But we are not left to such arguments alone to disprove the solvent powers ascribed to the intestinal juice by the above observers. They receive a still more direct contradiction from the experiments made by Frerichs* and by Lehmann.t These authorities concur to state, that neither in nor out of the body can it dissolve the protein cornpounds. And Leh mann's case may be regarded as affording much more than an ordinary negative re sult ; since in it, all communication between the stomach and the fistulous aperture made use of, seems to have been excluded.
Finally, we may recall to the reader that close parallel which was observed in the action of the gastric juice and the infusion of sto mach ;—how, allowing for dilution and im purity, we found the latter behaving just like the former. Now, in strilcing contrast to this significant fact, numerous observers t agree in representing the infusion of intestine as utterly incapable of that solvent action attributed by Bidder and Zander to its secretion. Indeed, Koelliker and Goll have found the capa city of digesting protein-compounds so inti mately connected with the structure of the proper gastric tubes, as to be almost lost in the pyloric extremity of the Dog's stomach ; where these begin to assume the characters of intestinal tubes by losing their oval gastric cells.
Hence all these circumstances throw great doubt on the alleged solvent powers of the in testinal juice ; and render it impossible for us at present to decide what is the exact digestive office which it fulfils. And we are almost as ignorant of its quantity as of its quality. But it is probably secreted by the small intestine in much greater amount than by the large. According to Bidder and Schmidt, it is poured out most freely about five or six hours after a meal. And drinking soon increases its amount, without causing any converse diminution of its concentration. Its strongly alkaline reaction may be conjectured to have some relation to that large quantity of acid, which is appa rently withdrawn from the chloride of sodium contained in the blood of the stomach, in order to furnish the gastric juice. Indeed, a liberation of soda or some other alkaline base, appears almost implied in that of the hydro chloric acid. But hitherto no exact analysis has informed us to what particular substance the alkaline character of the intestinal juice is immediately due. And it is only after a
careful comparison of the composition and quantity of this secretion with those of the less alkaline bile and pancreatic fluid that we should be entitled to conjecture, how far the neutralization* of the acid peptone constitutes a special function of the intestinal juice. Still, from the great extent of secreting surface which yields this juice, we can hardly doubt, that it takes a large share in this neutralizing process, which was formerly attributed chiefly to the bile. It probably thus forms part of that cycle of alternate decomposition and recomposition, which appears to be under gone by the chloride of sodium.
The vascular arrangements by which these intestinal tubes are supplied with blood, so closely resemble those of the stomach-tubes, as to render any special description of them superfluous. Like the tubes themselves, the vessels are chiefly concerned with secretion. But while we are left in doubt as to the pre cise degree or kind of that absorptive function which the vessels of these tubes possess, in common with those of all such mucous sur faces, we are perhaps justified in attributing a special capacity of absorption to the plexus of large capillaries, which here, as in the former organ, lies immediately beneath the epithelium, around their open extremities. The loops of this superficial plexus are generally more simple than in the stomach. They encircle the month of each tube with what is often only a single ring of capillary (b ,fig. 258.) ; except in the neighbourhood of the solitary or agminate follicles, where they resemble the analogous gastric vessels in forming more complex meshes (a fig. 20.) They communicate very freely with the capil laries of the neighbouring villi. And the venous radiclesof these latter processes usually unite with the branches formed by their con for the most part disappears ; and the villi, which are still more or less flattened, have about twice the length, and half the width, of those present in the upper part. But it is in the upper part of the jejunum that they attain thew greatest number ; being placed so closely together that their interstices scarcely flux in a small vein ; that sinks vertically through the mucous membrane, to join the sub-mucous plexus which gives origin to the portal vein.