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more than common ardour in bis style of expression, arising from his admiration of that wonderful adaptation of parts and principles which is so exceedingly conspicuous in this portion of the animal structure and economy.
The gullet, passing down between the vertebra of the back posteriorly and the wind-pipe anteriorly, terminates in the stomach, at its left extremity. This organ, the stomach, is a membranous pouch, which lies across the
and left part of the abdomen, immediately under the diaphragm, and between the spleen, which is on its left side, and the liver on the right. It is not, properly speaking, at the left extremity of the stomach that the opening is made into it from the gullet; for there is a considerable curvature from the orifice, by which the food that passes into the stomach, is partly prevented from returning; while, at the opposite extremity,—that by which the organ is connected with the intestines, - we find a thickening or doubling of its coat, which so projects from the orifice towards the intestine, that a sort of valve is formed, also preventing regurgitation; and a ring of fibres is also found here, which constitutes a sort of sphinctes to the stomach, yielding and contracting according to the demands of the organ under different circumstances.
We shall not enter further into an anatomical description of the intestines, than by stating, that along a great portion of their internal surface, numberless small vessels arise by open mouths that are destined to convey the nutritious part of the food into the blood-vessels. These vessels, which are called lacteals from the milky appearance of their contents, pass, in their way on to the blood vessels, through a large number of glands, called the mesenteric glands, and which are often the seat and source of much disease, especially in the infantile period of life. Having traversed these glands, the lacteal vessels become fewer and larger, so as to form a set of trunks that ultimately unite into the Thoracic duct, which opens directly into one of the large veins of the body (the subclavian), and thus pours the chyle at once into the mass of circus lating blood.
This is not the whole of the digestive or assimilating organization; but we must here suspend our description, in order to point out the alteration which the aliment undergoes while yet in the stomach, which alteration constitutes the main portion of the digestive process.
Upon the internal surface of the stomach, a tine membrane is every where expanded, which secretes the Auid called the gastric uice, respecting both the quantity and quality of which, much discrepancy of statement has obtained. This has partly arisen from the extreme difficulty attendant upon
the collecting of the liquid unconnected with other secretions that are poured out from the same membrane which supplies the liquid in question.
• It is moreover by no means improbable,' remarks Dr. Paris, that this liquid may vary in different stomachs, or even in the same stomach under different circumstances. Majendie observes, that the contact of different sorts of food upon the mucous membrane, may possibly influence its composition. It is at least certain, that the gastric juice varies in different animals; for example, that of man is incapable of acting (readily) on bones, while that of the dog digests these substances perfectly. From the best authorities on this subject, the true gastric juice would seem to be a glairy fluid not very diffusible in water, and possessing the power of coagulating certain fluids in a very eminent degree. Dr. Fordyce states, that six or seven grains of the inner coat of the stomach infused in water, gave a liquor which coagulated more than a hundred ounces of milk. Some authors have regarded it as colourless and without taste or smell, while others have described it as being acidulous. Dr. Young, of Edinburgh, is stated to have found that an infusion of the inner coat of the stomach, which had been previously washed with water, and afterwards with a dilute solution of carbonate of potass, still retained the power of coagulating milk very readily. We see, therefore, how unfounded that opinion is, which attributes to the potation of water, , the mischief of diluting the gastric Auid, and thus of weakening the digestive process. The coagulating and efficient principle, whatever it may be, is evidently not diffusible in that liquid. After one fit of vomiting, should another take place after a short interval, the matter brought up will be little more than water with a slight saline impregnation and some mucus ; it will not be found to possess any power of coagulating; which, Dr. Fordyce observes, evidently shows, that even water, flowing from the exhalents, and which we should therefore expect would throw off the whole of any substance from the surface of the stomach, is incapable of detaching the gastric juice.
The gastric juice, Dr. Paris adds, is remarkable for three qualities—a coagulating, an antiputrescent, and a solvent power. The well-known experiments of Spallanzani, of Reaumur, and of Stevens, are sufficiently satisfactory as to the last of these qualities; and the coagulating principle is rendered evident, as well by what has already been advanced, as by the fact, that milk coagulates instantly upon being exposed to the action of the gastric Auid, even out of the body. But the experiments of Thackrah have thrown some doubts on the accuracy of Fordyce's inference with respect to the power of this fluid in correcting putrefaction. Upon the whole, the change operated upon aliment by the digestive juice, is more analogous to solution, than to any other principle influeneing inanimate matter; yet, it is a solution of a specific kind, accompanied with a peculiar kind of action; and all attempts at establishing an analogy between the action of chemical agents upon dead matter and the functions of the stomach, have proved completely abortive. Some physiologists,' said John Hunter, while addressing his pupils, will have it, that the • stomach is a mill; others, that it is a fermenting-vat ; others,
again, that it is a stew-pan. But in my view of the matter, it • is neither a mill, a fermenting-vat, nor a stew-pan, but a sto• mach, gentlemen, a stomach.'
When large masses of aliment are received into the stomach, and only part of it at a time can be exposed to the internal surface of the organ, so as to come under the influence of the gastric secretion, it seems difficult to conceive upon what principle, or in what manner, the several portions of the food are successively made to come into contact with it, and so to be acted upon as that the whole shall be duly changed into chyme.
Dr. Wilson Philip has made several observations on the stomachs of rabbits which had been killed at different periods after having taken food; and he remarks, that • the first thing which strikes the eye on examining the stomach of those animals which have lately eaten, is, that the new is never mixed with the old food. The former is always found in the centre, surrounded on all siles with the old food; except that, on the upper part, between the new food and the smaller curvature of the stomach, there is sometimes little or no old food.'
And he goes on to state" that, in proportion as the food is digested, it is moved along the great curvature, where the change in it is rendered more perfect, to the pyloric portion. Thus, the layer of food lying next the surface of the stomach is first digested, and, in proportion as this undergoes the proper change, and is moved on the muscular action of the stomach, that next in turn succeeds to undergo the same change.'
Mr. Thackrah, however, maintains that, the gastric secretion being called forth in proportion to the quantity of the aliment taken, the centre of the mass of food becomes at length per
* John Hunter here alludes to the futile attempts of former physiologists to refer digestion to attrition, fermentation, and heat. It was one of the great peculiarities of this great man, that he investigated the principles and phenomena of life upon their own grounds, without attempting any forced analogies with other departments of nature.
vaded in a direct way, without the necessity of those curiously successive movements which the experiments of Dr. Philip led him to infer, always have place., Even Mr. Thackrah, indeed, admits, that the portion of aliment which is in immediate contact with the stomach, will be most dissolved, and that, in order to move this onwards towards the right extremity of the organ, and thus to make way for that which is less affected, the fibres of the muscular coat are thrown into successive action, and the peristaltic or vermicular motion is produced; a motion which takes place through the whole length of the alimentary canal, and by virtue of which the gradual propulsion of the aliment is produced.
The right extremity of the stomach, or that by which the organ joins the intestines, is called by anatomists the Pylorus; and says Mr. Thackrah, with a liveliness of manner bor. dering. perhaps, on bad taste :
• The office of this door-keeper is not a sinecure. He must ex. amine the qualifications of every applicant, and allow those only that are in a suitable state to pass his portal. Accordingly, the muscular ring contracting, drives back all undigested matter, and compels it to perform again the round of the stomach. It appears, however, that the pylorus, like other officers, may, by repeated solicitation, be induced to transgress his orders; for clasp-knives, halfpence, and, I believe also, pence and crowns, have been sent through the aperture. It is related that Vaillant, when pursued by corsairs, swallowed twenty valuable gold medals, which at length passed the canal ; and that he even sold one of them by anticipation, before it had made its appearance. Several substances also, difficult of solution, but harmless either from their nature or their size, are permitted to pass ; sometimes indeed are early thrown into the intestines, in order, as it would seem, that the stomach might employ its energies on food more soluble or nutritious.'
The aliment thus prepared by the gastric juice and saliva, (for it ought to be recollected, that a part of the process of assimilation is performed by the saliva itself,) is termed chyme; 'a term,' remarks Mr. Hare, vague and indefinite, since
chyme (like its etymology xupos) means juice of any kind, and • alimentary pulp is something more than juice. The propriety of its name is, however, of small importance; but it becomes a question of great interest, whether this homogeneous paste'. be always the same, from whatever materials it may have been formed, or whether it varies with the variation of the food. M. Majendie has lately examined the subject with great precision, and it would seem to follow from his experiments, that · there are as many species of chyme as there are varieties of food. It may therefore be inferred, that the salivary and
gastric secretions, with the muscular motions of the stomach, effect but a part in the great and important business of assimilation.
It will now, then, be in order to proceed, in our anatomical sketch, to the mention of those parts and organs which have been with some propriety named the assistant chylopoietics. The first of the small intestines is named the Duodenum ; and it has been ascertained, that a sort of second digestion takes place in this reservoir of the chyme, partly, perhaps, effected by the secretion from the inner coat of the intestine itself, but more thoroughly or substantially from the admixture which it here receives with the fluid from the liver and the pancreas; the first being the largest and, apparently, the most important of the viscera that are subservient to the assimilating function; the second, the pancreas, being likewise necessary to the com. pletion of the chyliferous process,
The Liver, which is found on the right side and upper part of the abdominal cavity, is composed of a congeries of bloodvessels, nerves, cellular substance, and secretory pores, and is connected with other parts, as well as retained in its situation, by several membranous expansions that are with some impropriety termed ligaments. The secretory pores, of which we have spoken, become gradually larger, until they eventually form a considerable duct, which conveys the secretion from the gland into the duodenum. But this duct, before it constitutes what is called the common duct, divides into two branches; or rather, it receives a brancb from the gall-bladder; and the junction of the hepatic duct and the duct from the gall-bladder together, form the common duct for the conveyance of the bile into the intestine.
The Pancreas, which is usually called the sweet-bread in the inferior animals, lies across the upper and back part of the abdomen, between the stomach and the spine. Pores or acini, as they are termed by anatomists, likewise arise in this organ, which gradually become larger and larger, so as at length to form one common duct, which terminates also in the duodenum, either along with the common duct from the liver, or, in some cases, at a little distance from it. Thus is the chyme in the duodenum acted on, conjunctively, by the secretion from the coats of the intestine itself, by the gall or bile from the liver, and by the juice formed from the pancreas; which last considerably resembles, both in appearance and properties, the secretion, of which we have spoken, from the salivary glands, a similarity to which we shall afterwards have to refer.
In this organ, then, (the duodenum,) a sort of second digestion is effected; or, to say the least, the alimentary mass is not