32 



COMPARATIVE ANATOMY 



distal ends of the digits, the Mammalia may be subdivided into 

 Unguiculata and Ungulata, the former group including those with 

 claws or nails, and the latter those with hoofs. But no hard and 

 fast line can be drawn between these structures, which in all cases 

 are derivable from a simple form of claw, like that of Reptiles and 

 Birds. The terminations of the digits are without a horny 

 covering in Cetacea, though rudiments are present in the embryo 

 of Toothed Whales ; while among the Sirenia the Manatee possesses 

 vestigial and variable nails. 



The horny nail-plate is situated on the dorsal side of the 

 digit, while ventrally is the softer sole-horn, which is continuous 

 proximally with the pads or tori on which the foot partially or 



D 



FIG. 24. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE DISTAL ENDS 

 OF THE DIGITS OF A, ECHIDNA ; B, AN UNGUICULATE MAMMAL ; C, MAN ; 

 AND Z>, HORSE (after Gegenbaur and Boas). 



1 3, phalanges ; b, torus ; N, nail-plate ; S, sole-horn ; W, bed of claw or nail. 



entirely rests when on the ground. The essential relations and 

 chief modifications and reductions of these parts in various 

 Mammals are illustrated in Fig. 24. Tori are present in most 

 Mammals, and have a definite arrangement on the palms and soles 

 (apical, interdigital, and proximal), and in them the dermal 

 papillae are either irregular or are definitely grouped, and may 

 give rise to a series of concentric lines and arches. 1 



When pigment is present (e.g. on the snout, external genitals, 

 and teats), it is chiefly situated in cells of the Malpighian layer 



1 Compare those of Man, which yield the characteristic "finger-prints." 



