chap, xiil] DIGITS : MAN. 325 



The number of phalanges in the digits in these Conditions 

 may be represented thus. The || marks the metacarpal space. 

 (The hand is supposed to be a right.) 



Distinct from these Conditions are the states sometimes 

 described as " double-hand." In the full form of this there are 

 eight digits, each of three phalanges. The eight digits are 

 arranged in two groups, four in each group. The two groups 

 stand as a complementary pair, the one being the optical image 

 of the other ; or in other words, the one group is right and the 

 other is left. 



Besides the double-hand with eight digits there are also forms 

 of double-hand with six digits, arranged in two groups of three and 

 three. 



Lastly, there are cases of double-hand having seven fingers, 

 an external group of four and an internal group of three. Thus 

 expressed these cases seem to come very near that mentioned as 

 a variant on Condition IV, but in one and perhaps both of these 

 double-hands there was in the structure of the fore- arm and 

 carpus a great difference from that found in the only recorded 

 skeleton of Condition IV. 



At first sight it would naturally be supposed that these double- 

 hands in one or all kinds stand to the other Conditions in the 

 some relation that Condition IV of the pes in the Cat does to the 

 other polydactyle conditions in the Cat. But the matter is 

 complicated by the fact that the evidence goes to shew that in the 

 human double-hands the bones of the arm and carpus may be 

 modified, and in Dwight's example of seven digits (No. 489) at all 

 events, and perhaps in other double-hands, an ulna-like bone takes 

 the place of the radius, or in other words, the internal side of the 

 fore-arm is fashioned like the external side. In the polydactyle cats 

 the bones of the fore-arm were normal, as are they also substantially 

 in cases of the human Conditions III and IV, which have been 

 dissected. Further, in some of the human cases of eight digits 

 the abnormality was confined to one hand, which is never the case 

 in the higher condition of polydactylism in the Cat, so far as I 

 know. These circumstances make it necessary to recognize the 

 possibility that some at least of the human double-hands are 

 of a different nature from the lower forms of polydactylism. 

 This subject will be spoken of again after the evidence as to the 

 variation of digits has been given (Chap. xiv. Section (4).) 



