In addition to these important relations between the agminatc follicle on the one hand, and the vascular and intestinal cavi ties on the other, recent observations have shown that there is a third, which is perhaps quite as intimately connected with its func tion : — namely, the connection of the follicle with the commencement of the lacteal system. For the general analogy of the intestinal follicle to the vascular gland is far surpassed by that dose structural resemblance which Koelliker has shown that it possesses to the follicles of the lymphatic glands. The latter, indeed, exhibit a remarkable similarity to the structure of the agminate follicles. Like them, they enclose vessels as well as cells, within the cavity formed by their limitary wall. Hitherto it has certainly been found impossible to verify the presence of lacteals within the agminate follicles; or to establish the existence of any direct communication between their cavities and that of the lacteal vessels them selves. But in spite of this, it seems certain, both that the lacteals occupy the patches in numbers quite disproportionate to the small and few villi here present ; and that they possess the closest proximity with the contents of the follicles. Such a* conclusion must, I think, be dravvn from Bruecke's researches ; in which the cavity of the follicle soon became slightly coloured with reddened turpentine, which had been injected into the lacteals by compres.sing the distended intestine.
It remains, however, for future researches to determine how far this view is correct, and whether the agminate follicles do really par take of the nature and office of lymphatic glands. In any case, their very variable number, and their occasional absence, would seem to indicate, that (like the similar struc tures always present in the tonsils, and some times found in large numbers within the mucous membrane of the stomach) their function is either not very important ; or—what is far more likely — can be more or less replaced by that of other kindred organs. And from the number and size of these follicles, we may perhaps corjecture, that their merely quanti tative effect on the chemistry of the organism is not very great.
The little that is known of their changes in health and disease confirms what has just been stated respecting their relations to the vascular and lymphatic systems. Thus, dur ing digestion, they become so swollen, as to project from the inner surface of the intes tine:— a condition that may, perhaps, be due to increased absorption from the intestine, but is better ascribed to that energetic determina tion of blood to the vihole of the intestinal structures which then takes place. During the violent drain of cholera the same tumid condition obtains : probably from a similar cause. And finally, the remarkable parallel between the disease of these follicles and that of the neighbouring mesenteric glands, which is seen both in phthisis and typhoid fever, is a strong additional argument for the reality of that analogy which physiology indi cates to exist between the two structures.
Solitaty follicles. — The solitary follicles are so completely what their name implies — isolated structures of the same kind as those which are aggregated to form the " patch" that any further description of their mi nute anatomy would be quite superfluous. Indeed, those smaller patches which are formed by. two or three follicles, may be almost regarded as a transition between the "solitary" and " agminate" arrangement. But these scattered solitary follicles are seldom or never surrounded by a definite circle of the apertures of intestinal tubes. And they often sustain villi of the usual size and shape. Their number is extremely variable. Some dines they seem to be altogether absent. But a very careful examination will now and then show, that such a deficiency is one in appear ance only ; and is due to the very slight degree of distention which obtains in the follicles really present. Whether this explanation would always hold good is more doubtful: though the remarkable constancy with which these structures are found in most animals and in the human fietus is, to say the least, a strong confirmation of its gencral truth. In other instances they are strewn thickly over the whole intestinal canal, from the cesophagus to the anus. Such au excessive development is perhaps strictly a morbid phenomenon. But it is also capable of ex planation as a mere collective hypertrophy ;— an overgrowth which results in an increased number of these minute organs, instead of an increased size of each individual follicle. Which of these two views is the more correct, will only be decided when we know more respecting their office.
They usually occupy the whole of the small and large intestine in considerable numbers. They are, however, more numerous in the latter of these two segments of the diges tive tube. And here they also present a larger size, as well as what is generally a deeper situation in the sub-mucous areolar tissue. Hence the depression that indi cates the follicle in the small intestine is exaggerated, in the large intestine, into a deep fossa ; which, commencing by an aperture over each follicle, widens as it passes downwards between the opposed sides of a few contigu ous intestinal tubes, to terminate, near their extremities, on the bulging surface of the fol licle.
Racemose, or Brunn's glands.— The re maining constituent of the compound intes tinal membrane is one which, unlike all the preceding minute organs, is limited to a very small segment of the canal. It consists of a number of highly ramified tubes, which are usually termed the glands of Brunn, but might preferably be named the raceniose or duodenal glands. These glands occupy that upper part of the small intestine already distinguished as the duodenum ; of which segment they are thus, as it were, the natural index, or the characteristic structure.