320 



THE HIND LIMB. 



The nearer a horse approaches the heavy draught type, 

 the thicker is the growth of the callosities on his legs ; 

 possibly on account of the comparative slowness of his 

 circulation (pp. 323 and 324). Thus, Shire horses, which 

 have an enormous amount of long hair on their legs, often 

 have chestnuts that take the form of small horns (Fig. 

 396) ; but the callosities on the limbs of light breeds of 

 horses, are relatively thin. 



As comparatively recent ancestors of the horse walked 

 on their fetlocks, ergots are undoubtedly vestiges of pads, 

 similar, as pointed out by Professor Ewart, to the middle 

 portion of the trilobed sole-pads of the dog and cat. Even 

 horses of the present day sometimes use their fetlocks as 

 a means of support, when galloping (Fig. 184). 



Careful examination shows that chestnuts and ergots 

 are purely epidermal (p. 323) growths, similar in structure 

 to corns on our toes, and the horny excrescences which 

 form on the hands of oars-men, for instance. Professor 

 Ewart, to whom we are indebted for the elucidation of 

 this subject, has strengthened the theory of the skin origin 

 of the chestnuts and ergots, by proving that in the 

 equine foetus they sometimes contain hair rudiments, 

 which are also occasionally present in the hind chest- 

 nuts of equine hybrids. Such callosities arise only 

 from pressure, and play the part of foot-pads in many 

 animals. The kangaroo, like man, is a plantigrade 

 animal, and has, on each hind leg, an epidermal pad 

 (Fig. 396A) which extends from the fetlock to the point 

 of the hock, and wliich is a modified counterpart of 

 the rough skin on the sole of our feet ; for it is largely 

 developed at the fetlock (first row of the toe-joints) 

 and at the hock (heel), but is only of narrow width 

 under the bones (metatarsal bones) that correspond to the 

 bones of our instep. Similar pads are found in other 

 marsupials, such as the Australian bandicoot, and in the 

 common rat and other rodents. In assuming that the 

 chestnuts are vestiges of foot-pads wliich existed in 



