PRENTISS : POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 287 



This abnormality may be accounted for in two ways : Either (1) digit 

 I" represents the developed pollex, and i* is formed by the duplication 

 of dio-it II, or (2) digits i" and i* are duplications of the pollex, and the 

 metacarpal of i'' is secondarily fused to that of digit ii. The first 

 hypothesis is supported by the similarity in structure of digits i* 

 and II, their symmetry with reference to each other, and the differences 

 existing between i" and i^ The second view, however, is supported 

 (1) by the fact that the extra digits are enclosed in the same hoof, and 

 therefore probably developed together, (2) by the fact that the trape- 

 zium articulates with the compound metacarpal, and (3) by the structure 

 of the muscles and nerves. 



To another type belong two cases in which digit i" is completely 

 developed and articulates with the carpus (Plate 20, Fig. 28). Digits 

 I*" and II are borne on a single large metacarpal, but i** is much the 

 larger. The phalanges of ii are of normal size and unsymmetrical 

 with those of i^. The ungual phalanges of both i" and i* are enclosed 

 in separate hoofs, and are symmetrical with each other, although differ- 

 ing somewhat in size. The trapezium is large, and articulates with 

 metacarpal i" and with a portion of the compound metacarpal. The 

 musculature and innervation of this manus are similar to those of the 

 foregoing case. 



Our view that these abnormalities are due to duplication of the 

 pollex and the subsequent fusion of the metacarpal of i^ to that of ii, 

 is favored by the structure of a manus figured by Otto ('41, Tab. 

 26, Fig. 12). In this case there are two extra digits of three pha- 

 langes ; i" is borne on a distinct metacarpal, which articulates with 

 the trapezium, and i* on a metacarpal which is almost completely fused 

 to metacarpal ii. Digit ii is of normal size. The phalanges of i" and 

 i^ form a single series of three bones, each of which is incompletely 

 divided into two ; the ungual phalanx evidently bore a single hoof. 

 The trapezium articulates with metacarpal i" and with two-thirds of 

 the pi'oximal surface of the compound metacarpal. The trapezoid is 

 smaller and articulates with the remaining third of the proximal facet 

 of the large metacarpal bone. In this manus, therefore, the digits 

 i" and i^ evidently developed together, and the fusion of metacarpal 

 i^ to that of II was of subsequent occurrence. This being the fact, it 

 is very probable that the foregoing cases which we have examined were 

 produced in a similar manner. 



Having now briefly described the types of digital variation in the manus 

 of the pig, we shall next attempt to determine their significance. 



