TWINNING (DUPLICITY) IN LIMBS 201 



finger, though a little larger. It has three joints like 

 the little finger and takes its origin from the palm at 

 the same level as the latter (Fig. 68, p. 202). 



2. We next have an instance in which the physio- 

 logical isolation of the two sides of the hand was more 

 pronounced, so that partial twinning has occurred. This 

 is a case of a woman who had six digits on each hand 

 and foot. In each hand the thumb has three joints like 

 a little finger. There are, however, some irregularities 

 that complicate the case. 



3. Another case is cited of a hand with six digits 

 arranged in two groups that were somewhat opposable 

 to each other (Fig. 69). Digits II, III, IV, V stand in 

 normal position as the ulnar set, while the radial group 

 consists of two normal fingers, each with three joints, 

 and neither one thumblike. Here we have partial 

 twinning and partial mirror-imaging, but the radial side 

 is still somewhat inferior. This case seems to indicate 

 that in a hand of normal type the thumb stands over 

 against the rest of the fingers as a reduced or inferior 

 half of the appendage. The physiological isolation of 

 the weaker from the stronger side allows the thumb 

 side to become more nearly equal to the little-finger 

 side. 



4. A still more nearly double hand is one described 

 from a specimen in the Harvard Medical School Museum. 

 The forearm consists of two ulna bones instead of a radius 

 and an ulna. In other words, the superior of these twin 

 bones is repeated on both sides. The hand of this 

 double arm consists of seven digits in two groups, an 

 ulnar group of four very normal digits and a radial or 

 thumb group of three nearly normal fingers (Fig. 70) , 



