Ruminantia — The Ruminants
These differ materially from the foregoing section of the Artiodactyla by the construction of their digestive organs. Instead of the food being masticated and passed at once into the stomach, each mouthful is but slightly bruised and passed into the paunch, whence at leisure it is regurgitated into the mouth to be chewed. For such an operation the machinery is of course more complicated than in other animals, and I must therefore attempt to describe briefly and as clearly as I can the construction of the ruminating stomach. Taking the ox as a typical specimen, we find four well-defined chambers varying in size. The first of these is the rumen or paunch, in which the unmasticated food is stored; it is a large sac partly bent on itself, and narrowing towards its junction with the oesophagus or gullet, and the entrance into the second chamber. It is lined with a mucous membrane, which is covered with a pile or villous surface, and this membrane is what is sold in butchers’ shops as tripe. From this bag (the paunch) in the act of rumination a certain portion of the food is ejected into the second chamber, which is termed the reticulum (i.e. a little net) from the peculiar arrangement of its inner or mucous surface, which is lined with a network of shallow hexagonal cells. The functions of this receptacle are probably the forming of the food into a bolus, and by a spasmodic contraction the forcing of it back through the gullet into the mouth for mastication. Here it is well chewed, and, being thoroughly mixed with saliva passes back; on being swallowed in a soft pulpy state it passes the groove or valve communicating with the chamber from which it issued, and goes straight into the psalterium or manyplies, as the third chamber is called. This is globular, but most of its interior is filled up with folds like the leaves of a book, more or less unequal. It is not quite clear what the peculiar functions of this chamber are, but the semi-liquid food, passing through it, goes into the proper stomach (abomasum or reed) and is here acted upon by the gastric juice. Professor Garrod thus describes the probable order of events in the act of rumination: “The paunch contracts, and in so doing forces some of the food into the honeycomb bag, where it is formed into a bolus by the movement of its walls, and then forced into the gullet, from which by a reverse action it reaches the mouth, where it is chewed and mixed with the saliva until it becomes quite pulpy, whereupon it is again swallowed. But now, because it is soft and semi-fluid, it does not devaricate the walls of the groove communicating with the manyplies, and so, continuing on along its tubular interior, it finds its way direct into the third stomach, most of it filtering between the membrous laminæ on its way to the fourth stomach, where it becomes acted on by the gastric juice. After the remasticated food has reached the manyplies, the groove in the reticulum is pushed open by a fresh bolus, and so the process is repeated until the food consumed has all passed on towards the abomasum or true digestive stomach.”
The ruminants are peculiar also in their dentition; in the so-called true ruminants there are no incisors or cutting teeth in the upper jaw, but the teeth of the lower jaw are opposed to a hard callous pad; the herbage is cropped by being nipped between these teeth and the pad, and detached by an upward motion; in some few, such as the musk deer, Chinese water deer and the rib-faced deer or muntjac the upper canines exist, and are largely developed.
The camels and llamas possess two cutting teeth in the upper jaw, and in this respect they differ from the true ruminants, as also in some internal features.
The grinding teeth are six on each side of the jaw, and are composed of alternate convolutions of enamel, dentine and cement, which wear unequally by the lateral motion of grinding, and so form the necessary inequality of surface.
The centre metacarpal bones in the Ruminantia are fused into one common bone, except in the deerlets, which also have the two outer fore and little finger metacarpals distinct, whereas they are but rudimentary in the rest of the true ruminants, and totally absent in the camels.
The following is the classification at present adopted: Sub-order Ruminantia, containing two sections, viz. True Ruminants and the Camels (Tylopoda). Section True Ruminants, containing two divisions, viz. Horned Ruminants and Hornless Ruminants, such as the chevrotians or deerlets (Tragulidæ). Division Horned Ruminants, containing two groups, viz. Hollow-horned Ruminants (Bovidæ), and Solid-horned Ruminants (Cervidæ). The deerlets possess no psalterium or third stomach, except in a rudimentary form, and their feet approximate to those of the pigs, and they are destitute of horns. The hollow-horned ruminants are those which bear a persistent sheath of horn on a bony core; the others bear solid antlers which are periodically shed, and grow afresh.
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