But the more accurate researches which have recently been made on the nature and amount of these secretions, confirm a suspi cion that has long been entertained with respect to some of them by physiologists. Comparing their quantity and quality with that of the fwces and the food, we can now confi dently state, that but a very small fraction of their whole mass leaves the canal with the excrements ; by far the greater part of it being reabsorbed into the vessels of the ali mentary canal.
This proposition —so important to a cor rect appreciation of the true office of the in testinal canal, and of-the relation of digestion to nutrition—has lately been placed in the clearest light by the admirable researches of Bidder and Schmidt upon animals. From their toilsotne and accurate experiments, it would appear, that the total quantity of matter which thus leaves and returns to the cir culation of an adult man, may be esti mated at little less* than 20 pounds of liquid daily ; of which about 3 per cent. consists of solids in solution. The importance of these "reerementitious " secretions to the system, is well shown by the results which follow the establishment of an artificial biliary fistula. Unless the ensuing loss of bile is compen sated by the digestion of a much larger quan tity of food,the animal so operated on soon dies of inanition. And it is probable that the ex haustion produced by diarrhma, or by the discharge of the intestinal contents through an abnormal opening in the bowel, may be partially due to a similar loss of this and other rich organic fluids, which ought to be reab sorbed.
Whether the secretions experience any change prior to absorption —whether any of them are really modified, and thus far digested by their colleagues —rernains at present in doubt. It may be conjectured, however, that they are so altered. At any rate, it would seem that, by provoking these secretions 1-, the whole system of a starving animal may be for a time invigorated and restored. But the chain of these phenomena is at present too indistinctly seen, and their connection with various other organic processes much too obscure, to justify us in doing more than offer ing this conjecture, as one of the most imme diate explanations of certain well-known facts.
But we know enough to state that, within the limits of ingestion and egestion, lie two corresponding acts of absorption and secre tion. Each of these is, so to speak, the co
efficient of two elements. Absorption takes up food and secretions : secretion pours out, not only materials newly devoted to this purpose by the system, but others which have, in all probability, already subserved it many times before. The great mass of the intesti nal secretions is thus continually revolving in a cycle :—forming a circulation the channel of which, placed in the intestinal canal, leaves and returns to the blood that flows in its walls ; and only allows a very small offohoot of its current to reach the outer world, bear ing with it certain of its effete particles.
The important chemical details of this cir culation have yet to be won by sedulous and thoughtful " questionings of nature." But since, for the acquisition of such results, the liver offers what will probably be the easiest prize, it may be useful to point out how little even the vast progress of modern chemistry has hitherto been able to establish respecting its true physiological itnport. The portal blood, charged with the water, fat, albumen, salts, and extractive, which it has taken up from the food, and from the secretions of the diges tive organs, reaches a large gland. There it breaks up, as it were, into two streams of fluid: bile, and hepatic-venous blood. And hence, the composition of these two fluid products, compared with its own, might be expected to give us a clue to the process by which they originate, if not to the action of the secreting structure itself.
Such an examination would show that the hepatic blood has lost almost all the fibrin, half the albumen, much of the water, and half the fat (even more of the clain) present in the portal vein. It has gained in extrac tive, and especially (ten to sixteen times as much) in sugar. And its pale corpuscles are increased in number.
On the other hand, the organic constituents of the bile are chiefly fatty substances, espe cially the fatty cholic acid and its congeners. The quantity and quality of most of these sub stances show that they have probably been formed in the liver : and hence that their pre sence in the bi e is not to be explained as a mere transudation of certain dissolved con stituents of the blood, followed by their con centration in the gland, such as might be alleged in the case of most of its salts.