THE ANTERIOR RADIOULNAR REGION 477 



longus pollicis, the ulnar vessels and nerve, and the median nerve. In the hand its tendons 

 are in relation, superficially, with the palmar fascia, superficial palmar arch, and the branches 

 of the median nerve; deeply, with the tendons of the deep Flexor and the Lumbricales. 



The Deep Layer (Fig. 358). 



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 (m. flexor digitorum profundus} (Fig. 358) 

 is situated on the ulnar side of the forearm, immediately beneath the superficial 

 Flexors. It arises from the upper three-fourths 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 three-fourths of the posterior border of the ulna, 

 in common with the Flexor and Extensor carpi ulnaris; 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 'digitorum. Opposite the first 

 phalanges the tendons pass through the openings in the tendons of the Flexor 

 sublimis digitorum, and are finally inserted into the bases of the last phalanges. 

 The portion of the muscle for the index finger (Fig. 362) is usually distinct 

 throughout, but the tendons for the three inner fingers are connected by cellu- 

 lar tissue and tendinous slips as far as the palm of the hand. The tendons of 

 this muscle and those of the Flexor sublimis digitorum, while contained in the 

 osseoaponeurotic canals of the fingers, are invested in a synovial sheath, and are 

 connected to each other and to the phalanges by slender tendinous filaments, 

 called vincula accessoria tendinum (vincula tendineae). Of these there are two 

 sets: (a) The ligamenta brevia two in each finger are composed of triangular 

 bands of fibres which connect respectively (1) the tendon of the Flexor sublimis 

 digitorum to the front of the first interphalangeal joint, and (2) the head of the 

 first phalanx and the tendon of the Flexor sublimis digitorum to the front of the 

 second interphalangeal joint and to the head of the second phalanx. (6) The 

 ligamenta longa connect the under surfaces of the Flexor profundus digitorum 

 to those of the subjacent Flexor sublimis digitorum after the tendons of the former 

 have passed through the latter (Fig. 363). 



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 superficial 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 deep surface, in the forearm, with the ulna, 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 inter- 

 posed. 



The Flexor longus pollicis (m. flexor pollicis longus} (Fig. 358) is situated on 

 the radial side of the forearm, lying on the same plane as the preceding. It arises 

 from the grooved anterior surface of the shaft of the radius, commencing above, 



