July, igo6.] 



KNOWLEDGE & SCIENTIFIC NEWS 



473 



the case; and it may be regarded as a fortunate cir- 

 cumstance that the total elimination of such super- 

 fluous structures appears to be an exceedingly difficult 

 process, so that their rudiments, or vestiges, are fre- 

 quently left to tell the tale of their gradual degeneration. 



As regards some of the rudimentary organs occurring 

 in the horse, one of the most interesting is the rem- 

 nant in the skull of the Eastern breeds, of the cavity in 

 front of the eye, which, in the extinct three-toed hip- 

 parions probably contained a gland similar to the 

 larmier, or face-gland, of deer and many antelopes. 

 Fuller reference to this rudimentary gland-cavity, which 

 is often very faintly marked, will be found in " Know- 

 ledge & Scientific News " for August, 1904. 



At first sight there may seem to be little, if any, 

 connection between this last vestige of the hipparion's 

 face-gland, and those curious warty structures on the 

 inner side of the limbs of the horse, which are com- 

 monly known as callosities, or chestnuts (Fig. i). It 

 appears to be a very genera! belief that these structures 



Fig. I. -The Left Fore 

 the Callo 



tid Hind Limhs of a Horse, to 

 ties, or "Chestnuts." 



are for the purpose of serving as cushions, or pads, to 

 ease the pressure on the limbs when the animal is lying 

 down. This, however, is obviously out of the question; 

 and it is quite certain that the callosities are now use- 

 less remnants of structures that were once functional. 

 The question is, what those structures were. One 

 theory is that they were foot-pads, or cushions, com- 

 parable to those on the foot of a dog or a cat; and in 

 order to support this hypothesis, it has been stated that 

 they arc situated much lower down in the foetus than in 

 the adult, so as to be situated on what corresponds to 

 the foot of other mammals. This, however, is not the 

 case, as is demonstrated by specimens exhibited in the 

 Natural History Museum. 



A much more probable theory is that these callosities 

 represent scent-glands, comparable tO' those on the 

 limbs of deer. Strong support to this is afforded by 

 the fact (as I am informed) that the secretion which 

 exudes from these callosities when cut will cause a 

 horse to follow any substance anointed therewith; and 

 also by the poacher's practice of carrying a fragment 

 of one of them to keep his dog quiet. Tliat a rudi- | 



mentary foot-pad would have any effect of this kind is, 

 of course, quite out of the question, although nothing 

 is more likely than that such emanations should pro- 

 ceed from a decadent foot-gland. 



In regard to the connection between the rudimentary 

 face-gland of certain horses, and the callosities, it may 

 be noted that both face-glands and foot-glands appear 

 to be for the purpose of aiding animals in finding the 

 whereabouts of their fellows; the leg or foot glap'"'~ 

 leaving a scent on the grass or jungle through which 

 they pass. If, however, animals live on open plains, as 

 is the case with horses and zebras, where they can see 

 one another at long distances, such aids may be quite 

 unnecessar)'. We know that the horse and its kindred 

 have lost the facial scent-glands of their ancestors, and 

 what is more likely than that they should at the same 

 time have discarded their leg-glands, of which the 

 callosities are the last remnants? 



That the horse does retain vestiges of the foot-pads 

 of its ancestors, who applied a portion of the sole of 

 their foot, instead of only the nail (hoof) of the middle 

 toe, tO' the ground, appears, however, to be undoubted. 

 At the hinder basal extremity of the second joint of 

 the pastern is a curious little horny spur (very con- 

 spicuous in the foetus), known to veterinarians as the 

 ergot; and this ergot seems to represent the central pad 

 of the foot of the tapir. .As this part of the foot of the 

 horse does not touch the ground, the pad is of no 

 functional importance, and has consequently degen- 

 erated to this curious little horny spur. 



Other rudimentary organs in the horse are the splint- 

 bones lying on either side of the upper end of the fore 

 and hind cannon-bones, and representing the functional 

 metacarpal and metatarsal bones, and sometimes even 

 the lateral toes of the hipparion. In domesticated 

 horses not only are these bones useless, but they are 

 actuallv harmful, producing the disease called splint. 

 How this accords with the theory that the horse has 

 been specially evolved for the use of man, may be left 

 to those who hold that theory to explain. Even this 

 does not exhaust the list of rudimentary structures in 

 the horse. In the " knee," or carpus, of the hipparion 

 exists a bone kncmn as the trapezium, which supports 

 one of the aforesaid metacarpal bones of the lateral 

 toes. In the horse this bone is functionless and very 

 minute, and is present only in about fifty out of every 

 hundred individuals; so that it is evidently about to 

 follow in the wake of the lost lateral toes. 



In the hor.sc only certain elements of the limbs have 

 become rudimentary, in order to permit the greater 

 development of other elements of this part of the 

 skeleton. In some groups of animals on the other 

 hand, one or both pairs of limbs are, in many instances 

 at any rate, completely wanting; and had it not been 

 that they arc occasionally represented by minute 

 vestiges, we should have had no direct evidence that 

 they ever existed in the group. As it is, wc are 

 absolutely certain (if evolution be the true explanatior 

 of the resemblanceof animals to one anotheri that snakes 

 and whales are descended from creatures with four limbs. 



In regard to snakes, most members of the group, 

 show no' traces of limbs, either externally or internally; 

 but in the family groups which include the boa-con- 

 strictors and pythons (the Boid^ of naturalists) it 

 fortunately happens that in manv species, at any rate, 

 minute vestiges of the hind-limbs are retained, as if 

 for the very purpose of telling us the story of their 

 ancestry, for it is quite certain that in most instances 

 .It all events, these ludiments are absolutely useless. 



