Sub-Order 10. ARTIODACTYLA.
Fig. 138.—Bones of the Manus—A, of Pig (Sus scrofa). × ⅓. B, of Red Deer (Cervus elaphus). × ½. C, of Camel (Camelus bactrianus). × ⅛. c, Cuneiform; l, lunar; m, magnum; m2, m5, second and fifth metacarpals; R, radius; s, scaphoid; td, trapezoid; u, unciform; U, ulna; II-V, second to fifth fingers. (From Flower's Osteology.)
The Artiodactyle or "Even-toed" Ungulates are to be distinguished from the Perissodactyla, and from other Ungulate groups, by a number of trenchant characters. The most salient
of these, and that which has given its name to the group, concerns the arrangement of the digits. Instead of there being but one prevailing digit—the third, in the hand and foot, through which the axis of the foot passes, there are two, numbers three and four, between which the same axis passes, and which are perfectly symmetrical with each other. This type of foot has been termed "paraxonic," as opposed to the "mesaxonic" Perissodactyle foot (see Fig. 121 B, p. [235]). It has been attempted to prove that the single prevailing digit of the Horse's foot is a fused pair of digits, and the state of affairs which characterises the Camel, where the two metacarpals or metatarsals are to an almost complete extent united, has been urged in proof; so, too, certain abnormalities, such as those called "solid-hoofed pigs."[[171]] These latter are simply Pigs in which the two central metacarpals and the terminal hoofs are completely fused with one another. In some of such cases there is not the slightest trace of the union of the separate metacarpals and phalanges. Even the sesamoid bones, attached behind to the toes, are two in number instead of four. And, furthermore, the tendon supplying the bones is single, though showing traces of its double origin. Such Pigs often show the abnormality from generation to generation, and they proved convenient for those whose scruples would not allow them to eat the flesh of a beast "dividing the hoof" and not chewing the cud. More singular still, as showing a pathological approach from another side to the Perissodactyle condition in an Artiodactyle, is a calf, where the foot ended in three equi-sized digits, of which the middle one lay in the longitudinal axis of the limb. From the opposite side cases are known of a Horse with a split hoof and phalanges, thus presenting the most striking likeness to a Camel.
There is, furthermore, in certain groups of Artiodactyles (e.g. the Tragulidae) a tendency for the two middle metacarpals to unite, quite apart from such "sports" as those illustrated by the cases just set forth. And, as already mentioned, the union of the two middle metacarpals culminates in the Camel, Ox, etc. There is, however, absolutely no trace of such a fusion in the series of Perissodactyle animals known to us; and it would be by fusion rather than dismemberment that, as it would appear on this theory, the modern Ungulate foot has been arrived at. Of course
the facts of Ungulate descent are absolutely destructive of any such comparisons.
As is the case with the Perissodactyles, the Artiodactyles show a historical series, the primitive five-toed condition being almost preserved in Oreodon, up to the most modern modification exemplified by the Ox, Sheep, etc., in which animals there are not even vestiges of the fourth and fifth toes. It has been stated, however, that the foetal Sheep has traces of those rudiments. The so-called cannon bone (the fused third and fourth metapodia) is accompanied in its fusion by an increase in length. At the same time the functional middle metacarpals push aside the rudiments and, forming a broad surface for that purpose, articulate with the magnum and unciform bones to the exclusion of the rudiments. This has been termed an "adaptive reduction." In the "inadaptive reduction" there is the same reduction of the metacarpals, but the rudiments still articulate as in the primitive Artiodactyle foot, i.e. Mc II with trapezium, trapezoid, and magnum; Mc III with magnum and unciform; Mc IV and V with unciform. This would appear to give greater solidity and consequently greater strength to the foot.
Fig. 139.—Dorsal surface of right tarsus of Red Deer (Cervus elaphus). × ⅓. a, Astragalus; c, calcaneum; c3, cuneiform; cb, cuboid; mIII, mIV, metatarsals; n, navicular. (From Flower's Osteology.)