402 THE MUSCULATURE 



Variations. — There is considerable variation in the extent of the radial origin and in the 

 extent of the independence and fusion of the different fasciculi. In the prosimians a common 

 tendon extends as far as the hand. The division in the higher forms is associated with refine- 

 ment of movements of the fingers. One or more special fasciculi not infrequently join the 

 muscle from the flexor sublimis, the flexor poUicis longus, the medial epicondyle, or the ulna. 

 The accessorius ad flexorem digitorum profundum is a fasciculus which arises from the coro- 

 noid process of the ulna and sends a tendon to join the tendon of one of the fingers, most fre- 

 quently the middle or index. It is found in 20 per cent, of bodies. 



The flexor pollicis longus (fig. 372). — Origin. — The attachment extends along the oblique 

 hne and the ventral border of the radius from slightly below the bicipital tuberosity to within 

 5 cm. of the wrist. ]\Iedially it is continued into the interosseous membrane. Proximally the 

 tendon frequently extends to the distal radial margin of the coronoid process of the ulna and 

 gives rise to fibre-bundles connected with the muscle, as well as to a fasciculus of the flexor 

 profundus. 



Structure and insertion. — The fibre-bundles descend obliquely to be inserted in a penni- 

 form manner on a tendon which begins high up on the volar surface near the ulnar border 

 of the muscle, and descends as a broad band which near the wrist becomes cylindroid. The 

 insertion of fibres continues nearly to the point where the tendon passes under the transverse 

 carpal ligament. Here the tendon enters the carpal canal radial to the tendons of the flexor 

 profundus, and passes beneath the superficial head of the short flexor of the thumb, then between 

 the thumb sesamoids into the osteo-fibrous canal of the thumb, in which it is continued to the 

 base of the terminal phalanx. 



Nerve-supply. — Usually from two branches of the volar interosseous ramus of the median 

 nerve. These enter the proximal half of the ulnar margin of the muscle. The nerve fibres 

 arise from the sixth, seventh (and eighth) cervical nerves. 



Action. — It is a strong flexor of the second phalanx on the first and has less powerful action 

 on the metacarpo-phalangeal joint and on the wrist. 



Relations. — It lies beneath the flexor sublimis, the flexor carpi radialis and brachio-radialis 

 muscles, and the radial artery. Near the wrist it crosses over the insertion of the pronator 

 quadratus. In the hand the tendon runs beneath the opponens poUicis and the superficial head 

 of the flexor brevis, and across the deep head of the latter. 



Variations. — It may be fused or united by fasciculi with the flexor profundus, the flexor 

 sublimis, or the pronator teres. It may be partially doubled, giving rise to an accessory ten- 

 don which extends to the index finger. The origin may extend to the medial epicondyle of 

 the hum.erus (epitrochlear bundle). 



d. Fourth Layer 



This layer consists of a single quadrilateral muscle which passes transversely 

 across the lower part of the forearm from the ulna to the radius. In the leg there 

 is no corresponding muscle. 



The pronator quadratus (fig. 377). — Origin. — Medial side of the volar surface of the lower 

 fourth of the ulna. 



Structure and insertion. — From the ulna a strong aponeurosis extends a third of the way 

 across the volar surface of the muscle. From this membrane and from the bone fibre-bundles 

 extend transversely to be inserted on the distal quarter of the volar surface of the radius and on 

 the triangular area above the ulnar notch. The deeper fibre-bundles which arise directly from 

 the ulna are inserted into the radius by means of an aponeurosis. The superficial and deep 

 portions of the muscle are often separated. The muscle is thicker distally than proximally. 



Nerve-supply. — The volar interosseous nerve descends along the interosseous membrane, 

 passes behind the middle of the proximal margin of the muscle, and sends branches into its 

 deep surface. The nerve fibres arise from the (sixth), seventh and eighth cervical and first 

 thoracic nerves. 



Action. — To pronate the forearm. 



Relations. — The muscle lies immediately beneath the muscles of the third layer and upon 

 the radius and ulna, the interosseous membrane, and radio-ulnar joint. The radial artery and 

 ulnar nerve pass in front of it, the volar interosseous artery behind it. 



Variations. — It may be missing or may extend furtlicr up the forearm than usual or down 

 upon the carpus. It may be triangular or divided into parts the fibre-bundles of which take 

 different directions. It may send fasciculi to the carpus or metacarpus or be fused with the 

 flexor carpi radialis brevis (see below). 



Abnormal Muscles of the Volar Side of the Forearm and Wrist 



The epitrochleo-olecranonis (anconeus internus). — A muscle fasciculus, distinct from 

 the distal margin oi the triceps, which runs from the medial epicondyle to the olecranon over 

 the groove for the uhiar nerve, by a branch of wliich it is sui)plicd. It takes the place of the 

 fibrous arch normally extending between tJic cijicondylar and ulnar heads of the flexor carpi 

 ulnaris. It occurs in about 2/3 jjer cent, of bodies (Testut), and represents an adductor of the 

 olecranon of the lower nianimals. Occasionally the medial head of the triceps may descend 

 over the ulnar groove, but tliis forms another type of mus(fle variation. 



The flexor carpi ulnaris brevis (uino-carpeus). — An abnornuil muscle which arises from 

 the distal quarter of the volar surface of the ulna and is inserted into the hamatum (unciform), 

 the pisiform, the abductor of the little finger, or the superior extremity of the fifth metacarpal. 



