In the ensuing description these circum stances will be enlarged upon, and the excep tions and local peculiarities pointed out, as far as I have been able to ascertain them. At present, I would say that these two elements are generally present. The most interesting questions in animal physiology are involved iii the determination of the nature and offices of these two elementary parts of the mucous tissue. The discovery of them is, however, too recent, and our knowledge of their history as yet too incomplete, to allow of any certain conclusions on the subject. Both present various modifications in different situations, the study of which is of great importance with reference to their function. It will now be our business to descend to a particular account of each.
Of the basement membrane.—The basement membrane of the mucous tissue, as displayed in the kidney, is an extremely thin, transparent, and homogeneous lamina, simple and entire, without any aperture or appearance of structure (fig.273,c). It forms the parenchymal wall of the uriniferous tubules ; gives them their size, shape, and stability ; is in relation, on the one hand, with the vascular system of the organ, and on the other with the epithelial lining. It is simply in contact with the capiflar plexus, which is fixed chiefly by their mu tual interlacement; but the epithelium adhere to it by an organic union. When detache from the vascular rete which it traverses, any deprived of its epithelium, it readily wrinkle (fig. 273, c); and such is its tenuity, that i is sometimes only by the folds thus occasione. that it becomes visible at all. The epithelitti readily separates from it after a slight macera tion, and also in many diseased states of th organ, such as inflammation and Bright's di' ease. Though this basement tissue is so deli sate, its presence or absence in any fragmen of a separate tubule may always be ascertainei by the aspect of the marginal outline ; if thi be linear and well defined, the basement mem brane is present, but if irregular and broken the epithelium only (fig. 273, a, b). It some times happens that when the epithelium may seem to be altogether detached, the basement membrane retains, scattered evenly over iti surface and at some distance apart, a flambe of roundish marks, of the size and aspect 0 the nuclei of epithelium particles. These are appear to form an integrant part of the wall of the canal.
most probably the early condition of the new or advancing series of these particles. Its thickness in the kidney certainly does not ex ceed the of an English inch. I have dis covered, that in the Malpighian bodies of the kidney, which are the dilated extremities of the uriniferous ducts, with an enclosed tuft of arterial capillaries, the basement membrane is often, to some extent, naturally bare, i.e. w.thout a covering of epithelium. This is the only situa tion of the body in which such an arrangement is known.
In the testis, the same membrane may be shewn without difficulty to he that which gives to its secreting tubules their peculiar strength ; and here, as might be expected, it is somewhat modified. The difference is principally as regards its thickness, which here reaches of an English inch, and in some animals even exceeds that amount, its essential charac ters, however, remaining the same. In the
larger tubes, emerging from the gland, this tunic becomes gradually invested by a delicate fibrous layer, by which the vascular network is attached to it, and which at first sight may In the salivary and all the allied glands, the basement membrane admits of being easily demonstrated. A very thin slice of the fresh organ should be torn by needles, gently washed, and inspected under a high power. The termi nal vesicles of the duct will then be brought into view and their outline 'seen to be perfectly sharp and linear (fig. 275, a a a). In parts where the epithelium which they contain has been loosened, the basement membrane will be left in relief. It is of extreme delicacy ; and, as in all other situations, its capillary plexus (when well filled with coloured material) may he seen ramifying, not in its substance (for its tenuity renders such a disposition impossible), but on its parenchymal surface.
I have sought in vain for the basement mem brane in the lobules of the liver, and I am in clined to think that it does not exist in this gland, except in the excretory part of the bile ducts.
In the air-cells of the lungs the basement membrane assumes a most interest;ng and re markable developement, for it constitutes almost the entire thickness of their walls, the epithe lium being of extreme delicacy. It appears to be here strengthened by interlacing arches of elastic fibrous tissue, but to he itself transpa rent and homogeneous, as elsewhere. It is on its parenchymal surface that the close vascular web is spread out. (See Putmo.) But this membrane may be also detected in every part of the alimentary tube, which is more characteristically mucous, in that, viz. in tervening between the cardia and the lower ex tremity of the canal. Here it deserves an attentive study on account of the apparent com plexity of its foldings, and because its exist ence here offers the most unequivocal proof which we possess, of the anatomical identity of the true glands with the membranes usually called mucous. As it is more delicate in this part than any other, and difficult of detection by reason of the enormous preponderance of its epithelial investment, I shall describe the man ner in which it may be best observed. The specimen should be as fresh and healthy as possible, or should have been immersed in alcohol immediately on its removal from the body ; a fragment of the tubes of Lieberkiihn should then be scraped off, pulled to pieces, and examined in a fluid medium under a high power. The margins and rounded extre mities of the tubes will then be seen to be sharply defined, as in the cases already men tioned, and to be formed by a structure inde pendent of the epithelium, which latter forms atlis of their thickness. This structure is the basement membrane. When masses of epithe lium, escaped from the tubes and bearing their form, are met with floating around, their out line is uniformly irregular, and, as it were, woolly. Sometimes, as in the kidney (fig. 273), the basement membrane is seen up to a certain point only, beyond which it has been detached; and in less recent specimens a tube of basement membrane is sometimes seen, containing a mass of broken-down epithelium.