Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/186

174 H R S E [ZOOLOGY. the crowns of the teeth gradually become longer, the valleys deeper, and the ridges not only more elevated but more curved and complex in arrangement. To give support to these high ridges and save them from breaking in use, the valleys or cavities between them became filled up to the top with cementum, and as the crown wore down an admirable grinding surface consisting of patches and islands of the two FIG. l.-r-a. Grinding surface of unworn molar tooth of Anchitherium ; 6, cor responding surface of unworn molar of young horse; r, the same tooth after it has bem some time in use. The uncoloured portions are the dentine or ivory, the shaded parts the cementum filling the cavities and surrounding the exterior. The black line separating these two structures is the enamel or hardest constituent of the tooth softer substances, dentine and cement, separated by variously reduplicated and contorted lines of intensely hard enamel, resulted (fig. 2, c). The crown continued lengthening until in the modern horses it has assumed the form called &quot;hyp- sidont&quot; (fig. 2, 6). Instead of contracting into a neck, and forming roots, its sides continue parallel for a considerable depth in the socket, and as the surface wears away, the whole tooth slowly pushes up, and maintains the grind ing edge constantly at the same level above the alve olus, much as in the per petually growing rodent s teeth. But in existing horses there is still a limit to the growth of the molar. After a length is attained which in normal conditions supplies sufficient grinding surface for the lifetime of the animal, a neck and roots are formed, and the tooth is reduced to the Condition of FIG. 2.^, Side view of second upper t-hif- nf rlip bnplwnrlnnr nn molar footh of ^n^ i ^rmm (brachyo- Dracnyoaoril an- dont f orrn ). fi) corresponding tooth of cestor. It is perfectly clear hoise (hypsidont form). that this lengthening of the crown adds greatly to the power of the teeth as organs of mastication, and enables the ani mals in which it has taken place to find their sustenance among the comparatively dry and harsh herbage of the open plains, instead of being limited to the more succulent vegetable productions of the m;;rshes and forests in which their predecessors mainly dwelt. The modifications of the limbs which took place pari passu with those of the teeth must have been associated with increased speed, especially over firm and unyielding ground. Short, stout legs, and broad feet, with numerous toes, spreading apart from each other when the weight of the creature is borne on them, are sufficiently well adapted for plodding deliberately over marshy and yielding surfaces, and the tapirs and the rhinoceroses, which in the structure of the limbs have altered but little from the primitive Eocene forms, still haunt the borders of streams and lakes and^the shady depths of the forests, as was probably the habit of their ancient representatives, while the horses are all inhabitants of the open plains, for life upon which their whole organization is in the most eminent degree adapted. The length and mobility of the neck, position of the eye and ear, and great development of the organ of smell, give them ample means of becoming aware of the approach of enemies, while the length of their limbs, the angles the different segments form with each other, and especially the combination of firmness, stability, and lightness in the reduction of all the toes to a single one, upon which the whole weight of the body and all the muscular power are concentrated, give them speed and endurance surpassing that of almost any other animal. When surprised, however, they are by no means helpless, both fore and hind feet becom ing at need powerful weapons of defence. If we were not so habituated to the sight of the horse as hardly ever to consider its structure, we should greatly marvel at being told of a mammal so strangely constructed that it had but a single toe on each extremity, on the end of the nail of which it walked or galloped. Such a forma tion is without a parallel in the vertebrate series, and is one of the most remarkable instances of specialization, or deviation from the usual type, in accordance with special conditions of life. It can be demonstrated, both by the structure of the foot itself, and also by an examination of the intermediate forms, that this toe corresponds to the middle or third of the complete typical or pentadactyle foot, the &quot;ring finger&quot; of man; and there is very strong evi dence to show that by a gradual concentration of all the power of the limb upon this toe, and the concurrent dwindling away and final disappearance of all the. others, the present condition of the horse s foot has been produced. The small horse-like animals of the Eocene period with four, or rudiments at least of five, toes on the fore foot have been already mentioned. In the early Miocene period the animal most like an existing horse was the Anchitherium , the remains of which are found in a fossil state both in Europe and in America. In this genus there were three well-developed toes reaching the ground on each foot, and Fir,. 3. Successive stages of modification of the feet of extimt forms of hoise-like animals (chiefly from Marsh), showing gradual reduction of the outer and en largement of the middle toe (in), n, Oiohippus (Eocene); b, Mesohippus (Early Miocene); c, Miohippvt and Anchilherivm (Late Miocene); d Hippa- iton and P/iohippus (Pliocene); e, Equus (Pleistocene). the radius and ulna, and the tibia and fibula of the hind leg, were complete and distinct. This was succeeded on the European and Asiatic continent by Hipparion and in America by Pliohippus, perhaps more directly in the line of descent, as Hipparion has some special characters of its own in the teeth and skull, which make it probable that it is a collateral branch which became extinct without leaving descendants. In these and other forms which flourished at this period, the lateral toes, though containing the full number of bones, were much reduced in size, and did not reach the ground, but were suspended to the outside of and rather behind the large middle one, like the rudimentary ou er toes of the deer, or the short first digit (&quot; dew-claw &quot;) of the dog. Horses, or rather horse-like creatures, with this structure of feet were no longer met with in the Pleistocene period, but then for the first time appeared the true horse in its development exactly or very nearly as we know it now. The outer toes were reduced to rudiments of the metacarpals or metatarsals only, the so-called &quot;splint bones&quot; entirely con cealed beneath the skin (fig. 3, e, n. and iv.), the middle