(2304). The stomach itself presents such endless diversity of form, that merely to enumerate all the details that have been amassed relative to this part of our subject would fill many volumes, without perhaps at all advancing our real knowledge concerning the progress of digestion; we must therefore content ourselves with a very general view of the organization of this important viscus, and regard the Mammalia as possessing either simple, complex, or compound stomachs, each of which will deserve a distinct notice.

(2305). In the simple form of stomach the organ consists of a single cavity, as is the case in the human species: let the shape of the viscus be elongated, pyriform, or globular (for in this respect there is every possible variety); whatever its form, or the relative positions of the cardiac and pyloric orifices, its structure corresponds with that of Man in all essential particulars. This kind of stomach exists in by far the greater number of Mammals.

(2306). In the complex stomach the viscus is made up of several compartments communicating with each other, but without presenting any difference of organization such as in the present state of physiological knowledge would lead us to suppose them to possess different functions: neither are we at all able to find any connexion between such an arrangement and the nature of the substances used as food. The Kangaroo (Macropus major), the Kangaroo Eat (Hypsiprymnus), the Porcupine (Hystrix), and the Hyrax are amongst the most striking examples.

(2307). The compound stomach is that possessed by the Ruminantia, or animals that chew the cud, and consists of four distinct cavities, differing very materially both in their size and in the arrangement of their lining membranes. The first and by far the largest cavity (fig. 402, d) is called the paunch (rumen), and is of very great size, occupying a considerable portion of the abdominal cavity, and forming the great receptacle into which the crude vegetable aliment is received when first swallowed: this chamber is lined with shaggy villi. The second cavity (reticulum) (c) is much smaller, and its walls are covered with numerous polygonal cells, from whence it derives the name it bears. The third chamber (e), called the psalterium, has its lining membrane disposed so as to form deep lamellae, arranged longitudinally in alternating large and small layers, and thus presenting a most extensive surface. The fourth stomach (abomasus) (f) also exhibits very numerous folds of mucous membrane: it is of a pyriform shape, and by its smaller end terminates at the pylorus (g.) The three first stomachs are lined internally with a thin cuticular investment; but the last, apparently the representative of the single stomach of those quadrupeds that have but one stomachal cavity, is coated with a soft membrane that furnishes abundantly the ordinary gastric secretions, and appears to be more especially the digestive stomach.

(2308). The passage of the food through these different chambers will be easily understood on referring to the preceding figure, in which the course of the aliment before and after rumination is indicated by the direction of the probes a, b. The oesophagus, it will be observed, communicates on the one hand with the paunch (d), and on the other with the cavities c, e,f; and, moreover, by means of a muscular fold formed by the walls of the second cavity, a passage may be formed leading directly into the third stomach (e) without communicating with the second (c.) The process of rumination, therefore, would seem to be effected in the following manner: - The herbage when first swallowed in an unmasticated condition passes into the capacious paunch (d), where it accumulates, and undergoes, no doubt, a kind of preliminary maceration. When the Ruminant has done grazing, and is at leisure, the food is again regurgitated into the mouth, to undergo more careful and complete mastication: for this purpose, a part of it is admitted into the reticulum (o), and there formed into a smooth and lubricated bolus, which, being expelled into the oesophagus, is immediately seized by the spiral muscles surrounding that canal and forced forwards into the mouth.

After undergoing a thorough trituration, the aliment is once more swallowed, and it then enters into the third stomach (e), passing along the muscular fold that leads from the oesophagus into that compartment. Here it is spread out over the extensive surface formed by the laminated walls of the psalterium, and is prepared for admission into the last cavity (f), which, as has been said, is the true digestive stomach.

Stomach of the Sheep.

Fig. 402. Stomach of the Sheep.

(2309). While the young Ruminant continues to be nourished by its mother's milk, the three first cavities are undeveloped and comparatively very small; so that the milk passes on immediately into the fourth stomach, to be at once appropriated as aliment.

(2310). In the Camel, the Dromedary, and the Llama, the walls of the reticulum and of a portion of the paunch are excavated into deep cells or reservoirs bounded by muscular fasciculi, wherein water may be retained in considerable abundance, unmixed with the contents of the stomach: it is in consequence of this arrangement that these animals are able to subsist for many days without needing a fresh supply of water even during long journeys in a tropical climate.

(2311). In the Cetacea the stomach consists of several bags that communicate with each other. These bags vary from five to seven in number; but in the present state of our knowledge concerning the physiology of digestion it is difficult to divine what is the purpose of such an arrangement, more especially as rumination is here out of the question. The first stomach of the Whale, however, is no longer merely a reservoir *, as the food undergoes a considerable change in it. The flesh of its prey is entirely separated from the bones, which proves that the secretion of this cavity has a solvent power. This was found to be the case in the Bottle-nose Porpoise and in the large Bottle-nose Whale, in both of which several handfuls of bones were contained in the first cavity, without the smallest remains of the fish to which they had belonged. In others the earth had been dissolved, so that only the soft parts remained; and, indeed, it is only partially-digested materials that can be conveyed into the second and third cavities, the orifices being too small to permit bones to pass.