320 THE MUSCLES 



extreme obliquity of itj^ insertion. Aftt-r liending the third phalanx, the muscle 

 will assist in the Hexion of the other })halanf!;es and the wrist. 



Relations. — Superticially, the flexor sublimis digitorum and flexor carpi 

 ulnaris, the ulnar vessels, the median and ulnar nerves; deeply, the pronator quad- 

 ratus and wrist-joint. In the hand, it lies beneath the tendons of the flexor sub- 

 limis digitorum and the luml^ricales, and upon the adductor of the thumb, the 

 interossei muscles, and the deep palmar arch. 



Variations. — The flexor profundus digitorum frequentlj' receives, like the flexor longus 

 pollie-is, a slip from the superficial muscles of the forearm. Sometimes the part which goes to 

 the index finger may take part of its origin from the radius; and occasionallj' slips may pass from 

 the flexor profundus to the flexor longus pollicis, or vice versa. A slip has been observed to end 

 in the synovial membrane of the palmar bursa. 



2. FLEXOR LONGUS POLLICIS 



The flexor longus pollicis — named from its action upon the thumb and its 

 length as compared with the short muscles of the ball of the thumb — is a fusiform 

 sheet. 



Origin. — (1) The anterior surface of the radius below the oblique line, with the 

 exception of the last two inches (5 cm.) of the surface; and the adjacent surface 

 of the interosseous membrane. (2) A second head arises from the inner border 

 of the coronoid process of the ulna, but occasionally it may come from the internal 

 condyle of the humerus. 



Insertion. — The front of the base of the last phalanx of the thumb. 



Structure. — A penniform muscle arising fleshy from the bone and the inter- 

 osseous membrane. The tendon first appears upon the anterior surface near its 

 ulnar border about the middle of the forearai. It receives the fleshy fibres upon 

 its outer border and posterior surface and becomes free at the level of the wrist- 

 joint, where it enters the canal formed by the anterior annular ligament and the 

 front of the carpus. It is here invested by a special compartment of the great 

 synovial bursa, and this compartment is continuous with the synovial sheath of 

 the tendon as it lies in the thumb. After entering the palm of the hand, the 

 tendon passes beneath the outer head of the flexor brevis pollicis, then through the 

 groove formed by the two sesamoid bones which belong to the tendons of this 

 muscle, and after lying in close contact with the concave palmar surface of the first 

 phalanx of the thumb, it is inserted into the front of the base of the second 

 phalanx. 



Nerve-supply. — From the outer and inner cords of the brachial plexus 

 (through the eighth cervical and first thoracic nerves) by means of the anterior 

 interosseous Ijranch of the median nerve, which sends filaments to it upon its 

 anterior surface near its ulnar border about the middle of the forearm. 



Action. — It is a powerful flexor of the last phalanx of the thumb. It Avill also 

 flex its metacar})o-phalangeal joint, and afterwards it will assist the other flexors of 

 the wrist. 



As the bones of the thumb are not in the same plane with those of the fingers, 

 but rotated so that their palmar surface looks towards the rest of the hand, their 

 flexion will be accompanied by adduction; as when the thumb meets the other 

 fingers in ])icking up any small object, or in grasping anything between the fingers 

 and thunil). 



Relations. — Su]K'rficially, the flexor sublimis digitorum, flexor carpi radialis, 

 bracliio-radialis (supinator longus), and the radial vessels; deeply the jironator 

 (piadratus and wrist-joint. In the hand, after passing beneath the anterior annular 

 ligament, it is covered l>y the o])])onens pollicis and the outer head of the flexor 

 brevis pollicis, and it lit-s on the inner head of the flexor brevis pollicis. 



Variations.— Besides the communication with the flexor profundus digitonim, we occasionally 

 find shps passing from the tendon of tlie flexor longus i)olhcis to join the first lumbricalis muscle. 

 The second head may be absent, as in fig. 275. 



