entirely. Thus Wingradoff has shown that frogs, in which this disease has been produced, recover if put in a cold place, a low temperature serving to check fermentation; but the disease reappears if the animal be replaced in an atmosphere sufficiently warm to allow of fermentation taking place.1 The most remarkable case, however, of diabetes artificially produced is that in which it is caused by special modifications wrought in the nervous system. Cl. Bernard discovered that if a puncture be made in the floor (in P', Fig. 71) of the fourth ventricle of an animal (a rabbit), between the roots of the

P

auditory and those of the pneumogastric nerves, sugar is found a short time afterwards (an hour and sometimes less) in the urine of the animal. (A puncture made a little higher up, as at P, produces glycosuria, accompanied by polyuria; a little higher up, the puncture produces albuminuria.) This glycosuria is caused by the hepatic function, Wingradoff having shown that if the fourth ventricle of a frog be pricked, thus producing diabetes, the disease will disappear if the liver, which is the sugar-producing organ, be removed. We know, on the other hand, that after a long course of slow poisoning by arsenic the liver loses its glycogenous matter and thus the power of producing sugar; and, in this case, a puncture in the fourth ventricle of an animal does not produce diabetes. The nerve-tract which unites the fourth ventricle to the liver appears to belong, not to the pneumogastric, but to the great sympathetic nerve, as was imagined by Cl. Bernard, and directly proved by Schiff and Moos: the latter, espe

Fig. 71.-Fourth ventricle (rabbit) and experimental punctures.*

1 See Cl. Bernard, “Cours du Collége de France." (In Revue des Cours Scientifiques, avril, 1873.)

*The lobes of the cerebellum are separated: below are seen the restiform bodies whose divergence surrounds the point of the calamus scriptorius and the fourth ventricle. The puncture P', which produces glycosuria, is situated a little above the point of the calamus. The puncture P is made at the level of the tubercles of Wenzel; that is to say, the origin of the auditory nerves. (Cl. Bornard.)

cially, has shown that, if all the sympathetic nerves leading to the liver of a frog be tied, diabetes can no longer be produced, either by puncture of the fourth ventricle or by electrical excitation of the spinal cord. In all these cases violent hyperemia of the liver appears to be necessary to the excitement of its glycogenic functions; indeed, if the inferior vena cava below the liver in a frog be tied, an increase of circulation in the portal vein is produced, followed by diabetes. This increase of circulation is caused by the anastomoses existing in this animal, between the venous system in general and the system of the portal vein. The congestion of the liver and excitation of its glycogenic function which follow a puncture made in the fourth ventricle do not, however, appear to be produced simply by a (nervous) paralytic hyperæmia, arising from the abolition of the vasomotor innervation; because the artificial diabetes thus produced is but temporary (lasting, at the most, twenty-four hours). This diabetes appears rather to arise from the excitation of certain nerves included in the network of the great sympathetic nerve, and which are to the liver what the chorda tympani is to the sub-maxillary gland (Cl. Bernard).

D. Organs of absorption. - Function of the chyliferous vessels.

We have seen how the digested matters reach the very substance of the villus by means of the epithelium. While the epithelium is being renewed (desquamation, etc.), the body of the villus empties its contents, and the absorbed elements are diffused into or through the vessels.

These vessels, however, are of two kinds: we have seen that there is a vascular blood network, forming the origin of the portal vein, and a central chyliferous vessel, the origin of the chyliferous vessels, which open into the principal trunk of the lymphatic circulation (thoracic duct. See lymphatic system, p. 156). The blood current, being placed so near the surface, is evidently in the most favorable situation to absorb whatever is brought to it by the epithelium: it is, therefore, generally supposed that the greater part of the absorbed matters are carried along by the blood; and it is true that we find the peptones and glucose again in the portal vein. But, while the fat is disappearing from the villus, we find that the central chyliferous vessel becomes quite white, and that a large number of delicately emulsionized fat molecules make their appearance in it; this seems to show that

the fats do not pass through the same organs as the preceding substances, and that the chyliferous vessel is especially appointed for their absorption.

We may, indeed, suppose that the fat contained in the intestine, is absorbed by the cells of the villus (epithelial and plasmatic cells), and that it is excreted by them into the central chyliferous vessel. We have already considered the lymphatic vessels as appointed to collect the deeper residuum, the waste produced by the life of the epitheliums (see p. 194).

The fat does not, however, pass through the lymphatic organs only; it is also found in the blood, although the quantity there is very small. The other matters which have been absorbed are also met with in the chyliferous vessels, but their quantity, compared with that of the fat, is infinitesimally small.

Some authors, however, entirely deny that the vessels of the portal circulation have the power of absorbing and carrying off the fat. This is because the fat found in the blood is not in the same state as in the chyle: in mammal's blood the fat is never in a free state, but always saponified; it is, no doubt, saponified by the choleate of soda in the bile.

Most poisonous substances are absorbed by the veins; intoxication taking place so rapidly that the poisons can scarcely be supposed to pass through the lymphatic organs.

Metals absorbed in the form of metallic salts, accumulate in the liver. This is an important fact, for it shows that the liver retains a large proportion of the alimentary substances for the purpose of modifying them. The albumen is transformed, because it comes in contact with the hepatic cells by means of the portal circulation.

We find, in short, that our knowiedge of this interior process of absorption is still very incomplete. We have been occupied in studying these phenomena in reference to the living cells in which absorption takes place, and we have considered the process of absorption as an essential feature of these globules. We have, therefore, paid little attention to the physical theories of absorption, or to experiments made with membranes deprived of life. Experiments of this kind have led to the belief that absorption is simply a phenomenon of osmosis. Thus J. Béclard considers the current of absorp

See Béclard, "Recherches Expérimentales sur les Fonctions de la Veine Porte." (Arch. Génér. de Médecine, 1848.)

tion as produced by the difference in the specific heat of those fluids which surround the membrane to be traversed: he looks upon the osmosis which then takes place as a physico-chemical property, in virtue of which the miscible fluids have a tendency to mix in the membrane, one current predominating over another. All other things being equal, the direction and intensity of the current are determined by the differences in specific heat. The figures given by J. Béclard, in support of this theory, showing the specific heat of the different fluids, agree perfectly with what we know of their flowing towards each other. However plausible this theory may appear, it is only a physical theory of osmosis; and knowing, as we do, the important function of the living cell, we cannot imagine that, in the phenomenon of intestinal absorption, it simply plays the part of an inert membrane.

V. LARGE INTESTINE.

THE aliments that pass out from the stomach form a fluid mass; we have seen that they become still more fluid by the addition of the pancreatic and enteric juices. However, as these matters pass through the small intestine, their consistency increases, while their bulk diminishes, the greater part being absorbed. The small intestine, therefore, delivers to the large intestine only a solid substance, or waste, which is to be thrown off, and is prevented from passing back again by the ileo-cæcal valve, which renders any reflux impossible. In man, very little digestive action takes place in the large intestine; the small amount which has escaped absorption are here, however, drawn into the blood current, and the large intestine may even absorb fluids directly introduced into it. After injection, by the rectum, of fatty substances (fats in a state of emulsion), the lymphatic vessels leading from the large intestine exhibit the same features, the same chyliferous appearance, as those of the small intestine. The villi are not found here, but their place is supplied by numerous folds in the mucous membrane. In herbivorous animals, whose cæcum is very much developed, this part of the intestinal tube is the seat of actual digestive phenomena: the cœcum may therefore be considered as a sort of second stomach; it contains acids which suffice for the digestion of the vegetable albuminoids. It is not certain that these acids are secreted from its walls: they are, more probably, produced by the aliments themselves. They increase in

quantity with the increase of substance in the canal. These acids are generally the lactic and butyric acids, arising from the fermentation and decomposition of the sugars and the fats.

Half-way through the large intestine, however, all digestion and absorption cease: the tube contains only those matters which are to be thrown off, - the faces, in short. The fæces have been wrongly considered as principally formed of that part of the food which cannot be assimilated: if this were true, if all the nourishment received can be absorbed, there ought to be no fæces, and yet they appear, even in this case. Thus the foetus, whose digestive tube is as yet empty, immediately after birth expels fæces which are well known under the name of meconium: the meconium is formed of remains of epithelial cells, colored yellow by the bile, which, not having yet become decomposed, preserves its natural color. This explains why the principal product thrown off, and of which the fæces are chiefly composed, consists of remains of the desquamated epithelium: sometimes, even in the adult, these remains alone form the substance of the fæces. They appear either as entire or as mutilated globules of a whitish color, variously tinged by the decomposed bile. These epithelial remains somewhat resemble the fine scales which fall from the cutaneous epidermis, but they are more numerous and important than this; for we have seen that the shedding of the epithelium is the fatal termination of the series of phenomena of absorption, and that the principal use of the bile is to regulate and to hasten its production.

Those parts of the aliments and of the digestive fluids which cannot be assimilated can only be classed as secondary elements in the constitution of the fæces. Among these are cholesterine and the coloring matter of the bile which are precipitated when this fluid enters the intestine; also fatty substances, when ingested in too large quantities; amylaceous substances protected by too thick a covering of cellulose; and cellulose, in general, with its derivatives. Indeed vege table aliments contain the largest quantity of substances which resist digestion, and the fæces of the herbivorous animals are, therefore, much more abundant than those of the carnivora. Animal food, however, also contains elements which long resist the influence of the digestive juices: thus the horny growths of the epidermis (hair, nails, etc.), and the yellow or elastic tissues (parts of tendons, of arterial coats, etc.), are found in the fæces almost entire.