CHAPTER IV.. Organs of Digestion--the Stomach--the Gastric Juice.

ORGANS OF DIGESTION--THE STOMACH--THE GASTRIC JUICE.

Surprising power of digestion--Variety of sources of food--All structures, however different, formed from the same blood--General view of digestion, chymification, chylification, sanguification, nutrition--The stomach in polypes, in quadrupeds, and in man--Its position, size, and complexity, in different animals--Its structure; its peritoneal, muscular, and villous coats; and uses of each--Its nerves and bloodvessels, their nature, origins, and uses--The former the medium of communication between the brain and stomach--Their relation to undigested food--Animals not conscious of what goes on in the stomach--Advantages of this arrangement--The gastric juice the grand agent in digestion--Its origin and nature--Singular case of gunshot wound making a permanent opening into the stomach--Instructive experiments made by Dr Beaumont--Important results.

If, in the whole animal economy, where all is admirable, there be one operation which on reflection appears more wonderful than another, and which evinces in a higher degree the prodigious resources and power of the Creator in fashioning every thing to His own will, it is perhaps that by which the same kind of nutriment is extracted from the most opposite varieties of food consumed by living beings. For, singular as it may appear, recent researches tend to establish the fact, that, even in animals differing so widely in their aliment as the herbivorous and carnivorous quadrupeds, the ultimate products of digestion in both--the chyle and the blood--are identical in composition, in so far at least as can be determined by their chemical analysis.

Remarkable, however, as this uniformity of result undoubtedly is, it becomes still more striking when we contemplate the variety of sources from which food is derived for the support of animal life. To use the words of an able writer already quoted, “There is no part of the organized structure of an animal or vegetable, however dense its texture or acrid its qualities, that may not, under certain circumstances, become the food of some species of insect, or contribute in some mode to the support of animal life. The more succulent parts of plants, such as the leaves or softer stems, are the principal sources of nourishment to the greater number of larger quadrupeds, to multitudes of insects, as well as to numerous tribes of other animals. Some plants are more particularly assigned as the appropriate nutriment of particular species, which would perish if these ceased to grow: thus the silk-worm subsists almost exclusively upon the leaves of the mulberry-tree; and many species of caterpillars are attached each to a particular plant, which they prefer to all others. There are at least fifty different species of insects that feed upon the common nettle; and plants of which the juices are most acrid and poisonous to the generality of animals, such as euphorbium, henbane, and nightshade, afford a wholesome and delicious food to others.” Nor are the precision and accuracy with which the same fluid--the blood--affords to every structure of the body the precise species of nourishment or secretion which its elementary composition requires, however different each may be from the rest in chemical qualities, less admirable and extraordinary than its own original formation from such a variety of materials. To bone, the blood furnishes the elements of bone with unerring accuracy; to muscle the same blood furnishes the elements of muscle,--to nerves the elements of nerve,--to skin the elements of skin,--and to vessels the elements of vessels;--and yet, while each of these differs somewhat in composition from the others, the constituent elements of the blood by which they are furnished are everywhere the same.

Similar phenomena, indeed, occur in the vegetable world; but this, instead of diminishing our wonder, tends rather to augment it. The same elements, extracted from the same soil, are converted into every variety of vegetable product--into leaves of every shade of green, flowers of every form and tint, and juices of every quality, from the deadly poison up to bland and life-supporting milk. Nay, even in the same plant--as in the poppy--we sometimes find the seeds and the capsule which covers them endowed with the most opposite properties.

It would be very interesting to discover by what resources Nature thus effects the production of the same kind of nutritive fluid or chyle, from so great a variety of substances, and apportions to every part the precise elements of which it stands in need; but it is doubtful whether the human faculties were ever designed to penetrate so far into the modes of vital action, and, in the mean time, it will be better for us to confine our attention to that branch of the inquiry which we can easily comprehend, and which bears a direct reference to our own welfare. We know already that certain organs are concerned in the processes above mentioned, and that these organs act under the regulation of certain general laws. If we make ourselves acquainted with, and carefully fulfil, these laws, we reap a rich reward in the enjoyment of sound and vigorous digestion. Whereas, if, either from ignorance or from carelessness, we neglect their fulfilment, we bring upon ourselves a severe punishment in the form of dyspeptic or nervous disease. Assuredly, then, alternatives like these ought to excite some desire for information in the minds even of the most indifferent.

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Before, however, commencing a description of the organs concerned in digestion, it will be useful to take a general view of the different stages of preparation through which the food passes, between its reception into the stomach and its assimilation, or ultimate conversion into a constituent part of the animal body, and becoming endowed with the properties of life. The reader will thus be better enabled to understand the meaning of various terms and expressions, the frequent use of which it is almost impossible to avoid, even in the beginning of our exposition.

When the food is received into the stomach, it is there subjected to the action of a solvent fluid, called the gastric or stomach juice (from γαστηξ, gaster, stomach), by which it is gradually converted into a soft greyish and pultaceous mass, called chyme, (from χυμος, chymos, humour or juice); whence the process is called chymification, or chyme-making. The chyme, as fast as it is formed, is expelled by the contractile power of the stomach into the duodenum (from duodenus, consisting of twelve, because it is supposed to be about twelve inches long), or first portion of the intestines. It there meets with the bile from the liver, and with the pancreatic juice, which very much resembles the saliva, from the pancreas or sweet bread (πας, pas, all, and κρεας, kreas, flesh, it being of a fleshy consistence), a large gland which lies across the spine a little below the stomach, and is marked P in the wood-cut given in the chapter on Chylification. By the action of these two fluids, the chyme is converted into two distinct portions,--a milky white fluid named chyle (from χυλος, chylos, chyle), and a thick yellow residue. This process is called chylification or chyle-making. The chyle is then sucked in by absorbent vessels, extensively ramified on the inner membrane or lining of the bowels, and sometimes named, from the white colour of their contents, lacteals or milk-bearers (from lac, milk). These lacteals ultimately converge into one trunk, named the thoracic duct or chest-pipe (from its course lying through the thorax or chest), and which terminates, as will be seen in a cut in