PKENTISS: rOLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 297 



trzA 



h. Four metacarpals ; digit i is represented by a splint radial to 

 digit 11, wliicli is fully developed and composed of three phalanges 

 (Fig. Y). 



In these cases there are four large bones present 

 in the distal row of carpals. Digit ii is large, and 

 its metacarpal is fused throughout most of its length 

 to that of digit iii. Four cases are cited by Marsh 

 ('92), and one is carefully described by Bateson ('94). 



A different interpretation from that here as- 

 sumed may be brought forward in explanation of 

 these cases. The digit designated as ii in Figure 

 Y may be regarded as a duplication of digit iii, and 

 the so-called trapezoid of the carpus may represent 

 a duplication of the os magnum. Then the bone 

 designated as trapezium must be the true trapezoid, 

 and its splint bone the second, not the first, meta- 

 carpal. Only by a careful examination of the 

 skeletal, muscular, and nervous structures can we 

 determine which interpretation is correct ; whether 

 digit II is of vestigial origin, or due to a duplication 

 of digit III. The fact that in phylogeny the pollex 

 disappeared long before the iifth digit is a strong 

 argument against the former interpretation. For 

 by that interpretation we should here have the pol- 

 lex reappearing, and the second digit almost as large 

 as the third, while the fourth digit is unmodified 

 and the fifth is entirely absent. 



c. Five metacarpals ; one supernumerary digit, 

 home on metacarpal ii. One case i^ described by 

 Piitz ('89) in which the trapezoid bears digit ii ; 

 this consists of a well-developed metacarpal bone and three phalanges. 

 Radial to this is a large trapezium, articulating with the scaphoid and 

 trapezoid and bearing a splint six cm. long ; metacarpal iv is normal, 

 and on its ulnar side is another metacarpal element supposed to rep- 

 resent digit V. The supernumerary elements in this case can only be 

 exi)lained as of vestigial origin. 



(2) Two digits borne on metacarpal iii. 



These are clear cases of dui)lication, and have been described in the 

 manus only. The doubling may extend to the metacarpal bone, but is 



Fig. F. — Anterior 

 view of left polydac- 

 t^'le manus of horse. 

 I, metacarpal of first 

 supernumerary digit 

 (pollex) ; II, second 

 supernumerary digit; 



III, functional digit; 



IV, metacarpal (splint) 

 of fourth digit; r, 

 radius; tvz., trape- 

 zium; ««., unciform. 

 (After Marsh.) 



