296 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



VII. Polydactylism in the Equidae. 



A. Literature. 



The anatomy and diseases of tlie liorso have been studied almost as 

 thoroughly as those of man, and consequently we find that polydactylism 

 in the Equidae has received considerable attention. Aside from the classi- 

 cal allusion of Suetonius ('86) to the horse of Julius Caesar "which had 

 feet that were almost human, the hoofs being cleft like toes," the first 

 account of polydactylism is that of Winter (1703), who describes two cases. 

 Geoffroy St. Hilaire ('32-37) records a foetus whicli Avas polydactyle 

 in the fore feet, the left foot bearing three nearly equal digits, and the 

 right two. Numerous instances have since been noted, tlie more im- 

 portant descriptions being tlioso of Arloing ('67), Wood-j\Iason ('71), 

 Marsh ('79, '92), Ercolani ('81), Boas ('85), Putz ('89), and Ewart ('94). 

 Blanc ('93), and Bateson ('94) review the general subject. 



The normal functional digit of the Equidae is iii of the typical 

 mammalian mauus ; it consists of a long metacarpal bone and three 

 phalanges. The ungual phalanx is completely enclosed in a massive 

 hoof. Two splints, representing the metacarpals of digits ii and iv, 

 articulate at each side of the cannon bone posteriorly and with the 

 carpus. The trapezium is a small pea-shaped rudiment lying posterior 

 to the trapezoid and often wanting. The os magnum is very large, and 

 ■with it, chiefly, the cannon bone articulates. 



The polydactyle cases cited by various investigators fall into two 

 groups, the first of which may be subdivided into three: 



(1) Supernumerary digits representing the development of digital 



vestiges. 



a. Three metacarpals, the extra digits being home on ii and iv. The 

 condition of an extra digit borne on metacarpal ii may occur on all four 

 feet (Marsh, '92) or be limited to the raanus (Arloing, '67). The extra 

 digits are always smaller than in and do not function in locomotion ; 

 this condition is of quite frequent occurrence. A single case is cited by 

 Wood-Mason ('71), in which an extra digit of three phalanges occurs on 

 metacarpal iv ; the radial splint bone (ii) was also somewhat better 

 developed than in a normal nianus. Cases of three digits (both n 

 and IV being developed) are cited by Geoffroy St. Hilaire ('32-37) and 

 Marsh ('92), but no good anatomical descriptions are given. 



