296 THE MANUS. [CHAP. 



each side of it the nearly equal second and fourth. In the 

 Rhinoceros there is a rudiment only of the fifth metacarpal. 



In the Horse (Fig. 101), the three bones of the first row 

 of the carpus are subequal. The second row consists of a 

 very broad and flat magnum (;//), supporting the great third 

 metacarpal, having to its radial side the trapezoid (td), and 

 to its ulnar side the unciform (?/), which are both small, and 

 articulate distally with the rudimentary second and fourth 

 metacarpals. The pisiform is large and prominent, flattened 

 and curved ; it articulates partly to the cuneiform, and 

 partly to the lower end of the radius. The single digit 

 consists of a moderate-sized proximal, a very short middle, 

 and a wide, semilunar, ungual phalanx. There is a pair of 

 large nodular sesamoids behind the metacarpo-phalangeal 

 articulation, and a single, transversely extended, " navicular " 

 sesamoid behind the joint between the second and third 

 phalanx. 1 



2. The Ariiodaetyla have the third and fourth digits 

 almost equally developed, and their ungual phalanges 

 flattened on their inner or contiguous surfaces, so that each 

 is symmetrical in itself, but when the two are placed together 

 they form a figure symmetrically disposed to a line drawn 

 between them. Or, in other words, the axis or median 

 line of the whole manus is a line drawn between the third 

 and fourth digits, while in the Perissodactyles it is a line 

 drawn down the centre of the third digit. 



In the S/iina, Pigs (Fig. 102), Peccaries, and Hippopotamus, 



1 With reference to the interesting cases of atavistic polydactylism, 

 see Gegenbaur's " Critical Remarks on Polydactylyas Atavism," Journ. 

 of Anat. and Physiol. xvi. p. 615 ; Boas, " Bemerkungen ueber die 

 Folydactylie des Pferdes," Morphol. Jahrb. x. p. 182. By far the 

 larger number of cases of polydactyly in the Horse as in other 

 Mammals, are merely due to a teratological multiplication of digits, 

 and are not reversions to an ancestral condition, as frequently supposed. 



