CHAP. XIII.] 



DIGITS : HORSE. 



3G5 



given the digit II is extensively developed and the trapezium 

 bears a splint bone representing the metacarpal I, like that which 

 in the normal represents the digit II. This is a phenomenon 

 illustrating the principle seen in the case of teeth and other p.-irt- 

 in series (see p. 272), namely, increase in the degree of development 

 of the normally last member of a series correlated with the appear- 

 ance of a new member beyond it, 



Nevertheless the same cases have sometimes been described 

 (e.g. Gated. Mus. Coll. Surg.) on a different hypothesis. This is 

 illustrated by the lettering of Fig. 108. On this other view 

 the innermost carpal is considered to be the trapezoid and its 

 splint-bone is regarded as the original metacarpal II. The second 

 digit, ac, and its tarsal bone are supposed to be " accessory " or 

 "intercalated." To these terms it is difficult to attach any 

 definite meaning. The proposal that some digits are to be 

 reckoned in estimating homologies and that others are to be 

 omitted is arbitrary, and, if allowed, would make nomenclature 

 dependent on personal choice. It is, as has been often pointed 

 out in foregoing chapters, simpler to number the parts in order as 

 they occur and to accept the visible phenomena as the safest index 

 of the methods and possibilities of Variation. Nevertheless, to 

 illustrate the point at issue I have introduced two cases of the 

 same Variation, the one, No. 536, lettered on the view advocated 

 by the Catalogue of the College of Surgeons, &c., the other, No. 537, 



acm 



FIG. 108. Eight fore foot of Horse No. 53G from 

 behind. The upper surfaces of the carpal bones of the 

 distal row are separately shewn above. Specimen in 

 Coll. Surg. Mus., Ter. Cat., 304. 



T, trapezoid. J/, magnum. U, unciform. ac, ac- 

 cessory carpal bone. II, III, IV, metacarpals. acm, 

 accessory metacarpal. 



This figure is lettered to illustrate the hypothesis 

 adopted in the Catalogue, which is alternative to that 

 adopted in Fig. 107, A. 



