486 



MUCOUS MEMBRANE. 



becomes necessary to guard against confound- 

 ing such compound membranes with the simple 

 mucous tissue, of which these and all other por- 

 tions of the mucous system consist. 



Of the ultimate structure of the mucous 

 system. It has been already stated that the 

 mucous tissue is essentially an uninterrupted 

 membrane in which the other tissues of the 

 animal are contained. A very cursory attempt 

 will serve to shew how much more easily it 

 admits of being separated and examined in 

 certain situations rather than in others. This 

 variety depends chiefly on the difference of its 

 arrangement and connexions in different regions. 

 In the testis and kidney, the capillary vascular 

 rete, spread over its parenchymal surface, has 

 no intimate attachment to it, and the appended 

 areolar tissue is in very small quantity. In 

 the testis especially, this latter may be said 

 to be almost wanting as an investment to the 

 individual tubules, being, as it were, disposed 

 as a collective covering to the entire organ, 

 sending partial septa into its interior, and 

 bearing the name of Tunica Albuginea. In 

 the kidney, a more intricate vascular rete in 

 a great measure supplies the place of areolar 

 tissue. Hence, in these viscera, the simple 

 mucous element allows of being isolated with 

 remarkable facility. In the liver, its isolation 

 is almost impracticable, owing to its lying in 

 the interstices of a capillary plexus that may be 

 termed solid, from its being extended uniformly 

 in every direction. The intricacy of the inter- 

 lacement of the mucous and vascular elements 

 in this organ is sufficient to explain the total 

 ignorance that prevails concerning the mode 

 of termination of the biliary ducts, and con- 

 cerning their size, shape, and connexions in the 

 lobules of the gland. In many other true 

 glands, the mucous tissue may be submitted to 

 examination without much difficulty; examples 

 of which may be seen in the pancreas and 

 salivary glands, with those allied to them, such 

 as the duodenal glands of Brunner, the buccal, 

 palatal, arytenoid, tracheal, &c. ; and in the 

 sudorific glands of the skin. In the compound 

 membranes also, as that of the alimentary 

 canal below the cardia, and the more highly 

 developed parts of the skin, the mucous ele- 

 ment may be generally distinguished from the 

 tissues in connexion with it in a satisfactory 

 manner. But in the plane expansions of the 

 simple membrane which line the bony cavities 

 of the nose and ear, its isolation and the demon- 

 stration of its structure are far more difficult, 

 for reasons which will be afterwards explained, 

 and our knowledge of it here still rests partially 

 on the ground of analogy. 



In the mucous tissue there are two structures 

 that require to be separately described, viz. 

 the basement membrane and the epithelium. 

 The basement membrane is a simple homoge- 

 neous expansion, transparent, colourless, and 

 of extreme tenuity, situated on its parenchymal 

 surface, and giving it shape and strength. This 

 serves as a foundation on which the epithelium 

 rests. The epithelium is a pavement composed 

 of nucleated particles adhering together, and 

 of various size, form, and number. The fol- 



lowing general observations on these elementary 

 parts will receive illustration as we advance. 

 Neither the one nor the other is peculiar to the 

 mucous tissue in the sense either of being 

 invariably present in it, or of not being found 

 elsewhere. There are certain situations of the 

 mucous system where no basement membrane 

 can be detected, and others from which the 

 epithelium is absent. Both, however, are never 

 absent together. Airain, a structure apparently 

 identical with the basement membrane is met 

 with in numerous textures besides the mucous, 

 and all internal cavities, whether serous, syno- 

 vial, or vascular, or of anomalous kind, (as those 

 of the thymus, and thyroid body) are lined by 

 an epithelium. 



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 in 

 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 

 tiie urimferous 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 capillary 

 plexus, which is fixed chiefly by their mu- 

 tual interlacement; but the epithelium adheres 

 to it by an organic union. When detached 

 from the vascular rete which it traverses, and 

 deprived of its epithelium, it readily wrinkles 

 (fig- 273, c) ; and such is its tenuity, that it 

 is sometimes only by the folds thus occasioned 

 that it becomes visible at all. The epithelium 

 readily separates from it after a slight macera- 

 tion, and also in many diseased states of the 

 organ, such as inflammation and Bright's dis- 

 ease. Though this basement tissue is so deli- 

 cate, its presence or absence in any fragment 

 of a separate tubule may always be ascertained 

 by the aspect of the marginal outline; if this 

 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 its 

 surface and at some distance apart, a number 

 of roundish marks, of the size and aspect of 

 the nuclei of epithelium particles. These are 



