PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 291 



cular attachments and the innervation of the extra digit are entirely- 

 distinct from those supplying and innervating digit m. We can only 

 conclude, therefore, that in these cases the supernumerary digit is not a 

 duplication of digit ii. If it were such a duplication, why should not 

 the fifth digit be affected as often as the second 1 On the contrary, in 

 every polydactyle manus so far observed the siipernumei'ary digit is found 

 on the radial side of digit ii. 



There is no doubt that abnormalities due to the duplication of a func- 

 tional digit may occur in the manus of the pig as in other mammals; 

 but in the majority of cases the origin of the extra digit must be vestigial. 

 By variation and duplication of this vestige in its development, two or 

 more supernumerary digits may be formed. Whether or not the develop- 

 ment of this digital vestige is due to reversion, we will discuss in the 

 theoretical portion of this paper. 



Summing up the facts obtained as to polydactylism in the pig, it is 

 found that — 



1. Polydactylism is confined almost entirely to the manus. (This fact 

 is interesting, as the condition restores that found in fossil swine. In the 

 pes of Ancodus the hallux is entirely gone, although in the manus the • 

 pollex is well developed. If we regard the extra digit as due to duplica- 

 tion of digit II we should expect this duplication to occur as often in 

 the pes as in the manus; but if the extra digit is vestigial in its origin, 

 the early and complete reduction of the hallux in fossil swine is good 

 reason for its never being developed in the pig of the present day.) 



2. The supernumerary digits in every case occur on the radial side 

 of the second normal digit. 



3. In nineteen of the thirty-six cases examined, a single super- 

 numeraiy digit is present ; in five instances this digit is composed of two 

 phalanges ; in nine cases, of three ; and in five instances its metacarpal 

 is fused to that of digit ii. 



4. In the remaining seventeen specimens thirteen are hexadactyle, 

 although in three cases the metacarpal of one supernumerary digit (i^) 

 is fused to that of digit ii ; in three instances two supernumerary digits 

 are present, but digit ii is entirely wanting ; and in one specimen there 

 are evidences of tliree extra digits. 



5. In more than a third of the cases examined, the skeletal, muscular, 

 and nervous organs of the manus give some evidence that the extra 

 digit is vestigial. 



6. The trapezium (so-called) may represent this carpal element plus 

 the rudiment of digit I. 



VOL. XL. NO. 6 4 



