394 THE MUSCLES AND FASCIAE. 



Eelations. By its superficial surface, in the forearm, with the Flexor sublimis 

 di<ntorurn, 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 lorigus pollicis, the anterior interosseous vessels and nerve 

 being interposed. 



The Flexor longus pollicis 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, immediately below the tuberosity 

 and oblique line, and extending below to within a short distance of the Pronator 

 quadratus. It also arises from the adjacent part of the interosseous membrane, 

 and generally by a fleshy slip from the inner border of the coronoid process or from 

 the internal condyle of the humerus. The fibres pass downward, and terminate in 

 a flattened tendon which passes beneath the annular ligament, is then lodged 

 in the interspace between the outer head of the Flexor brevis pollicis and the 

 Adductor obliquus pollicis, and, entering an osseo-aponeurotic canal similar to 

 those for the other flexor tendons, is inserted into the base of the last phalanx 

 of the thumb. 



Eelations. By its superficial surface, with the Flexor sublimis digitorum, 

 Flexor carpi radialis, Supinator longus, and radial vessels ; by its deep surface, 

 with the radius, interosseous membrane, and Pronator quadratus ; by its ulnar 

 border, with the Flexor profundus digitorum, from which it is separated by the 

 anterior interosseous vessels and nerve. 



The Pronator quadratus is a small, flat, quadrilateral muscle, extending trans- 

 versely across the front of the radius and ulna, above their carpal extremities. It 

 arises from the oblique or pronator ridge on the lower part of the anterior surface 

 of the shaft of the ulna ; from the lower fourth of the anterior surface and the 

 anterior border of the ulna ; and from a strong aponeurosis which covers the inner 

 third of the muscle. The fibres pass outward and slightly downward, to be 

 inserted into the lower fourth of the anterior surface arid anterior border of the 

 shaft of the radius. 



Relations. By its superficial surface, with the Flexor profundus digitorum, 

 the Flexor longus pollicis, Flexor carpi radialis, and the radial vessels; by its 

 deep surface, with the radius, ulna, and interosseous membrane. 



Nerves. All the muscles of the superficial layer are supplied by the median 

 nerve, excepting the Flexor carpi ulnaris, which is supplied by the ulnar. The 

 Pronator radii teres and the Flexor carpi radialis derive their supply primarily 

 from the sixth cervical ; the Palmaris longus from the eighth cervical ; the Flexor 

 sublimis digitorum from the seventh and eighth cervical and first dorsal, and the 

 Flexor carpi ulnaris from the eighth cervical and first dorsal nerves. Of the deep 

 layer, the Flexor profundus digitorum is supplied by the eighth cervical and first 

 dorsal through the ulnar and anterior interosseous branch of the median. The 

 remaining two muscles, Flexor longus pollicis and Pronator quadratus, are also 

 supplied by the eighth cervical and first dorsal through the anterior interosseous 

 branch of the median. 



Actions. These muscles act upon the forearm, the wrist, and hand. The 

 Pronator radii teres helps to rotate the radius upon the ulna, rendering the hand 

 prone: when the radius is fixed it assists the other muscles in flexing the 

 forearm. The Flexor carpi radialis is one of the flexors of the wrist ; when 

 acting alone it flexes the wrist, inclining it to the radial side. It can also 

 assist in pronating the forearm and hand, and, by continuing its action, to bend 

 the elbow. The Flexor carpi ulnaris is one of the flexors of the wrist : when 

 acting alone it flexes the wrist, inclining it to the ulnar side, and, by continuing 

 to contract, it bends the elbow. The Palmaris longus is a tensor of the palmar 

 fascia. It also assists in flexing the wrist and elbow. The Flexor sublimis 



