In the act of rumination, the following is the probable order of events:—The paunch contracts, and in so doing forces some of the food into the honey-comb 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 divaricate 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 numerous 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.
BRAIN OF A SHEEP.
There are other features also which are characteristic of the ruminating animals. Their symmetrical four-toed feet (in which the thumb on the fore and the great toe on the hind are entirely absent) have the toes so proportioned that the axis of the limb runs down between the two middle toes at the same time that both the inside and outside toes are much reduced in size, and lost entirely in the Camel tribe, the Giraffe, and the Cabrit.
Another peculiarity which exists in all ruminating animals is the absence of cutting-teeth in the middle of the upper jaw; and it is only in the Camels and their intimate allies, the Llamas, that there are any upper cutting-teeth at all, they being replaced in all the others by a callous pad, on which the lower cutting-teeth impinge in mastication.
The canine teeth, which correspond to the tusks of the Lion and Dog, also deserve attention. Those of the lower jaw are always present, and are modified so as to appear like lateral cutting-teeth. In the upper jaw they are most often absent, but are enormous, projecting far down outside the lip, in the Musk, the Chinese Water Deer, and the Muntjacs. In some other Deer they are present, but small, and generally they are wanting.
The grinders are six on each side of each jaw, and are so formed that their surfaces wear down unevenly by the lateral movement to which they are subject during mastication. As in the Elephant, this depends upon each tooth being made up of alternate layers of enamel, dentine, and cementum, which, being of different degrees of hardness, are differently affected by the grinding action.
The ruminating animals exhibit a fair amount of intelligence, never, however, attaining that power of perception and memory exhibited by the Carnivora and other higher forms. The figure of the surface of the brain of the Sheep indicates that the convolutions of the brain are far from inconsiderable in number, and its allies of the same size agree with it in this respect, whilst larger species have more, and smaller less elaborate brain-markings, as is nearly always found to be the case in every group.
The accompanying table gives an outline sketch of the classification of the ruminating animals which has been adopted by zoologists:—