230 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



unattached on one of their surfaces, whilst the other is continuous with the 

 tissues which they overlie. The principal part of the substance of all these 

 membranes is made up of the Simple Fibrous tissues described in the preceding 

 section, interwoven so closely as to form a sort of condensed areolar tissue, with 

 which bloodvessels, lymphatics, nerves, and smooth muscular fibres may be 

 blended in varying proportions. The fibres of this tissue are continuous with 

 those of the looser texture that lies beneath its attached surface, and there is con- 

 sequently no definite boundary to the membrane on that side. But the free sur- 

 .face is covered by a layer of basement membrane, which forms a complete limit, 

 not only to the fibres, but also to the vessels, i^rves, &c., of the subjacent tissue. 

 This membrane, it is true, cannot always be distinguished; but there is strong 

 analogical ground for believing in the universality of its presence. Supported 

 by this basement membrane, and covering what would otherwise be its exposed 

 face, we find one or more layers of cells ; and these may have very different 

 endowments in different situations, so as to impart very diversified characters 

 to the surfaces which they form. Whilst all the membranes now under con- 

 sideration agree in consisting of the foregoing elements, they differ amongst 

 each other in regard alike to the relative proportions of their components, and 

 to the mode in which they are arranged. There are three principal categories, 

 however, under which they are capable of being grouped together, viz, the Skin, 

 the Mucous Membranes, and the Serous Membranes ; the first of these forming 

 the external integument ; the second being continued from it at various points, 

 so as to line all the open cavities of the body ; and the third forming closed 

 sacs, which intervene between surfaces that rub or glide one over the other. 

 Of these, the Serous Membranes are the least distinguished by the speciality of 

 their endowments ; and they may, therefore, be advantageously considered in 

 the first instance. 



227. Serous and Synovial Membranes. These membranes, which are so 

 named from the nature of the fluid with which their free surface is moistened, 

 are thin and transparent, so as to allow the colour of subjacent parts to be seen 

 through them. They are endowed, however, with a considerable amount of 

 strength, and possess much elasticity in situations where mobility is particularly 

 required. Their free surface, which is smooth and glistening, is found, when 

 examined with a microscope, to be covered with a single layer of flattened poly- 

 gonal cells, usually of a tolerably regular hexagonal shape, constituting what is 

 designated as a "tessellated" or a "pavement epithelium" ( 230); and beneath 

 this a layer of basement membrane is affirmed by Messrs. Todd and Bowman to be 

 clearly distinguishable. The principal part of the substance of the membrane is 

 composed of what may be considered as condensed areolar tissue, into which the 

 yellow fibrous element largely enters, its filaments interlacing into a beautiful 

 network, and thus imparting a high degree of elasticity to its texture. This 

 gradually passes into that laxer variety, by which the membrane is attached to 

 the parts it covers, and which is commonly known as the " sub-serous" tissue ; 

 here fat-cells are not unfrequently found. The bloodvessels of Serous mem- 

 branes usually have a simple plexiform arrangement, parallel to the surface, and 

 are seldom very copious ; but those of Synovial membranes are far more nume- 

 rous, and their minutest ramifications are remarkable for their length and tortu- 

 osity (Fig. 53) a disposition which seems to have reference to the nutrition of 

 the Cartilage beneath. The Synovial membranes are further distinguished by 

 the presence of numerous fringe-like processes, of extreme vascularity, hanging 

 down loosely into the cavity of the joint; these are covered with an epithelium 

 of a very different character from that already described, its cells being large, 

 spheroidal, and very loosely attached to the surface beneath ; and there can be 

 little doubt that they constitute the secreting apparatus for the synovial fluid. 

 The JBursse interposed between the prominences of bones and the tendons or in- 



