124 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



is usually much more complicated. These line the three great cavities 

 of the body, the head, chest, and abdomen, together with their sub- 

 divisions ; enveloping the viscera which these contain, so as to afford 

 them an external coating over every part save that by which they are 

 suspended ; and being then reflected over the interior of the cavity, so as 

 to form a shut sac intervening between its outer walls and its contents. 

 The chief purpose of this appears to be, to facilitate the movements of 

 the contained organs, by forming smooth surfaces which shall freely 

 glide over each other ; this is evidently of great importance, where such 

 constantly moving organs as the heart and lungs are concerned. 



198. The free or unattached surface of these membranes is covered 

 with a layer of cells ; but these constitute a distinct tissue, the Epithe- 

 lium, of which an account will be given hereafter. The epithelium lies 

 upon a continuous sheet of membrane, of extreme delicacy, in which no 

 definite structure can be discovered ; the nature of this, which is called 

 the basement or primary membrane, will be presently considered ( 206). 

 Beneath this is a layer of condensed Areolar tissue, which constitutes 

 the chief thickness of the serous membrane, and confers upon it its 

 strength and elasticity ; this gradually passes into that laxer variety, 

 by which the membrane is attached to the parts it lines, and which is 

 commonly known as the sub-serous tissue. The yellow fibrous element 

 enters largely into the composition of the membrane itself; and its fila- 

 ments interlace in a beautiful network, which -confers upon it equal elas- 

 ticity in every direction. The membrane is traversed by blood-vessels, 

 nerves, and lymphatics, in varying proportions ; some of the synovial 

 membranes, especially that of the knee-joint, are furnished with little 

 fringe-like projections, which are extremely vascular, and which seem 

 especially concerned in the secretion of the synovial fluid. The fluid of 

 the serous cavities is so nearly the same as the serum of the blood, that 

 the simple act of transudation is sufficient to account for its presence in 

 their sacs ; on the other hand, that of the Synovial capsules, and of the 

 Bursse Mucosae which resemble them, may be considered as serum with 

 from 6 to 8 per cent, of additional albumen. 



199. The elements of Areolar tissue enter largely also into two other 

 textures, which perform a most important share in both the Organic and 

 the Animal functions ; namely, the Mucous Membranes and the Skin. 

 These textures are continuous with each other ; and may, in fact, be con- 

 sidered as one and the same, modified in its different parts according to 

 the function it is destined to perform-. Thus it is everywhere extremely 

 vascular ; but the supply of blood in the Skin is chiefly destined for the 

 nervous system, and is necessary to the act of sensation ; whilst that of 

 the internal skin or Mucous Membrane is rather subservient to the pro- 

 cesses of absorption and secretion. This tissue is continued inwards 

 from the external surface of the body, by the several orifices and outlets 

 of its cavities ; and it is further continued most extensively from its 

 primary internal prolongations, into the inmost recesses of the glandular 

 structures. 



200. Thus the G-astro-intestinal mucous membrane commences at the 

 mouth, and lines the whole alimentary canal from the mouth to the anus, 

 where it again becomes continuous with the skin ; and it sends off as 



