Warren Harmon Lewis 173 



also partly from the ulna and is inserted into the condensed tissue at the 

 proximal end of the fifth metacarpal. It is quite separate from the 

 common extensor for the greater part of its length. 



Of the second group, the hracMoradinlis is quite distinct from the 

 extensor- carpi radialis longior et hrevior for most of its length, but at 

 their origin, however, the two are closely connected. Both muscles are 

 broader and larger than in the preceding stage. The extensor passes 

 beneath the third gTOup and ends in the condensed tissue near the 

 proximal ends of the second and third metacarpals. 



The tJiird group, which arises beneath the first from both radius and 

 ulna, has split more or less into four parts. The proximal one, which is 

 the most completely separated, is the supinator and passes from the ulna 

 and external condyle to the radius. It is united with rest of this group 

 along their ulnar origins, forming thus a continuous sheet for a short 

 distance. The next two pass over the extensor carpi radii tendon, and 

 fuse with the condensed tissue of the first digit. They are the abductor 

 pollicis longits, extensor poUicis brevis and the extensor poUicis longus 

 muscles. The fourth division is broad and thin and soon joins the 

 deep surface of the tendon of the extensor communis and goes with it 

 to be inserted into the condensed tissue of the second digit. 



The Kerves. — By the migration of the arm posteriorly the brachial 

 plexus has been pulled caudally and given a decided posterior inclin- 

 ation. It has also divided into the various cords more than in the pre- 

 ceding stage. 



The distribution of the muscle and cutaneous nerves is much as in. 

 the adult and as in the next stage. 



Embryo XXII. 



Embryo XXII measures 20 mm. V. B. and 18 mm. X. B. It is about 

 seven weeks old. The entire arm has a more posterior position. The 

 lower angle of the scapula is at the level of the sixth rib, its anterior 

 limit is about at the seventh cervical vertebra. The entire arm as well 

 as its various parts have increased in size. The muscles are sharper and 

 better developed than the preceding stage. Every muscle that the 

 adult arm presents can now be recognized and each one now contains 

 muscle fibers. The tendons are better formed and can be traced farther 

 towards their final insertions. The ligaments and fasciae are also more 

 distinct. The process of ligament and tendon formation from the con- 

 densed mesenchyma is still in progress at the distal ends of the digits. 

 The skeletal elements are for the most part fairly well formed in car- 

 tilage except the distal row of phalanges. 



