l74 The Development of the Arm in Man 



It is very broad and spreads out over the middle of the forearm and 

 carpus, where it divides into fonr broad, thin tendons which fnse with 

 the condensed tissue surrounding the distal end of the four ulnar meta- 

 carpals and first row of phalanges. The muscle fibers continue distal 

 as far as the middle of the carpus, where the muscle becomes wider 

 and thicker. The tendons do not show the split which is later to 

 appear and enclose the deep flexor tendon. The strongest part of the 

 tendons lie on the ulnar side of digits. 



The deep layer of the preceding stage has undergone marked changes. 



The flexor carpi ulnaris muscle is quite distinct. It arises partly 

 from the internal condyle superficial to the sublimis and closely con- 

 nected with it and the palmaris longus and partly from the ulna. The 

 muscle at its origin is broad and thin but narrows into a condensed 

 tissue tendon which is inserted into the os pisiform. 



The flexor digitorum profundus and the flexor polUcis longus muscles 

 arise from the surfaces of the radius and ulna and the internal condyle. 

 They are closely imited and pass to the carpal region where division 

 takes place into five well-formed oval tendons, which pass beneath the 

 tendons of the sublimis, and fuse with the condensed tissue about the 

 ends of the digits. 



The pronator quadraius muscle is a small, oval mass connecting the 

 distal ends of the ulna and radius. 



The lumhride muscles are fonned. They arise from the profundus 

 near the angles formed by the iive tendons. They are short and contain 

 distinct muscle fibers which end in tendons that fuse with the con- 

 densed tissue on the radial side of the ulnar four digits. 



The intrinsic muscles of the hand, the interossei, and muscles of the 

 thumb and little finger, are represented by a late premuscle tissue in 

 which a few muscle fibers are beginning to appear. These masses are 

 more or less continuous with each other and lie on the palmar surface 

 of the carpus and metacarpus and partially in between the latter. The 

 distal ends of these masses fuse with the less differentiated condensed 

 tissue about the digits. 



The extensor muscles of the forearm show considerable advance over 

 the preceding stage, but the development does not seem to have been 

 as rapid as in the case of the flexor muscles. 



Of the first group, the extensor communis digitorum and the extensor 

 minimi digiti are united into a broad, thin sheet which divides in the 

 metacarpal region into four broad, thin tendons that end in the con- 

 densed tissue of the four ulnar digits. The extensor carpi ulnaris closely 

 associated with this muscle at its origin from the external condyle arises 



