The force and rapidity of these muscular contractions are modified by the more or less stimulant nature of the food, the state of health, exercise, and other circumstances; but, according to Dr Beaumont, the ordinary direction in which they take place, and the course which they impart to the food, are as follows.
The alimentary bolus or morsel, on entering the cardiac orifice, turns to the left, follows the line of the great curvature of the stomach towards the pylorus, returns in the line of the smaller curvature, makes its appearance again at the cardia, and then descends as before to the great curvature, to undergo similar revolutions till digestion be completed. Each revolution occupies about from one to three minutes, and its rapidity increases as chymification advances.
In treating of muscular action in the former volume, I pointed out (p. [122]) the necessity of the co-operation of a nervous stimulus to produce the result; and remarked that there are two kinds of muscles, one called the voluntary, which contract at the command of the will, and the other the involuntary, over which the will has no control, and which act only in obedience to their own peculiar stimuli. Of the latter description are the muscular fibres of the stomach. They contract when the stimulus of food is applied to them, but we can neither contract nor relax them by an effort of the will, nor are we even conscious of their existence.
It is, indeed, fortunate for us that the necessary motions of the stomach are not entrusted to our guidance, like those of the hand or foot. Supposing that we were to eat three meals a-day, the digestion of each requiring three or four hours,—and that its management depended entirely upon our superintendence,—our whole attention would be required to the process, to the exclusion of every other duty, for ten or twelve hours a-day; and every time that our thoughts wandered for a few minutes, digestion would stand still, and the stomach be disordered by the chemical decomposition of the food which would ensue, so that it would be impossible for us to dedicate any time either to business or to social enjoyment. But from all these inconveniences we are entirely freed by the stomach being placed under the dominion of the involuntary nerves, and so constituted as to perform its functions without any aid from our will.
The third and innermost coat, called the mucous or villous, is that smooth, unequal, velvety membrane, of a reddish-white or pale pink colour, which lines the internal surface of the stomach. From being of much greater extent than the other two coats, its surface is thrown into rugæ, plicæ, folds, or wrinkles, which are simple in man, but very marked in some animals, as seen familiarly in tripe. The subjoined wood-cut, from the Library of Useful Knowledge, will give some notion of their appearance. Near the pyloric orifice the villous coat is doubled on itself, so as to form a ring, called the valve of the pylorus, the object of which is to prevent the too early exit of the food; this object, however, it accomplishes, not by any contractile power of its own, but by the aid of a layer of muscular fibres lying behind it. The villous coat is constantly covered with a very thin transparent viscid mucus, and its folds are always best seen in those who die suddenly. After disease, when the stomach is relaxed, they frequently disappear.
In addition to the folds just described, the mucous coat contains a great number of spheroidal glandular bodies or follicles, some of them scarcely larger than pin-heads, which lie immediately beneath and almost incorporated with it, and which are most numerous near the pylorus. Physiologists are not entirely agreed whether the fluid secreted by these follicles be the gastric juice or merely the mucus already referred to as lubricating the internal surface of the stomach. The latter, however, is the opinion generally entertained, and the one which is supported, as we shall afterwards see, by the strongest evidence; the gastric juice being, in fact, secreted directly from the capillary or hair-sized vessels in which the minute branches of the arteries terminate.
Of the nerves and bloodvessels supplying the stomach it is unnecessary to say much. We shall afterwards have occasion to notice the former at some length, and to the general reader the origin and distribution of the bloodvessels are as unimportant as they would be difficult of comprehension; for the nature of the red blood is the same by whatever artery it is supplied, and that of the dark blood the same by whatever vein it is returned to the heart. All that it is important to know is, that the stomach receives a large supply of blood by means of numerous bloodvessels, the principal of which, as represented on the wood-cut at page [67], follow the course of GG the greater and SS the smaller curvatures, and send off innumerable small branches as they proceed to every part of the stomach. The coronary artery and the pyloric branch of the hepatic or liver artery go to the smaller curvature, while another branch of the hepatic, and one from the splenic or spleen artery, are ramified on the larger curvature.
In determining the uses of the internal or villous coat of the stomach, we must begin by considering separately that of each of the elementary structures of which it is composed—its follicles, bloodvessels, and nerves—and the nature of the peculiar secretion, the gastric juice, to which it gives rise.