420 JAMES CRAWFORD WATT 



demonstrated the presence of a myoplasm or muscle forming 

 substance. 



In the forearm the extensor and supinator group differentiate 

 before the flexor and pronator set. As the muscle formation 

 follows closely upon the definition of the skeleton, if the growth 

 suppressing influence which acted on the skeleton lasted long 

 enough to influence the muscles it is to be expected that the exten- 

 sor group would exhibit the greatest amount of damage. Such 

 is actually the case. Only four extensor muscles are present as 

 against seven flexors and pronators plus one palmar muscle. 

 Only one extensor muscle reaches as far as the lower end of the 

 radius, nearly all the flexors reach that level. No extensor ten- 

 don reaches the digit, a flexor tendon passes right out to the 

 terminal phalanx, in addition to bearing a lumbrical muscle to 

 the digit. It is to be noted that in the members of the extensor 

 group here present the muscle masses are of about normal pro- 

 portion, covering half of the length of the radius but in only one 

 case is a long tendon developed, the other muscles inserting at 

 once on the middle of the shaft of the radius. This failure of 

 the long tendons to differentiate out after the appearance of 

 these muscles is a further example of the greater suppression of 

 growth in this region. Grafenberg ('11) describes the muscula- 

 ture in a case of absence of the radius and the thumb. Here 

 the radial musculature is present as a common mass high up in 

 the forearm, possessing no tendons, and so appearing very much 

 like the extensor muscles I have described. The other muscles 

 both flexors and extensors, in Griifenberg's case are present and 

 normal in extent. 



Regarding the muscle that I have called the common super- 

 ficial extensor mass, as separation into separate portions begins 

 at the carpus after the appearance of the tendons, it is not pos- 

 sible here to have such a division into its component muscles, 

 because its tendon is entirely absent. 



Absence of the thumb is not enough to cause disappearance of 

 the abductor pollicis longus and extensor pollicis brevis, the 

 radial members of the deep extensors, for there is still oppor- 

 tunity for the muscles to develop over the radius. The triceps 



