Before noticing the particular features of the equine group, it will be necessary to define the position which its members occupy in nature.
The whole of the Equidae or horse family belong to the Vertebrate kingdom and to the class Mammalia, which is separated by old writers into two great orders or divisions, the Ungulata or hoofed mammals, and the Unguiculata, including all animals with claws. This classification originated with John Ray in his Synopsis Methodica Animaliam, published in 1G93. Sir William Flower in his work on the horse remarks on the artificial character of the mode of division, but adds that some portion of the system has survived, especially the group Ungulata, which has been resuscitated of late years and used as a convenient designation for the group of quadrupeds that are distinctively hoofed.
1 The Tertiary is the third of the great life-periods known to geologists, being followed by the Post-tertiary or Quaternary, to which present-day life belongs.
Ungulate mammals are described by Sir W. Flower as animals which are eminently qualified for a life on land, and in the main for a vegetable diet. Their molar teeth have broad crowns with tuberculated or ridged grinding surfaces, and they have a perfect set of milk teeth, which are changed for permanent ones as the animals advance towards maturity. A very important point in their anatomy is the absence of collar-bones (clavicles). Their toes are covered with horny material, which usually encloses them completely, forming broad blunt nails or hoofs.
Cuvier, and after him Owen, distinguished two well-marked groups of ungulates, the fossil remains of which are found throughout the Tertiary period, the Artiodactyla or even-toed, and the Perissodactyla or odd-toed animals, both still represented by living forms. To realize the significance of these divisions it must be borne in mind that the number of toes in mammals is limited to five on each extremity. Each toe is the end of a series of bones starting from a compound joint, the carpus or wrist in the front or upper extremity (arm or fore-leg), and the tarsus or heel in the hind or lower extremity. To the series of bones the name digits is applied to express either fingers or toes, and the term phalanges is used to indicate the separate bones of which the digits are composed.
The annexed diagram, with the description taken from Professor Sir W. Flower's work, will make the above remarks intelligible.
So far all is quite simple; but it happens in nature, and it may also occur by chance, that one or more of the digits may be missing. Still the biologist is expected to decide from those which remain whether the animal belonged to the odd-toed or even-toed group, and it will shortly appear that it is most essential that there should be no risk of error in the conclusion arrived at.
Fig. 654. - a, Diagram representing the Bones of the Right Forefoot of an Odd-toed or Perissodactyle Animal. B, Diagram of the Bones of the Foot of an Even-toed Artiodactyle Animal, c, The Carpus or Wrist (knee of quadruped), consisting of two rows of bones.
The upper row consists of c, cuneiform; I, lunar; and s, scaphoid; the lower row u, unciform; m, magnum; and td, trapezoid; with the trapezium, tm, behind the cuneiform. The shaded parts of the bones in A are those that are now present in the horse; in B, those that are present in the ox. In five-toed mammals the digits are numbered one to five, beginning from the inner side of the limb. Digit No. I in the upper or fore extremity is the thumb (pollex), and in the hind or lower extremity the great toe (hallex); the other digits are distinguished by the figures II, III, IV, and V.
It will be seen by referring to fig. 654 that there is a marked difference in the arrangement of the digits in the two figures in the diagram. In the first figure, a, which may be taken to represent the foot of an early ancestor of the horse, the five digits are shown. The shaded parts are the bones which are to be found in the horse now existing. The special feature of the perissodactyle or odd-toed animal is the one large middle digit, the third in situation. In the next figure, B, representing the foot of the ox, the plan of construction is that of the artiodactyle or even-toed group. The first digit is not present, even in the most ancient members of the group, the second and fifth are absent or rudimentary in the recent members, and instead of one large middle digit there are two of equal size. These are distinct, and form the so-called cloven hoof of the ox, which is, in fact, constituted of the two middle digits, the third and fourth in situation. The shaded parts of the bones in the second and fifth digits in the diagram show the portions which remain in the foot of the ox.
Even a tyro in the science of anatomy will be able to understand the value of the indications afforded by the middle digit or digits in assigning to the animal to which they belong its proper position in the order Ungulata. For further illustration it may be assumed that the bones of the foot of an imaginary animal are in question, and it is granted that the animal is an ungulate mammal, and must therefore belong to the odd-toed or even-toed group. To determine which, the enquirer proceeds to examine the bones of the extremities below the carpus or wrist, otherwise called the knee, and the tarsus or heel, known as the hock in quadrupeds. Finding below these joints one large digit, no matter what other bones are present or absent, it is at once decided that the animal is one of the perissodactyle or odd-toed mammals. If, however, there are two equal digits, it is as certain that the animal is one of the artiodactyle or even-toed mammals.