connected with the tendons of the 

 Flexor Profundus in the palm. They will be described with the muscles in 

 that region. 



Relations. By its anterior surface, in the forearm, with the Flexor Sublimis 

 Digitorum, the Flexor Carpi Ulnaris, the ulnar vessels and nerve, and the 

 median nerve ; and in the hand, with the tendons of the superficial Flexor. By 

 its posterior surface, in the forearm, with the ulnar, the interosseous membrane, 

 the Pronator Quadratus; and in the hand, with the Interossei, Adductor Pollicis, 

 and deep palmar arch. By its ulnar border, with the Flexor Carpi Ulnaris. 

 By its radial border, with the Flexor Longus Pollicis, the anterior interosseous 

 vessels and nerve being interposed. 



