circulation of the digestive fluids from the blood vessels to the alimentary canal, and from the alimentary canal back again to the blood vessels.

That this circulation really does take place is proved by the following facts: First, if a dog be killed some hours after feeding, there is never more than a very small quantity of fluid found in the stomach, just sufficient to smear over and penetrate the half digested pieces of meat; and, secondly, in the living animal, gastric juice, drawn from the fistula five or six hours after digestion has been going on, contains little or no more organic matter in solution than that extracted fifteen to thirty minutes after the introduction of food. It has evidently been freshly secreted; and, in order to obtain gastric juice saturated with alimentary matter, it must be artificially digested with food in test tubes, where this constant absorption and renovation cannot take place.

An unnecessary difficulty has sometimes been felt in understanding how it is that the gastric juice, which digests so readily all albuminous substances, should not destroy the walls of the stomach itself, which are composed of similar materials. This, in fact, was brought forward at an early day, as an insuperable objection to the doctrine of Reaumur and Spallanzani, that digestion was a process of chemical solution performed by a digestive fluid. It was said to be impossible that a fluid capable of dissolving animal matters should be secreted by the walls of the stomach without attacking them also, and destroying the organ by which it was produced. Since that time, various complicated hypotheses have been framed, in order to reconcile these apparently contradictory facts. The true explanation, however, as we believe, lies in this-that the process of digestion is not a simple solution, but a catalytic transformation of the alimentary substances, produced by contact with the pepsine of the gastric juice. We know that all the organic substances in the living tissues are constantly undergoing, in the process of nutrition, a series of catalytic changes, which are charac teristic of the vital operations, and which are determined by the organized materials with which they are in contact, and by all the other conditions present in the living organism. These changes, therefore, of nutrition, secretion, &c., necessarily exclude for the time all other catalyses, and take precedence of them. In the same way, any dead organic matter, exposed to warmth, air, and moist ure, putrefies; but if immersed in gastric juice, at the same temperature, the putrefactive changes are stopped or altogether

prevented, because the catalytic actions, excited by the gastric juice, take precedence of those which constitute putrefaction. For a similar reason, the organic ingredient of the gastric juice, which acts readily on dead animal matter, has no effect on the living tissues of the stomach, because they are already subject to other catalytic influences, which exclude those of digestion, as well as those of putrefaction. As soon as life departs, however, and the peculiar actions taking place in the living tissues come to an end with the stoppage of the circulation, the walls of the stomach are really attacked by the gastric juice remaining in its cavity, and are more or less completely digested and liquefied. In the human subject, it is rare to make an examination of the body twenty-four or thirty-six hours after death, without finding the mucous membrane of the great pouch of the stomach more or less softened and disintegrated from this cause. Sometimes the mucous membrane is altogether destroyed, and the submucous cellular layer exposed; and occasionally, when death has taken place suddenly during active digestion, while the stomach contained an abundance of gastric juice, all the coats of the organ have been found destroyed, and a perforation produced leading into the peritoneal cavity. These post-mortem changes show that, after death, the gastric juice really dissolves the coats of the stomach without difficulty. But during life, the chemical changes of nutrition, which are going on in their tissues, protect them from its influence, and effectually preserve their integrity.

The secretion of the gastric juice is much influenced by nervous conditions. It was noticed by Dr. Beaumont, in his experiments upon St. Martin, that irritation of the temper, and other moral causes, would frequently diminish or altogether suspend the supply of the gastric fluids. Any febrile action in the system, or any unusual fatigue, was liable to exert a similar effect. Every one is aware how readily any mental disturbance, such as anxiety, anger, or vexation, will take away the appetite and interfere with digestion. Any nervous impression of this kind, occurring at the commencement of digestion, seems moreover to produce some change which has a lasting effect upon the process; for it is very often noticed that when any annoyance, hurry, or anxiety occurs soon after the food has been taken, though it may last only for a few moments, the digestive process is not only liable to be suspended for the time, but to be permanently disturbed during the entire day. In order that digestion, therefore, may go on properly in the

stomach, food must be taken only when the appetite demands it; it should also be thoroughly masticated at the outset; and, finally, both mind and body, particularly during the commencement of the process, should be free from any unusual or disagreeable excite

ment.

INTESTINAL JUICES, AND THE DIGESTION OF SUGAR AND STARCH.

From the stomach, those portions of the food which have not already suffered digestion, pass into the third division of the alimentary canal, viz., the small intestine. As already mentioned, it is only the albuminous matters which are digested in the stomach. Cane sugar, it is true, is slowly converted by the gastric juice, outside the body, into glucose. We have found that ten grains of cane sugar, dissolved in 3ss of gastric juice, give traces of glucose at the end of two hours; and in three hours, the quantity of this substance is considerable. It cannot be shown, however, that the gastric juice exerts this effect on sugar during ordinary digestion. If pure cane sugar be given to a dog with a gastric fistula, while digestion of meat is going on, it disappears in from two to three hours, without any glucose being detected in the fluids withdrawn from the stomach. It is, therefore, either directly absorbed under the form of cane sugar, or passes, little by little, into the duodenum, where the intestinal fluids at once convert it into glucose.

It is equally certain that starchy matters are not digested in the stomach, but pass unchanged into the small intestine. Here they meet with the mixed intestinal fluids, which act at once upon the starch, and convert it rapidly into sugar. The intestinal fluids, taken from the duodenum of a recently killed dog, exert this transforming action upon starch with the greatest promptitude, if mixed with it in a test-tube and kept at the temperature of 100° F. Starch is converted into sugar by this means much more rapidly and certainly than by the saliva; and experiment shows that the intestinal fluids are the active agents in its digestion during life. If a dog be fed with a mixture of meat and boiled starch, and killed a short time after the meal, the stomach is found to contain starch but no sugar; while in the small intestine there is an abundance of sugar, and but little or no starch. If some observers have failed to detect sugar in the intestine after feeding the animal with starch, it is because they have delayed the examination until too

late. For it is remarkable how rapidly starchy substances, if previously disintegrated by boiling, are disposed of in the digestive process. If a dog, for example, be fed as above with boiled starch and meat, while some of the meat remains in the stomach for eight, nine, or ten hours, the starch begins immediately to pass into the intestine, where it is at once converted into sugar, and then as rapidly absorbed. The whole of the starch may be converted into sugar, and completely absorbed, in an hour's time. We have even found, at the end of three-quarters of an hour, after a tolerably full meal of boiled starch and meat, that all trace of both starch and sugar had disappeared from both stomach and intestine. The rapidity with which this passage of the starch into the duodenum takes place varies, to some extent, in different animals, according to the general activity of the digestive apparatus; but it is always a comparatively rapid process, when the starch is already liquefied and is administered in a pure form. There can be no doubt that the natural place for the digestion of starchy matters is the small intestine, and that it is accomplished by the action of the intestinal juices.

Our knowledge is not very complete with regard to the exact nature of the fluids by which this digestion of the starch is accomplished. The juices taken from the duodenum are generally a mixture of three different secretions, viz., the bile, the pancreatic fluid, and the intestinal juice proper. Of these, the bile may be left out of the question; since it does not, when in a pure state, exert any digestive action on starch. The pancreatic juice, on the other hand, has the property

of converting starch into sugar; but it is not known whether this fluid be always present in the duodenum. The true intestinal juice is the product of two sets of glandular organs, seated in the substance of or beneath the mucous membrane, viz., the follicles of Lieberkühn and the glands of Brunner. The first of these, or Lieberkühn's follicles (Fig. 31), are the most numerous. They are simple,

Fig. 31.

FOLLICLES OF LIEBERKUHN, from Small Intestine of Pig

nearly straight tubules, lined with columnar epithelium, and somewhat similar in their appearance to the follicles of the pyloric portion of the stomach. They occupy the whole thickness of the mucous membrane, and are found in great numbers throughout the entire length of the small and large intestine.

The glands of Brunner (Fig. 32), or the duodenal glandulæ, as

they are sometimes called, are confined to the upper part of the duodenum, where they exist, as a closely set layer, in the deeper portion of the mucous membrane, extending downward a short distance from the pylorus. They are composed of a great number of rounded follicles, or culs-desac, clustered round a central excretory duct. Each follicle consists of a delicate membranous wall, lined with glandular epithelium, and covered on its surface with small, distinctly marked nuclei. The

follicles collected round each duct are bound together by a thin layer of areolar tissue, and covered with a plexus of capillary blood vessels.

The intestinal juice, which is the secreted product of the above glandular organs, has been less successfully studied than the other digestive fluids, owing to the difficulty of obtaining it in a pure state. The method usually adopted has been to make an opening in the abdomen of the living animal, take out a loop of intestine, empty it by gentle pressure, and then to shut off a portion of it from the rest of the intestinal cavity by a couple of ligatures, situated six or eight inches apart; after which the loop is returned into the abdomen, and the external wound closed by sutures. After six or eight hours the animal is killed, and the fluid, which has collected in the isolated portion of intestine, taken out and examined. The above was the method adopted by Frerichs. Bidder and Schmidt, in order to obtain pure intestinal juice, first tied the biliary and pancreatic ducts, so that both the bile and the pancreatic juice should be shut out from the intestine, and then estab