318 THE MUSCLES 



the seventh and eighth cervical and first thoracic nerves) bv branches from the 

 median nerve which enter the deej) surface of the muscle at its upper part. 



Action. — To fiex the second })halanges of the four fingers. Being inserted very 

 oV)H(iuely, it acts under a consideral^le mechanical disadvantage, Init at the same 

 time tlie speed and range of movement corresjionding to a slight contraction of the 

 nuiscle are very great. The size of the angle -which it makes Avith the shaft of the 

 phalanx at its point of insertion is somewhat increased by the raising of its tendon 

 from the i)ahnar aspect of the first i)halanx l:)y the tendon of the deep flexor. 

 After Ijending the second phahmx of the finger, it will bend also the metacarpo- 

 phalangeal joint; then the three joints which together produce flexion of the wrist; 

 itnd finally it will feebly assist in the flexion of the elbow-joint. 



Relations. — Superficially, the four muscles of the first sheet and the radial 

 vessels and nerves; deeply, the flexor longus pollicis, flexor profundus digitorum, 

 and pronator quadratus,the ulnar artery and veins, and the median nerve. In the 

 hand, it lies beneath the anterior annular ligament, the palmar fascia, and the 

 superficial palmar arch, and ujion the tendons of the flexor profundus digitorum 

 with the lumbricales. 



Variations. — The flexor sublimis digitorum varies very little in its origin. It has been seen 

 to arise partly from the pronator radii teres. Occasiouall}' its tendon to the little finger fails, and 

 the place of this tendon may be taken by a lumbriealis or a special slip from the flexor profundus. 

 Sometimes accessory heads join the tendons in the hand from the flexor proi'undus or the annular 

 ligament. 



Third Layer 



The third layer consists of two muscles — the flexor profundus digitorum and the 

 flexor longus pollicis — which arise from the ulna and radius respectively. 



1. FLEXOR PROFUNDUS DIGITORUM 



The flexor profundus digitorum — named from its action upon the fingers and 

 Its relation to their superficial flexor — is a strong fusiform sheet of muscular fibres 

 Avhich Avraps round the anterior and inner surfaces of the ulna, and divides below 

 into four tendons for the fingers. 



Origin. — (1) The upper three-fourths of the anterior surface of the ulna; and 

 (2) the adjacent part of the interosseous membrane; (3) the upper two-thirds of 

 the inner surface of the ulna; and (4) the posterior ridge of that bone by the apo- 

 neurosis common to it, the flexor car})i ulnaris, and the extensor carpi ulnaris. 



Insertion. — The front of the bases of the third phalanges of the four fingers. 



Structure. — Arising directly from the bones and fibrous structures which form 

 the origin of the muscle, the fleshy fibres converge below upon the back of four 

 tendons, w^hich, appearing about the middle of the forearm, become free at the 

 upper border of the anterior annular ligament. That part of the muscle which is 

 inserted into the index finger is separable from the rest of the muscle in nearly the 

 whole of its extent; the part to the little finger is also generally more separalde than 

 that to the other two fingers. As they pass lieneath the anterior annular ligament, 

 the tendons are invested l)v the common synovial sheath. At the metacarpo- 

 phalangeal joints they enter the vaginal sheaths belonging to their respective 

 fingers. < )iip<»site the first phalangeal joint each tendon i)asscs through the open- 

 ing formed by the si)litting of the flexor sublimis tendon; and on the palmar aspect 

 of the second i)halanx it passes over a slight elevation formed by the union of the 

 two halves of the companion tendon before they are inserted into the sides of the 

 second phalanx. The deep tendon finally ])asses over the second ])halangeal joint, 

 to be inserted into the front of the base of the third phalanx. In the pahn of the 

 hand the lumbricales (which will afterwards bi> described) arise from the tendons 

 of the flexor ])rnfnndus. 



Nerve-supply. — By two sources from the l)rachial ])lexus (through the eighth 

 cervical and first thoracic nerves): — (1) By the anterior interosseous branch of 

 the median nerve, which sends a filament to" the portion of the muscle belonging 



