It is usual to call such a simple digestive cavity a "stomach." But though the etymo logy of the term quite allows of its being thus applied, still the definite character of this organ in the higher animals seems to sug gest that we should either restrict its applica tion, or recollect the doubtful meaning which it acquires by such an extended use. When ever an organ of this kind appears to effect the solution of substances which pertain to the albuminous groupt, it is entitled to rank as a true stornach. But in proportion as this fact is uncertain or improbable, the name becomes a vague designation, which ought never to be made use of without recollecting what it really means—a mere receptacle of food. In the instances before us, such a receptacle probably represents, not only the stomach of the higher animal, but a fusion of this with the succeeding1 portions of the tube, and with all the accessory organs of dige.stion,—such as the liver, pancreas, &c. And just as such simple cavities import more than a mere gastric function, so conversely we might find others bearing the same name, which com plicate a fully developed alimentary canal, and thus imply less. These, though called stomachs, are probably mere crops.
A complex digestive organ might at first sight seem to be the natural antithesis of the preceding. But though complexity forms a useful subjective contrast, without which we could indeed hardly conceive of simplicity— still, as already hinted, instead of a progressive evolution, corresponding to a gradual and suc cessive advance of development, the alimentary canal rather offers a variety of deviations. And most of these deviations appear to result from causes, the number and intricacy of which is such as to defy all analysis. We shall therefore only enumerate those, the influence of which seenis to be most direct and im portant.
1. It is scarcely necessary for us to dwell upon that advance of development, and gradual increase of complexity, which the reader must have observed in the preceding sketch. He has seen how, in progressing from the low est Iufusory to the highest Mammal, a simple excavation first became a membranous canal ; how it then acquired an additional orifice ; au organ of masfication ; a salivary appa ratus ; a stomachal dilatation ; a subdivi sion of intestine ; a liver ; a pancreas ; and, finally, a compound character of mucous membrane, by virtue of which the whole tube might be compared to one vast expanse or aggregation of glands. Some of these par ticulars will again force themselves upon our attention. Hence we may here limit ourselves to the remark, that the main elements of this advance consist in the evolution or separation of accessories, and the increase and subdi vision of surface :—and that both of these conditions imply a division of' labour which, here as elsewhere, enhances both the quantity and quality of its product.
2. Respecting the homologies of the intes tinal canal, scarcely anything can be said. As might be expected, form seems always subordinate to purpose :—in other words, neither general nor individual development offers us any permanent or temporary organs of digestion, from which we can deduce a shape that can be considered as a common pattern or archetype.* In rare instances,—as
in the Earth-worm and Arachnidan,—the form of the internal canal approaches that of the body and limbs respectively. But even this peculiarity of form is probably teleological.
3. Sufficient allusion has alreadyt been made to vegetative or irrelative repetition, as a pos sible explanation of the complex canal seen in many- of the lower Invertebrata. Some of these ramified canals— such as those of the Acalephw, and, with less probability, of the Distomze— may be conjectured to represent a vascular, rather than intestinal, system. But there are others —such as those of the Leech and Spider—which seem to be true processes of the digestive canal, used as reservoirs of food.
4. Some complications seem mainly de pendent upon circumstances which may be termed collateral or subordinate to digestion itself. Thus, the crops of many animals, like the cells of the Camel's stomach, are connected with the more or less necessary habit of gorging large quantities of food at distant intervals. While the gizzard, which is possessed by such very different orders as Polyps, Molluscs, Fishes, and Birds, appears to be closely related, not only to the food, but to the mechanical conditions of the animal. This is especially the case with the Bird, whose long neck, and habits of flight, could scarcely be rendered compatible with a heavy masticatory organ occupying the or dinary position.
5. The import of some of those numerous blind tubes or pouches which we have so often noticed as opening into the intestinal canal, has already been suggested in the preceding remarks. They are generally, and no doubt correctly, regarded as earlier developmental forms of the various conglo merate glands which are appended to the canal in higher animals. But as regards the principles of their diagnosis, and the limits of its application, it seems important to remind the reader, that, in the present state of organic chemistry, the situation of their apertures, and the order of their appearance, often constitute our only guides. Thus, for in stance, tubes which open into the commence ment of the canal, especially in connection with a higher development of the masticatory organs, are probably salivary. In like man ner, those which empty themselves in the neighbourhood of the pylorus are supposed to be biliary. And any which, by communi cating with the anus and exterior of the body, appear to aim at an immediate and direct extrusion of their contents, naturally remind the physiologist of the highly poisonous characters of the urinary secretion, and so far entitle him to suspect that they serve to expel this important product of animal life. Here, however, chemistry would often assist his de cision. The colour of the bile sometimes affords a less certain aid to the diagnosis of this secretion. The order of appearance only helps our conjectures by showing, that, of the two glands which open into the median por tion of the digestive tube, the liver is the more constant and important :—and hence, that it is probable a solitary set of tubes are chiefly hepatic. But it is obvious that, in many in stances, all these aids to conjecture may leave us in doubt as to the true nature of a set of secerning tubes.