PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 295 



71. m. — 



The nerves of this manus also show important modifications. The 

 normal manus, like that of swine, is innervated by four branches of the 

 median nerve ; the most radial and most ulnar branches (compare Fig. 

 X, 2, 5) give off small twigs to the rudiments of digits ii and v. 

 Brancli 5 is joined by the ulnar nerve im- 

 mediately before it divides to form 5 a and 

 5 b. In the polydactyle manus (Fig. X, 2, 

 5) the modification is in connection with 

 the small fasciculus (2°), which normally 

 innervates the radial accessory hoof (rudi- 

 ment of digit ii). This is no longer a 

 mere filament ending at the distal end of 

 the metacarpus, but a moderate-sized 

 branch, which continues to the hoof and 

 ungual phalanx of the supernumerary 

 digit. The condition of this nerve branch, 

 together with the fact that the accessory 

 hoof of this side is absent, affords most con- 

 vincing proof that this abnormality is not 

 a monstrosity, or a duplication of digit in, 

 but is due to the development of digit ii. 



The second case, a right manus, con- 

 firms by its structure the conclusion which 

 we have drawn from the first. 



The line of demarkation between the 

 second and third metacarpals is even more 

 distinct (Plate 22, Fig. 30) ; the first 

 and second phalanges of digit ii are fused 

 together and. are abnormally short. 



Rosenberg ('73) states that metacarpals 

 II and V are present in tlie embryo of 

 the sheep and ox, but later partially de- 

 generate and fuse to the cannon bone, a 

 small portion of v remaining distinct in 

 the ox. In the Cervidae the distal ends 

 of the metacarpals persist in the adult. 

 It is not surprising therefore that we find 

 these digital rudiments occasionally developed in the adult ruminant. 



Polydactylism in ruminants is thus of two types: (1) vestigial, due 

 to the development of either digit ii or v (or both) ; (2) teratological, 

 produced by the duplication of one of the functional digits (iii or iv). 



IV. III. II. 



Fig. X. — Posterior view of 

 left polydactyle manus of the calf, 

 showing innervation. ii, extra 

 digit; V, metacarpal of fifth digit; 

 n. m., median nerve; n. u., ulnar 

 nerve; 2-5, four branches of me- 

 dian nerve; 2«, division of second 

 branch which supplies the extra 

 digit (ii); 5", division of fifth 

 brancli which innervates the ac- 

 cessory hoof (digit v). i natural 

 size. 



