ANATOMY OF A MONODACTYLOUS FOETUS 421 



did not fail when the uhia disappeared. The same is true of 

 the ulnar members of this group, the extensor pollicis longus, 

 and extensor indicis proprius. All this group have been obliter- 

 ated by a specific suppressing agent during myogenesis. 



In the flexor muscles it seems strange that the pronator teres 

 is not present when so many of the other muscles are. Its com- 

 plete absence has never been noted as an anomaly although its 

 coronoid head has often been lacking. In lower vertebrates this 

 muscle is a part of a common muscular layer known as the pro- 

 natoflexor mass. In this foetus it may be present in the super- 

 ficial layer, included with the mass of the flexor carpi radialis, 

 having failed to obtain an insertion at the usual level on the 

 radius. 



It is interesting to note that in this foetus a definite tendency 

 in one direction is showai by all muscles, which are properly 

 developed and which show anomalies. This tendency, for in- 

 stance is shown by all the muscles on the front of the upper arm 

 and is a regression or atavistic change, the anomalies resembling 

 normal muscles of the low^er animals. Changes due wholly to 

 loss of normal skeletal parts lead to anomalous attachments 

 which of course cannot be properly included in this class as they 

 are in the nature of monstrosities. 



The question naturally arises as to what single digit it is that 

 has persisted in this hand, and also what carpal bones are present. 



It may be taken as a plausible working hypothesis that with 

 loss of the ulna would be associated loss of the ulnar side of the 

 carpus, with the fourth and fifth digits. 



This hypothesis is supported by the fact that the main cu- 

 taneous digital nerves ventrally are two strong branches from 

 the median while dorsally the radial reaches the base of the 

 digit. The ulnar nerve has no digital distribution, and as it 

 normally goes to the fourth and fifth digits while the median and 

 radial supply the other three, the digit here present certainly 

 ought to be one of the three on the radial side of the hand. 



This would leave three digits still to decide between. This 

 number can be further reduced to two as the thumb is certainly 

 absent, for the persistent digit has a metacarpal and three pha- 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 22, NO. 3 



