Class 6. Mammalia. 471 



somewhat similar obtain in the Pig, where, however, the sole of the foot reaches 

 f ai-ther back than in the Horse. Ruminants (Fig. 382 E) display an advance 

 on the Pig, since the true sole reaches far foi-wai-ds, and the homy sole is largely 

 suppressed, and only represented by a narrow rim along the lower edge of the 

 homy wall: fm-ther, the most anterior portion of the tme sole in many 

 Ruminants (Red-deer, Oxen, etc.) has attained greater firmness than in others, 

 becoming like the horny sole, whilst in others {e.g., the Roe and the Elk) it is as 

 soft as usual. 



As in the Crocodiles, Chelonians, and Birds, growth of the claws is the result 

 •of the fonnation of a new thin stratum comeum over the whole surface of the 

 subjacent Malpighian layer ; thus the cap-shaped claw is pushed fonvards (just 

 as in the growth of the horns of Ruminants, see below). In Mammals 

 (and Lizards), on the other hand, a large portion of the subjacent Malpighian 

 layer is sterile (Pig. 381 a — b), i.e., is not concerned in strengthening the wall 

 which is foi-med at the proximal margin of the layer (left of a), and pushed over 

 stei-ile part; at the tip (right of 6) a formation of homy substance (t) again 

 occurs. The homy wall, therefore, increases in thickness from the base up to a, 

 retains the same thickness up to 6, thence increasing in thickness again, except 

 in so far as it is prevented by wear.* The homy sole, on the other hand, becomes 

 oontinuously thicker from the base to the tip. 



The horn of the Rhinoceros is an enormous local thickening of the stratum 

 ■comeum ; into it extends a papilla from the dermis, covered of ooxu-se with the 

 stratum Malpighii. The hoi-ns of Ruminants are of quite a different structure ; 

 each is to be regarded as a large, naked projection of skin, which is internally 

 ossified, and covered superficially with a firm, thick layer of horn ; the structure 

 "therefore consists of a bony mass within, the core, fused to the frontal bone ; 

 outside this, there is a layer of connective tissue and of the stratum Malpighii, and 

 externally, the stratum comeum, which increases by new deposits from within, 

 and is thus pushed out distalLy ; the basal edges of the individual layers of horn 

 appear as rings on the surface. The antlers of the Stag are very like, but 

 they differ in having a comparatively thin layer of hoi-n, and a covering of 

 hair ("velvet"). In the GirafBe, where the antlers are of small size, the soft 

 parts persist round the core; in other animals, however, when they are fully 

 ■developed, the velvet shrivels over most of the stirf ace and is rubbed off ; only 

 the basal region, the pedicle, retains the integumentary covering. The bai'e 

 bony mass, the true antler, is loosened annually from the pedicle, and is thrown 

 off ; the adjacent skin then grows over the bare edges, and a new antler develops 

 at the same spot, covered at first with velvet. In the Giraffe, no shedding occurs. 

 The vertebrae are usually biplanar, rarely opisthoccelous ; 

 they are connected by thick ligamentous discs of fibrous 

 connective tissue, which contain a remnant of the notochord, the 

 so-called nucleus pulposus, centrally. The vertebral column is 

 composed of the same sections as in EeptiUa. There are almost 

 always seven cervical vertebr£e,t regardless of the length of the neck. 

 The first two of these are, as in the Reptilia, developed as atlas and 

 axis. Monotremes alone possess separate cervical ribs (on the 



* In the transparent nail of Man, the bright basal portion (" Ivmula ") corresponds 

 to the portion a in Fig. 381 ; a bright line near the free edge corresponds with the 

 spot b. 



f Exceptions : the Manatee has only six, so has one of the Sloths '{Cholaepus 

 Hoffmani), whilst another of the same genus {Oh. didactylus) has seven, and yet 

 another (genus SradA/pus), nine. 



