180 



The Development of the Arm in Man 



The interossei mnscles and the small mnscles of the thumb and little 

 finger are now fairly well developed. Muscle fibers are present. 



The extensor muscles of the forearm show considerable advance over 

 the preceding stage. The tendons of the extensor communis digitoriim 

 are longer and narrower. The muscle fibers continue to the base of 

 the metacarpus, where the splitting into the four tendons takes place. 

 The tendons are inserted into the condensed tissue tips of the digits. 

 The edge of the tendons near their insertions are more or less contin- 

 uous with the perichondrium about the digit. 



The tendon of the extensor carpi ulnaris is beginning to. form. One 

 branch of it seems to join the communis tendon. This may be the 

 tendon of the extensor minimi digiti. 



briich,ora,lml,s /and .■xl! indf 



Fig. 1-t. Lateral view of the arm of embrj'o XXII. from Plate VIII. 

 Bardeen and Lewis, Vol. I, this Journal. X 12 dia. 



The extensor carpi radialis longior et hrevior are not to be separated. 



The supinator muscle is well developed and has the posterior inter- 

 osseus nerve passing through it. 



The abductor pollicis longus and extensor pollicis brevis muscles are 

 only to be separated where the muscle fibres pass into tendons, which 

 fuse with the perichondrium of the first digit. The separation occurs at 

 the lower end of the radius. These two muscles are fairly distinct from 

 the supinator and the extensor pollicis longus and extensor indicis pro- 

 jiriiis muscles. The last two muscles are inseparable for part of their 

 course and shortly after dividing each forms a round tendon. The ex- 

 tensor pollicis longus then spreads out into the sheath about the first 



