Bloodvessels being merely pipes serving for the conveyance of nourishment from the place where it is stored up and prepared, to those more distant parts which stand in need of it, it is obvious that in animals of this very simple description, where every part of the internal surface of the body is already in immediate contact with its food, and itself imbibes or absorbs directly all the nourishment which it requires, they would be entirely superfluous, and accordingly none are to be found.
Owing to this extreme simplicity of organization, a hydra may be cut into pieces, each of which will become a perfect animal. But in proportion as we ascend in the scale of creation, the organization becomes more complicated, and the functions more numerous and also more dependent on each other; so that, when we arrive at man—the highest of all—we find that the loss or injury of any vital organ puts a stop to, or impairs, the action of all the rest. It is impossible, however, in an elementary work like this, to trace the gradation through the different series of animals. We must confine our examination to man, and only borrow from other creatures such illustrations as may be necessary for throwing light upon the human structure.
In man, then, the stomach is a large membranous and muscular bag, lying under the convexity of the lower ribs of the left side, and stretching towards the right a little beyond the hollow commonly called the pit of the stomach. In shape it somewhat resembles the bag of a bagpipe, as will be seen from the figure on page [67]; its left or larger extremity being in contact with the ribs, and its right or narrow extremity situated under the pit of the stomach.
The position of the stomach (Stm.) relatively to the chest, bowels, and liver, will be understood by inspecting the figure on the next page. It is separated from the cavity of the chest by the diaphragm or midriff DD, with which its upper surface is in immediate contact, and through which the gullet passes to enter its left extremity. Its right or pyloric extremity, marked P in the cut on the opposite page, lies close to the lower surface of the liver (Livr.), the latter being a little displaced to shew its situation. On the lower surface it has the appearance of resting on the intestines, as if imbedded among their folds.
The parts of the stomach which have received names and require to be noticed, are,—the cardiac orifice (marked C in the next figure, and so named from being near the κεαρ, kear, or heart), in which the gullet terminates, and through which food and drink are introduced; P, the pylorus, or pyloric orifice (from πυλωρος, pyloros, a gate-keeper, because it allows none but digested food to pass out), where the intestine called the duodenum begins, and through which the chyme passes after digestion is completed, and which, when the stomach is full, is nearly on a level with the cardia, although when empty it is lower; SS, the smaller arch or curvature; and GGG, the greater arch or curvature. The spleen is attached to that part of the larger arch marked with dotted points. From the situation of the cardia C, and its connection with the gullet, it will be at once perceived that this forms one of the points of attachment by which the stomach is retained in its place.
In size the stomach varies much in different individuals, as well as in different animals, according to the bulk and quality of their food. As a general rule, it is larger among the labouring poor than among the rich, as the former require a larger quantity of their inferior food to obtain from it an equal amount of nourishment. For the same reason, animals which subsist on vegetable substances have a very capacious stomach, while those subsisting on animal or concentrated food have it simple and small. In man its capacity may be diminished or augmented within certain limits, by corresponding modifications of diet. In some gluttons, and in cases of diseased appetite, it has been found of enormous dimensions; but this rule does not always hold, for the stomach is sometimes smaller than usual in immoderate eaters, and then its contents pass through only partially digested.
In accordance with this relation between the capacity and structure of the organs of digestion and the quality of the food, the stomach and intestines are found to be very small and short in carnivorous quadrupeds and birds of prey, which are intended to live on concentrated aliment. The same is the case with the granivorous or grain-eating birds, as their food also is contained in a small bulk. But in herbivorous animals—in the food of which the nutritive principle forms a very small proportion of the whole, perhaps not one-twentieth, and which consequently require a large bulk of it for their sustenance—the digestive apparatus is on a large scale, as any one may conceive in a moment, by comparing the portly protuberance of the cow with the lank paunch of the greyhound. The cow, in fact, is little else but a living laboratory for the conversion of vegetable into animal matter; and accordingly, not only is its stomach large and complex, but its intestinal canal is nearly twenty-four times the length of its body; whereas, in some carnivorous animals, the whole intestine does not exceed once their own length.
