190

PROPERTIES OF GASTRIC JUICE.

alimentary mass into contact with its sides, so that the whole is after a time equally exposed to the influence of the gastric secretion. If this movement were not to take place, only the outside of the mass would be digested, and the central portion would remain but little affected.

A

207. The nature of the gastric fluid, and the mode of its operation upon the food, have been studied by withdrawing a portion of it from the stomach, and by observing its properties and actions out of the body. A sufficient quantity for this purpose cannot be easily procured. Spallanzani, an Italian physiologist of the last century, contrived to obtain it, by causing birds and other animals to swallow sponges to which pieces of thread were attached; these, when they had remained long enough in the stomach to cause a secretion of the gastric juice, were drawn up again; and the fluid they had absorbed was pressed out into vessels, in which its properties could be examined. More recently, however, an advantageous opportunity has presented itself for obtaining supplies of gastric fluid in a less objectionable manner. young man, named Alexis St. Martin, received a very severe wound in his left side, by the bursting of a gun; and although this wound laid open the cavity of his stomach, he recovered his health completely, and subsequently married and had a family. There remained, however, an aperture in his stomach, which would not close up; and through this orifice, which was usually covered by a bandage, the contents of the stomach could be drawn out. The gastric juice was obtained by introducing an India-rubber tube into the stomach when it was empty, and by moving it about within the cavity; the contact of the tube then excited the follicles to secretion (on the principle already mentioned, § 204); and the fluid thus poured into the stomach was drawn off through the tube.

208. The Gastric Juice is very like saliva in its appearance, but it is distinctly acid to the taste; and it is found, by chemical examination, to contain a considerable quantity of muriatic acid * in an uncombined state. Besides this, it contains a considerable quantity of a peculiar animal substance which seems like altered albumen, and which has been designated pepsin; as well as other ingredients of less importance.

* Muriatic acid is commonly known as spirit of salt.

ACTION OF GASTRIC JUICE.

191

This fluid possesses the power of dissolving albuminous substances of various kinds, when these are submitted to its action at the constant temperature of 100° (which is about that of the stomach), and are frequently shaken-up with it. The solution appears to be in all respects as perfect as that which naturally takes place in the stomach, but requires a longer time. It does not seem, however, that the gastric juice has a special solvent power for any other than albuminous substances. Gelatinous and saccharine matters are taken-up by it, as by other watery fluids; but neither starchy nor oleaginous substances undergo any other change by its action, than consists in the separation of their particles by the solution of the membranes and fibres which held them together. There is every reason to believe that what is true of artificial is true of natural digestion; and that so far from the whole operation being performed in the stomach, as was formerly supposed, gastric digestion is limited to the solution of the albuminous, gelatinous, and saccharine constituents of the food.

209. With regard to the precise mode in which the gastric fluid acts in dissolving albuminous substances, there is yet some uncertainty; although there can be no longer any reasonable doubt, that the operation is of a purely chemical nature. An artificial gastric fluid, capable of effecting all that can be done by that which is secreted in the living stomach, may be made, by macerating (or soaking) a portion of the membrane lining the stomach of a pig, or of the fourth stomach of a calf (even after it has been washed and dried) in water, which dissolves a portion of the pepsin; and by then acidulating this solution with muriatic or acetic acid. It has been proved that both the acid and the pepsin are essential to the process of solution; for the acidulated fluid without the animal matter acts extremely slowly upon pieces of meat, hard-boiled egg, &c., submitted to it; and water in which the stomach has been macerated, but which contains no acid, will not act at all. But the acidulated water alone will readily dissolve the substances just mentioned, at a higher temperature; and thus it appears that the acid is the real solvent; and that the pepsin has for its office to produce some change in the albuminous substances, by which they are more readily dissolved. The recent inquiries of Liebig and other

192

GASTRIC DIGESTION : CHYMIFICATION.

Chemists, render it probable that this change is of the nature of fermentation.

210. It is a fact of great practical importance, that a certain quantity of the gastric fluid can act only upon a limited amount of alimentary matter; so that, if more food be taken into the stomach than the gastric fluid can dissolve, it remains there undigested. Now it has been already mentioned, that the quantity of the gastric fluid secreted at any one time, is proportional, not to the amount of food in the stomach, but to the wants of the system; so that, if more food be swallowed than is required to repair the waste of the body, it lies for some time unchanged in the stomach, and becomes a source of irritation which prevents the due discharge of its functions; and the evil goes on increasing with every addition to the contents of the cavity. This may not be felt by the individual at the time; but it leaves permanent effects, which manifest themselves sooner or later in derangement of the general health. The habit of taking more food than is really necessary, and of irritating the stomach by stimulating substances or fluids (such as pepper, mustard, spirits, &c.), is a fertile source of disease. The injurious effects of these are manifested by the thirst which is the consequence of their use, and which is a call (as it were) on the part of the stomach, to prevent their irritating action by diluting them with water.

211. By the solution of its albuminous portion, and the separation of its other component particles, the food is reduced in the stomach to a kind of pulp, which is termed chyme. The consistence of this will of course vary according to the nature of the food, and the quantity of fluid in the stomach; but in general it is grayish, semi-fluid, and uniform throughout. When the food has been of a rich character, the aspect of the chyme resembles that of cream; but when the food has consisted of farinaceous substances (rice, potatoes, &c.), the chyme is more like gruel. At the point where the stomach opens into the intestinal canal, which is called the pylorus, there is a kind of valve, which permits the chyme to pass as fast as it is formed, but closes against the portions of the food which are yet solid and undigested; and thus the chyme escapes from the stomach in successive waves, slowly at first, but afterwards more rapidly, as the digestive process approaches its completion.

INTESTINAL DIGESTION,

Intestinal Digestion; Chylification.

193

212. The process of digestion is by no means completed in the stomach; for much of the matter which escapes from it in the chyme, is destined to undergo a further change whilst passing through the intestinal canal; especially in the herbivorous tribes, whose food, being less digestible than that of the carnivorous races, requires to be longer delayed in the intestinal canal, in order that it may yield up its nutritious portion. Hence we find this canal of enormous extent in most animals whose food is vegetable, being in the Sheep about twenty-eight times the length of the body; in the purely carnivorous animals, on the other hand, it is comparatively short, being in the Lion only about three times the length of the body, while in the Serpent it runs almost straight from one extremity to the other; and in animals which live on a mixed diet, it is of medium length, being in Man about six times as long as his body. The intestinal tube is usually distinguished into the small and the large intestine; of which the. small is the first portion, and the large the second. The former, as shown in fig. 108, is disposed in a convoluted or twisted manner, so that a great extent of it may be packed within a small compass; it usually forms about three-fourths of the whole length of the canal. It is held in its place by a serous membrane termed the peritoneum, which forms an immense number of folds that suspend it (as it were) from the vertebral column; but these still allow it a considerable power of movement.

213. Soon after passing from the stomach into the intestinal canal, the food is mingled with three secretions, which have an important influence on the changes it is further to undergo; these are the Bile, the Pancreatic fluid, and the Intestinal juice. The two former are prepared by two large glandular masses, the Liver and the Pancreas (or sweetbread), which, in all the higher animals, are completely detached from the alimentary canal, and send their secretions into it through special ducts; the latter, like the gastric juice, is formed in little follicles lodged in the wall of the canal itself. The peculiar matter which forms the chief solid constituent of bile, is essentially a soap formed by the union of two resinoid acids, with soda as a base (§ 364). The composition of the

194 BILIARY, PANCREATIC, AND INTESTINAL SECRETIONS.

pancreatic fluid closely corresponds with that of saliva, which it much resembles in appearance. The intestinal juice, like the gastric, is a nearly colourless, somewhat viscid fluid, containing an organic compound not far removed from albumen; but it differs from the gastric juice in being alkaline instead of acid. The relative offices of these three fluids have not yet been determined with certainty; but there appears good reason to believe: (1) that the bile, by its alkalinity, neutralizes the acidity which the chyme derives from the gastric juice, and that this neutralization favours the metamorphosis of starch into sugar, which has been almost suspended in the stomach; (2) that the bile aids the pancreatic fluid in reducing the oleaginous particles to the condition of an emulsion, that is, in bringing them into a state of very minute division, in which they remain suspended in the albuminous solution; (3) that the pancreatic fluid aids the salivary matter which was swallowed with the food, in the transformation of starch into sugar; (4) that the intestinal juice has a solvent power for albuminous substances which is scarcely inferior to that of the gastric juice, with a power of converting starch into sugar which is scarcely inferior to that of saliva or pancreatic fluid. The fluid of the Small Intestine, compounded of the salivary, gastric, intestinal, biliary, and pancreatic secretions, appears to possess a far greater digestive power than that of the stomach, being capable of dissolving, or at any rate of reducing to an absorbable condition, nutritious substances of every class. This process goes on during the passage of the alimentary mass along the small intes`tine; and the nutritious materials are progressively withdrawn by absorption, partly into the blood-vessels, which appear to receive whatever are in a state of perfect solution (§ 218), and partly into the lacteal absorbents, which take up nothing but that peculiar emulsion of albumen and fatty matter which is termed chyle (§ 222).

214. At the extremity of the Small Intestine, there is a kind of pouch, called the cœcum; which in some animals seems almost like a second stomach, and which is furnished with one or more little appendages, termed coca." *This is very small in Man, and does not seem to perform any important * The word cœcum is used in Anatomy to denote a tube closed at one extremity.