The last section of the Artiodactyle Ungulates is the great and natural group of the Ruminantia, or Ruminant animals. This section comprises the Oxen, Sheep, Antelopes, Giraffes, Deer, Camels, etc, and is distinguished by the following characters:
The foot is what is called "cloven," consisting of a symmetrical pair of toes encased in hoofs and looking as if produced by the splitting into two equal parts of a single hoof. In addition to these functional toes, there are mostly two smaller supplementary toes, placed at the back of the foot. The metacarpal bones of the two functional toes of the fore-limb, and the metatarsal bones of the same toes of the hind-limb, except in Hyomoschus, coalesce to form a single bone, known as the "canon-bone." The stomach is complex, and is divided into several compartments, this being in accordance with their mode of eating. They all, namely, ruminate or "chew the cud" - that is to say, they first swallow their food in an unmasticated or partially-masticated condition, and then bring it up again, after a longer or shorter time, in order to chew it thoroughly.
This process of rumination is so characteristic of this group, that it will be necessary to describe the structure of the stomach, as showing the mechanism by which this singular process is effected. The stomach (fig. 405) is divided into four (rarely three) compartments, which are usually so distinct from one another that they have generally been spoken of as so many separate stomachs. The gullet opens at a point situated between the first and second of these cavities or "stomachs." Of these the largest lies on the left side, and is called the "rumen" or "paunch" (fig. 405, r). This is a cavity of very large capacity, having its interior furnished with numerous hard papillae or warts. It is the chamber into which the food is first received when it is swallowed, and here it is moistened and allowed to soak for some time. The second stomach, placed to the right of the paunch, is much smaller, and is known as the "reticulum" or "honeycomb-bag" (h). Its inner surface is reticulated, or is divided by ridges into a number of hexagonal or many-sided cells, somewhat resembling the cells of a honeycomb. The reticulum is small and globular, and it receives the food after it has lain a sufficient time in the paunch. The function of the reticulum, as usually believed, is to compress the partially-masticated food into little balls or pellets, which are then returned to the mouth by a reversed action of the muscles of the oesophagus; but this is now discredited. After having been thoroughly chewed and prepared for digestion, the food is swallowed for the second time. On this occasion, however, the triturated food passes on into the third cavity (p), which is variously known as the "psalterium," "omasum," or (Scottice) the "manyplies." The vernacular and the first of these technical names both refer to the fact that the inner lining of this cavity is thrown into a number of longitudinal folds, which are so close as to resemble the leaves of a book. The psalterium opens by a wide aperture into the fourth and last cavity, the "abomasum" (a), both appearing to be divisions of the pyloric portion of the stomach. The mucous membrane of the abomasum is thrown into a few longitudinal folds, and it secretes the true acid gastric juice. It terminates, of course, in the commencement of the small intestine - i.e., the duodenum. The intestinal canal of Ruminants, as in most animals which live exclusively upon a vegetable diet, is of great relative length.
Fig. 405. - Stomach of a Sheep : o Gullet; r Rumen or Paunch ; h Honeycomb-bag or Reticulum; p Manyplies or Psalterium ; a Fourth Stomach or Abomasum.
The dentition of the Ruminants presents peculiarities almost as great and as distinctive as those to be derived from the digestive system. In the typical Ruminants (e.g., Oxen, Sheep, Antelopes) there are no incisor teeth in the upper jaw, their place being taken by a callous pad of hardened gum, against which the lower incisors impinge (fig. 406). There are also no upper canine teeth, and the only teeth in the upper jaw are six grinders on each side. In the front of the lower jaw is a continuous and uninterrupted series of eight teeth, of which the central six are incisors, and the two outer ones are regarded by Owen as being canines. Upon this view, canine teeth are present in the lower jaw of the typical Ruminants, and they are only remarkable for being placed in the same series as the incisors, which they altogether resemble in shape, size, and direction. Behind this continuous series of eight teeth in the lower jaw, there is a vacant space, which is followed behind by six grinders on each side. The praemolars and molars are of the "selenodont" type (fig. 399), and have their grinding-surfaces marked with two double crescents, the convexities of which are turned inwards in the upper, and outwards in the lower teeth.
Fig. 406. - Skull of a hornless Sheep : i Incisors; c Canines; m Molars and praemolars. (After Owen.)
The dental formula, then, for a typical Ruminant animal, is:
0 - 0
0 - 0
3 - 3
3 - 3
3 - 3
1 - 1
3 - 3
3 - 3
The departures from this typical formula occur in the Camelidae, the Tragulidae, and in some of the Deer. Most of the Deer conform in their dentition to the above formula, but a few forms (e.g., the Muntjak) have canine teeth in the upper jaw. These upper canines, however, are mostly confined to the males; and if they occur in the females, they are of a small size. The dentition of the Camelidae (Camels and Llamas) is still more aberrant, there being two canine-like upper incisors and upper canines as well. The lower canines also are more pointed and stand more erect than the lower incisors, and slightly separated from them, so that they are easily recognisable. The group of the Ruminantia includes the families of the Camelidae (Camels and Llamas), the Tragulidae (Chevro-tains), the Cervidae (Deer), the Camelopardalidae (Giraffe), and the Cavicornia (Oxen, Sheep, Goats, Antelopes).