Page:Popular Science Monthly Volume 68.djvu/344

340 dog. Normally the hallux, or great toe, is represented only by the proximal end of the metatarsal bone (Fig. 2, A). Not infrequently, a claw and one or two phalangeal bones may appear at the point where the hallux should be; occasionally the distal end of the metatarsal bone is also represented, and sometimes a complete digit with all the bones and articulations of a functional hallux may be developed. Such cases, which may be regarded with certainty as reversions to the five-toed type of foot, occur not rarely on the pes of the Scotch collie, St. Bernard and Newfoundland (Fig. 2, B, C).

In the foot of the pig the hallux is gone and the pollex is normally represented by a small carpal rudiment (Fig. 3, C). A small pollex was, however, present in the manus of Ancodus, one of the fossil swine

 X-ray photographs of the pig's manus showing normal structure and reversionary polydactylism. A, bones of normal manus; B, manus in which the pollex is represented by two phalanges and the distal end of the metacarpal bone (I.); C, manus with pollex completely developed; trz., trapezium.

(Fig. 3, A). It is, therefore, an interesting fact that in the polydactyl swine observed by the writer the extra digits were in every case located upon the manus, and in most instances were undoubtedly developed from the rudiment of the pollex; for the extra digit was attached to the carpal bone as a normal pollex would be, and careful dissections of muscles and nerves gave no evidence of duplications. This does not support Gegenbaur's assertion that the extra digits of swine were developed by the splitting of the second toe. His conclusion was based on the dissection of two 'pig's knuckles' cut off below the carpus. Consequently he could not tell how the extra digit was attached. In any case this was scanty evidence on which to base a general conclusion. The writer was fortunate enough to obtain for study thirty-six perfect specimens. In one type observed, a small hoof, two phalanges and the