Intestinal tubes.— The structure of the remaining constituents of the intestinal mu cous membrane can only be seen distinctly by the aid of the microscope. Amongst these minute organs, the intestinal tubes — or, as they are commonly called, the folli cles of Lieberkuelaz —are the first to de mand our notice. For, with slight modifica tions, they occupy the whole of the small and large intestine. An- allusion has already been made to the fact that, in many animals, they appear to usurp a portion of the gastric cavity. While the importance which this wide distri bution would imply, is confirmed by their im mense number ; which is such that we may estimate their aggregate surface as from ten to fifteen times that of the cylinder of intes tine into which they open.
Each tube may be described as a hollow cylinder, having a length which is about five times its width, and ending below in a rounded a diameter which amounts to about one-fourth the width of the entire tube.
The arrangement of these tubes so precisely recalls that of the gastric glands into which their structure appears sometimes to merge, as scarcely to require any separate descrip tion. Like these, they are placed vertically side by side, in a sparing quantity of dense fibrous matrix ; and are imbedded by their lower extremities in a layer of a similar ap pearance. The latter contain much unstriped muscle, the characters of which can be seen blind extremity. Its average diameter is about 3.,11,th of an inch, except at its orifice, where it is somewhat wider. The lower part of the tube is often slightly enlarged : and rarely it appears to bifurcate. But while it is doubtful whether these appearances can be depended upon*, it is certain that they are not sufficiently marked or frequent to alter the above general description.
This cylindrical tube is composed of base ment membrane and epithelium. The former constituent needs no special description. The latter is a single layer of short columnar cells. It clothes the whole interior surface of the tubes ; and becomes continuous, at their upper extremities, with the epithelium covering the villi, the constituent cells of which generally appear to be somewhat longer in shape. The cjlindrical cavity bounded by these cells has even more distinctly than in the analogous ga.stric structure. The agereg,ate mass of these vertical tubes forms the great bulk of the mucous membrane. So that a vertical section of this tunic exposes a dense pallisade of tubes, the depth of which corresponds to the thickness of the membrane: while a trans verse one shows that the interstices of the cylinders are only occupied by a scanty matrix enclosing numerous vessels.
The chief interruption to their presence is caused by the racemose glands, villi, and fol licles, which will shortly be described. The ducts of the first of these three structures merely occupy a certain portion of space which would otherwise be taken up by follicles. But the two latter encroach upon the surface of the mucous membrane much more con siderably. And since it is only between the villi that we find the intestinal tubes so the number of such tubes which occupy the inter vals of these processes must necessarily cor respond to the thickness with which the latter are strewed over the surface. Over the more projecting parts of the follicles, the tubes are also absent ; in a circular space which is sur rounded by a ring of apertures. The latter belong to the inflected upper extremities of those tubes which immediately encircle each follicle.
When fresh, these tubes always exhibit the structure just described ; their only contents being a clear, structureless, homogeneous fluid. But from their minute size, it is obvious that this fluid can never be obtained from their inte rior in sufficient quantity for any trustworthy analysis. While unless the secerning process were extremely rapid, even the secretion poured forth into the tube would often be mixed with those coarsely filtered contents of the intestine which can enter its upper orifice from the general cavity of the alimentary canal.
And as regards all fluids found in the gene ral cavity of the intestine, we ought never to forget that to procure them in a state of abso lute purity is impossible. Under normal cir cumstances, the fluid present in any part of the bowel can only be regarded as a complex mix ture of several ingredients, all of which are probably themselves undergoing a continual metamorphosis. Could we deduct from the contents of the intestine all chyme, bile, and pancreatic secretion, the residue would be strictly an intestinal juice. And by far the larger quantity of such a juice would be com posed of the secretion of the intestinal tubes. Now we shall hereafter find that analogy sup plies us with some plausible conjectures re specting the fluid secreted by the duodenal glands. While the closed follicles which abut on the cavity of the bowel can scarcely furnish sufficient fluid seriously to affect the composition of any mixture which it may contain. Hence whatever the share taken by the villi, the secretory office of these tubes might apparently be to some extent determined from an examination into the chemical and physiological properties of even such an im pure or mixed intestinal juice.