Warren Harmon Lewis 167 



the forearm and hand. It is wider in extent than the superficial and 

 shows indications of separations into muscles. Tlie portion for the 

 flexor carpi nlnaris shows most advance. The extension into the hand 

 probably constitutes the portion from which the interossei and lumbrical 

 muscles and flexor tendons develop. It is continuous with the con- 

 densed tissue of the digits. The portion on the forearm forms the flexor 

 profundus digitorum, flexor pollicis longus, flexor carpi ulnaris and 

 pronator quadratus muscles. Both the ulnar and median nerves supply 

 the deep layer. 



The extensor mass of the forearm is farther advanced than the flexor. 

 It can be differentiated into three groups of muscles which accord well 

 with the adult groups. The first group, the largest and most superficial, 

 extends from the lateral condyle to the proximal ends of the digits, 

 where it blends with the condensed mesenchyma. It is a thin layer 

 and spreads out over the ulnar two-thirds of the forearm and is quite 

 closely applied to the perichondrium and cpndensed mesenchyma of the 

 skeletal structures beneath. A portion of it overlaps the second and 

 third groups. It is the still undifferentiated extensor communis digi- 

 torum, extensor carpi ulnaris, and extensor minimi digiti. It is supplied 

 by branches of the posterior interosseus nerve. 



The second group occupies the proximal portion of the radial side of 

 the forearm. It arises in connection with the first group from the 

 external condyle and adjoining portion of the humerus. The muscle 

 mass passes distally along the radius and soon divides into two parts 

 between which the radial nerve passes. The radial part fuses with 

 the condensed tissue of the distal end of the radius. It is the 

 brachioradialis muscle. The second part passes beneath the third group 

 and fuses with the condensed mesenchyma at the proximal ends of the 

 second and third digits. It is the extensor carpi radialis longior et hrevior 

 muscle. Branches of the musculospiral nerve supply this second group. 



The third group arises beneath the first from the ulna and radius. 

 Its fibers pass toward the radial side of the forearm, passing from 

 beneath the first group and over the second group, and finally end in 

 the condensed tissue of the first and second digits. The portion to 

 the second digit is closely fused with the portion of the first group 

 which goes to this digit. This group is quite closely applied to the 

 underlying skeletal condensed tissue. The third group represents the 

 abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus 

 and extensor indicis proprius. Branches of the musculospiral nerve 

 supply this group. 



The supinator I believe must arise in connection with the third group, 

 judging from its position and the direction 'of its fibers. 



