To demonstrate the structure and arrange ment of the several constituent follicles of the agminate clusters, requires great care, and very delicate manipulation. At the free surface of the mucous membrane, they are extremely difficult to isolate ; both from their great tenuity, and their intimate union to the neighbouring tubes. Hence the best way of gaining access to them is from the outside of the intestine, where they may often be seen through the peritoneal and mus cular coats.t The removal of these tunics brings thern directly into view. To this method of examination must be added care ful section in the vertical and horizontal planes.
A proper combination of all these methods of investigation reveals the following facts.— Each follicle is a shut sac : having a roundi,h form, but a somewhat conical apex, which is directed towards the surface of the mucous membrane. Their diameter varies from 1 to 2 or 3-50ths of an inch. The base of each is in contact with the muscular coat (which is somewhat thinned in this situation) ; and is united to it by an areolar tissue that resembles the ordinary loose sub-mucous texture, in which the follicle is imbedded by the greater part of its bulk. The short apex of the follicle extends upwards between the lower extremities of the intestinal tubes : and, below their middle, it terminates in the immediate neighbourhood of the general mucous surface, which has already been described as depressed into a fossa in this situation. The thin stratum of tissue which intervenes between the follicle and the interior of the bowel (at i, fig. 270.) is so delicate, that its exact anatomy is not easily verified by .vertical sections. It appears, however, to con sist of a very small quantity of indistinctly fibrous tissue; which encloses some capillaries, and is covered by the ordinary layer of co lumnar epithelium.
The frequent rupture of the follicle in this situation has led many to regard it as either possessing a permanent orifice here, or ac quiring one by a kind of natural dehiscence. But later researches seem to show that this open state is quite exceptional and accidental; being due to disease, putrefaction, or mechanical violence. The author can at least express his own conviction that—as Boehm long ago stated,—the agminate follicles are closed sacs. This conclusion is much confirmed by the fact, that the follicles of some animals are altogether beneath the mucous membrane and the tubes, and quite distinct front both of them ; so as to lie wholly in the sub-mucous areolar tissue.* While the vascular arrange
ments which we are about to describe seem equally incompatible with any theory of their normal dehiscence.
Each of these follicles essentially consists of a capsule, enclosing a number of delicate capillaries, the interstices of which are occu pied by a cell-growth consisting of various forms.
The capsule is a structure which, though analogous to a basement membrane, differs from such a delicate homogeneous lamina, both in being much thicker, and in offering an in distinctly fibrous texture. Its smooth outer surfitce contains elastic fibres, and is attached by loose areolar tissue to the surrounding sub-mucous structures.
The vessels of the follicle offer a very peculiar arrangement. The small arteries in the sub-mucous areolar tissue give off branches that ramify amongst the several follicles of each " patch ;" and thus form a network of up into numerous capillaries of about solooth of an inch in diameter. The latter surround the membranous wall of the follicle vvith an irregular plexus. But where they- reach the middle of that part of it which projects into the interior of the intestine, they_are curved back upon themselves ; so as to form long loops, that radiate from a central space quite uncovered by vessels. And this appealance, which was depicted long ago by Boehm, does but represent, at the surthce of the follicle, what the more recent researches of Frei prove to be the vascular arrangement that per vades the whole of its interior. The injections of this anatomist show, that the capsule is not only surrounded by the network just men tioned, but is also penetrated by a number of minute capillaries. These, which are of a still smaller diameter than the former ves sels, leave them at right angles, and reach nearly to the centre of the follicle, before looping back again to its exterior. And finally, some of the uppermost of them have been traced by Koelliker uniting to form the radicle of one or two veins of about To-V.,th of an inch in diameter; which descend verti cally through the follicle, without receiving any further branches from the neighbouring capillaries.