MUSCLES AND FASCIA OF UPPER EXTREMITY 153 



(3) anterior oblique line of the radius; divided below 

 into four parts, ending in tendons inserted into the 

 second phalanges of the four inner digits. 



THE DEEP MUSCLES. Flexor profundus digitorum. 

 Origin, the upper three-fourths of the inner and 

 anterior surface of the ulna, from not quite the ulnar 

 half of the interosseous membrane for the same dis- 

 tance, and from an aponeurosis attached to the pos- 

 terior border of the ulna, common to it, the flexor and 

 extensor carpi ulnaris. Only one tendon (for the index 

 finger) separates above the wrist; in the palm, as the 

 tendons diverge, they give origin to the lumbricales; 

 over the first and second phalanges the tendon is 

 bound down by an osseo-aponeurotic sheath, and oppo- 

 site the first phalanx it passes through an opening in 

 the flexor sublimis tendon, and is finally inserted by an 

 expanded end into the base of the third or last phalanx. 



The sheaths of the flexor tendons are opposite the 

 first and middle phalanges, and formed of strong 

 transverse bands, ligamenta vaginalia; op'posite the 

 joints the bands change into a thin membrane, strength- 

 ened by oblique decussating fibers, so that there are 

 annular or transverse fibers, and crucial or oblique. 

 The sheath has a synovial lining containing small 

 folds, vincula tendinum or ligamenta mucosa, passing 

 between the tendons and bones. 



Flexor longus pollicis arises from the anterior surface 

 of the radius, below its oblique line to the edge of the 

 pronator quadratus, and from the adjacent part 

 of the interosseous membrane. The tendon passes 

 between the sesamoid bones of the thumb and enters 

 a canal similar to that of the other flexors, to be 

 inserted into the base of the last phalanx of the 

 thumb. Its complete separation from the flexor pro- 

 fundus is characteristic of man. 



Pronator quadratus, just above the wrist, close to 

 the bones behind the last two muscles, quadrilateral 

 and flat, arises from the pronator ridge and inner part 



