THE ANTIBRACHIAL MUSCLES. 595 



Relations. — By its deep surface this muscle is in relation with the sublimis and 

 profundus digitorum and with the ulnar vessels and nerve. The ulnar nerve and 

 posterior recurrent ulnar artery pass beneath a tendinous band which stretches across 

 between the two heads of the muscle, and towards the wrist the ulnar artery comes 

 to lie along the lateral border of the tendon. A mucous bursa (bursa m. flexoris 

 carpi ulnaris) is frequently to be found between the tendon and the upper part of the 

 pisiform bone. 



Variations. — The flexor carpi ulnaris frequently passes distally to be inserted into the base 

 of the fifth metacarpal. The conversion of the ulnar head into connective tissue has been 

 observed. 



5. Flexor Sublimis Digitorum (Fig. 577). 



Attachments. — The superficial flexor (m. flexor digitorum sublimis) arises from 

 the inner condyle of the humerus in common with the neighboring superficial mus- 

 cles, from an oblique line on the anterior surface of the radius, and from the tendi- 

 nous arch extending between these two bony points and beneath which the median 

 nerve and ulnar artery pass. The fibres arising from these origins form four bellies, 

 prolonged below into as many tendons, which at the wrist pass beneath the ante- 

 rior annular ligament and then diverge towards the bases of <Jie second, third, fourth, 

 and fifth fingers and enter the corresponding digital sheaths. Here each tendon 

 divides over the surface of the first phalanx into two slips, which pass one on either 

 side of the subjacent tendon of the flexor profundus digitorum and partially unite 

 beneath it to be inserted into the base of the second phalanx. Slight tendinous 

 bands, vincida tendimim, pass between the tendons of the profundus and the terminal 

 portions of those of the sublimis. 



Nerve-Supply. — By the median nerve from the seventh and eighth cervical 

 and first thoracic nerves. 



Action. — Primarily to flex the second phalanx of the four medial digits, but a 

 continuation of its action will flex the first phalanges of the same digits and eventually 

 the hand. 



Relations. — Superficially the flexor sublimis is covered by the remaining 

 muscles of the superficial layer ; deeply it is in relation with the flexor profundus 

 digitorum, the flexor longus poUicis, the ulnar vessels, and the median nerve. 



Variations. — Occasionally the portion of the muscle which gives rise to the tendon of the 

 fifth digit appears to be wanting, the tendon arising from the palmar fascia, the anterior annular 

 ligament, or the flexor profundus. An explanation of this anomaly is found in the developmental 

 history of the muscle, hi the lower vertebrates the superficial flexor inserts into the palmar 

 fascia, which gives origin to a set of superficial digital muscles whose relations are similar to 

 those of the digital portions of the sublimis tendons, hi the mammalia these digital muscles de- 

 generate into tendinous bands, with which the tendon of the antibrachial portion of the muscle 

 becomes continuous. The origin of the tendon for the fifth digit from the palmar aponeurosis 

 or transverse carpal ligament is, therefore, a persistence of a phyletic stage, as is also its origin 

 from the flexor profundus, since in the lower mammals the antibrachial portions of the two 

 muscles are united to form a single mass (page 597). 



{bb) The Middle Layer. 

 I. Flexor profundus digitorum. 2. Flexor longus poUicis. 



I. Flexor Profundus Digitorum (Fig. 578). 



Attachments. — The deep flexor (m. flexor digitorum profundus) arises from the 

 anterior and outer surfaces of the ulna and from the inner half of the interosseous 

 membrane. Its fibres are directed downward, and at about the middle of the fore- 

 arm are continued into four tendons, which pass beneath the anterior annular liga- 

 ment alone with the tendons of the flexor sublimis to enter the digital sheaths of the 

 second, third, fourth, and fifth fingers. Opposite the first phalangeal joint each 

 tendon passes between the two slips of the corresponding tendon of the flexor sub- 

 limis and is inserted into the base of the terminal phalanx. 



Nerve-Supply. — The lateral half of the muscle is supplied by branches from 

 the anterior interosseous branch of the median nerve and the medial half by the 



