TQ4 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 277. — The deep layer of the muscles of the flexor surface of the forearm after removal of the 

 superficial layer, seen from in front. 



Fig. 278. — The muscles of the forearm seen from the radial side. 



middle and lower thirds of the bone, and from the adjacent portion of the interosseous membrane. 

 Four parallel tendons are soon given off, the one situated nearest to the radial side coming from 

 a separate muscular belly composed of the fibers proceeding from the interosseous membrane. 

 These tendons run in the same synovial sheath as do those of the flexor sublimis and pass beneath 

 the transverse carpal ligament (the anterior annular ligament) to terminate upon the ungual 

 phalanges of the second to the fifth fingers (see page 205). 



The ulnar half of the muscle is supplied by the ulnar nerve, the radial half by the median ne've. It flexes the 

 terminal phalanges of the four fingers. 



The flexor pollicis longus (Figs. 275 to 277) is a spindle-shaped muscle placed imme- 

 diately alongside of the flexor digitorum profundis; it is penniform above and semipenniform 

 below. It arises from the volar surface of the radius between the insertion of the supinator 

 and the upper border of the pronator quadratus. It also usually receives a slender fasciculus, 

 frequently aponeurotic, from the coronoid process and from the internal condyle; this origin, 

 however, which is known as the humeral head, does not come directly from the bone but from the 

 muscular mass of the superficial flexors. The muscle becomes tendinous almost immediately 

 below its origin, the upper portion receiving the muscular fibers from either side, the lower portion 

 from trie radial side only, and the tendon runs in its own tendon-sheath beneath the transverse 

 carpal ligament and passes between the two heads of the flexor pollicis brevis to the ungual 

 phalrnx of the thumb. 



The muscle is supplied by the median nerve. It flexes the terminal phalanx of the thumb. 



a!^ -onator quadratus (Figs. 277 and 284) is a flat quadrilateral muscle which is con- 

 cealed by all the tendons of the flexor muscles and lies upon the volar surfaces of both bones of 

 the forea m toward their distal extremities. It arises from the volar border of the ulna and 

 is inserted into the volar surface and border of the radius, both its origin and insertion being 

 usually by short aponeuroses. 



The muscle is supplied by the median nerve (volar interosseous nerve) and pronates the forearm. 



THE RADIAL GROUP OF THE MUSCLES OF THE FOREARM. 



The three muscles of the radial group are placed at the radial side of the forearm and of 

 the lower portion of the arm in the so-called radial region, between the flexors and the extensors. 

 The brachioradialis belongs more to the flexor surface, but the other two muscles are upon the 

 extensor side of the forearm, and while the superficial layers of both flexor and extensor muscles are 

 adherent to the fascia of the forearm, the extensor carpi radialis brevis is the only muscle of the 

 radial group in which a similar relation obtains. 



The brachioradialis {supinator longus) (Figs. 270, 271, and 274 to 279) is a very long flat 



