The racemose glands are small " conglo bate " masses ; which in their size, structure, and position, closely resernble those accessory salivary organs that stud various parts of the mucous membrane lining the cavity of the mouth. Like these " labial " and " buccal " glands, they occupy the sub-mucous areolar tissue : and are therefore best examined by pinning out a piece of the duodenum on some flat surface, with its mucous side downwards, and then carefully removing the serous and muscular coats. Such a dissection easily ex poses them, as small roundish white granules of about the size of a millet seed.
They vary considerably in size and arrange ment. Immediately beyond the pylorus, they are of one-tenth to one-eighth of an inch in diameter ; and are present in such num bers, as to form what is almost a glandular layer around this part of the intestine. But lower down in the duodenum, their size dwindles to one-half or one-third of the above : and their scattered grains gradually become more sparing in number ; until, shortly before the termination of its inferior transverse portion, they cease altogether.
On tracing out the structure of an isolated duodenal gland under the microscope, it is seen to consist of numerous lobules ; which are aggregated into a single mass (c c,fig. 272.), by an enveloping layer of fibrous tissue. And on applying a still higher magnifying power, each of these lobules may be again resolved into smaller ones, which resemble a bunch of grapes, and constitute the true or ultimate acini of the gland. As seen in these vesicles have a globular or slightly polyhedral form; and a diameter which is about 3-,1,--6th to Auth (on an average, 31,0th) of an inch. But when separated from each other, they often exhibit more irregular shapes (d, .fig,. 273.). They are the terminal dilatations of tubes, which are themselves about two-thirds of this size. On tracing these minute tubes towards the general mucous surface, they will be found uniting with other similar ones, to form larger ducts. The successive union of these with other ducts formed in the same way, and of these larger ducts with their neighbours, gradually causes all their cavities to converge into a single canal of outlet, which is the proper efferent duct of the gland. This duct now passes between the intestinal tubes before described, to open on the free surface of the intestine, in the depres sions which intervene between the tary villi present here. The valvulm conni
ventes are not permeated by any such ducts. But in all other parts of its surface, the general mucous membrane is pretty evenly studded by their apertures, two or more some times'passing through it in company with each other.
Hence each of these glands may be briefly described as a tube, which branches repeatedly, and ends in very minute canals, with some what vesicular extremities.
The whole of this involution of mucous membrane is composed of the usual elements ; namely, limitary membrane and epithelium. The former constituent offers no peculiarity worthy of note. The latter consists of a single tesselated layer of cells. These have a poly gonal and slightly flattened shape, which some what resembles that of the cells lining the smaller branches of the straight urinary tubules. Where the smaller tubes converge to form the efferent duct that perforates the mucous membrane, these cells are exchanged for short cylinders, the structure of which rapidly merges into that of the ordinary columnar epithelium of the general intestinal surface.
As regards the secretion of these racemose glands, we can only state that their ducts contain a structureless mucus, which has an alkaline reaction.
With such an imperfect knowledge of its na ture, we can scarcely wonder that the office of this fluid remains unknown to us. Like many other animal matters, it converts starch into sugar. But until the precise rapidity and energy of this change have been established, it is im possible to determine how far this action is really comparable to that of the saliva. From the close resemblance between the sub-mu cous glands of the mouth and these of the duo denum, many have assumed them to prepare a salivary fluid. But, besides that we are not warranted in regarding the secretion of the buccal glands as identical with the saliva (of which it forms but a very small ingredient), a very tnoderate. knowledge of histology might suffice to indicate the danger of inferring the nature of any secretion from the mere arrangement of the structures by which it is furnished. Hence it must remain for the present undecided, whether this mucus is a salivary or pancreatic fluid ; or merely a more concentrated form of intestinal juice, secreted by glands which here reach a higher degree of development than that attained by the short cylindrical tubes of the rest of the bowel.