SEROUS AND SYNOVIAL MEMBRANES. 



523 



vaginales. Thus they are connected with 

 the organs of respiration, circulation, diges- 

 tion, generation, and innervation. Perhaps, 

 under this accurate allotment of serous mem- 

 branes to these several functions, many im- 

 portant analogies lie hidden, but the inter- 

 pretation of these hieroglyphics of nature 

 scarcely belongs to the present elementary 

 sketch ; and it will be both safer and more 

 profitable to regard their relations to the 

 three first of these functions as being deter- 

 mined mainly by the necessity of movement 

 which a high development of any one of them 

 implies, although the general protection 

 which mobility affords must not be lost sight 

 of. The relation of a separate membrane 

 to the function of generation seems, as it 

 were, the accidental result of position : the 

 tunica vaginalis, an offshoot of the perito- 

 neum, is prolonged from it by the testicle in 

 its descent from out of the abdominal cavity, 

 and is subsequently isolated by a degene- 

 ration of the serous membrane into areolar 

 tissue along the spermatic cord which con- 

 nects this gland with the interior of the 

 belly. So the arrangement of the arachnoid 

 around the nervous centre is, perhaps, more 

 related to the comparative delicacy of its 

 structure, and the movements inseparable 

 from circulation, than to the function of in- 

 nervation itself. 



A prominent feature in the anatomy of all 

 these structures is the remarkable continuity 

 of surface which they exhibit. With a single 

 exception, indeed, their interior surface, like 

 that of the subcutaneous bursas, is everywhere 

 a continuous one ; and hence the definition 

 of a serous membrane always includes the 

 statement, that it is " a shut sac," while this 

 peculiarity of arrangement is constituted 

 their " morphological character." 



A complete description of the serous mem- 

 branes would comprise two chief divisions of 

 the subject. One of these would include the 

 relative situation and arrangement of the 

 neighbouring textures, as well as the various 

 folds or processes by which the membranes 

 preserve their continuity in the intervals 

 between the viscera which they cover and the 

 cavities which they line. The other would 

 limit attention to their general structure ; and 

 to any variations in the nature, proportions, 

 or arrangement of their constituent tissues, 

 which may be obtained by a comparison of 

 the several membranes with each other. In 

 the present instance, the latter only of these 

 divisions will be briefly attempted ; for the 

 former of the two, the reader is referred to 

 the articles under the several headings of 

 PLEURA, PERITONEUM, HEART, NERVOUS 

 CENTRES, TESTICLE, &c. 



The epithelium of serous membranes con- 

 sists of flattened cells. The shape of most 

 of these is roundish-polygonal, and many of 

 them closely approximate to the hexagonal 

 form : and they are arranged in a single 

 layer, so as to form a tesselated pavement, 

 which everywhere constitutes the free sur- 

 face of the membrane. Their diameter varies 



considerably, but, generally speaking, is about 

 one 1000th of an inch. Their depth is 

 nearly one-fourth of this width; but it tapers 

 away towards the edge of the particle, and is 

 greatest at its centre, where it is usually 

 somewhat bulged by the presence of a tole- 

 rably large nucleus, which is contained in the 

 cavity of the cell, but is placed nearer its 

 inferior or attached surface than the opposite 

 or free one. This nucleus is of an oval or 

 spheroidal form, and contains a single bright 

 refractile spot or nucleolus ; but not tinfre- 

 quently there are two of these. Besides the 

 nucleus, the cell includes a small quantity of 

 contents, which are of somewhat viscid 

 consistence, and are usually almost trans- 

 parent, but sometimes, and especially after 

 exposure to the action of water, become 

 mottled or faintly granular. The attachment 

 of these cells to each other is very remark- 

 able, but their adhesion to the textures on 

 which they are placed is much less considera- 

 ble ; and this preponderance of their adhesion 

 in the horizontal direction renders it very 

 easy to strip off a number of them, and ex- 

 hibit the layer which they form by their 

 union. In this circumstance they offer a 

 marked and probably important difference 

 from the cells which clothe the interior of 

 bursas and synovial membranes. Acetic acid 

 exerts its ordinary effects, causing the cells to 

 swell out, and thus defining their polygonal 

 shape more accurately than before. 



The exceptions to these general cha- 

 racters are few. In one instance, namely 

 in the peritoneum of the female, the form 

 of the cellular covering is said to differ 

 from the above ; the ciliated epithelium, 

 which lines the Fallopian tubes, being con- 

 tinued for an exceedingly short distance 

 over the margins of their fimbriated extre- 

 mities. The size of the cells also expe- 

 riences slight variations : thus, they are 

 largest in the peritoneum, and smaller in the 

 pericardium, especially in its visceral layer. 

 Their arrangement as a single cellular stratum 

 is also interrupted in some parts : thus, the 

 arachnoid exhibits one or two layers, the 

 outer of which is composed of cells which are 

 more flattened and elongated than usual. 



Basement membrane. The existence of a 

 basement membrane immediately beneath 

 these cells is still a matter of doubt. It rests 

 chiefly on the affirmation of Professors Todd 

 and Bowman, and Goodsir high authorities 

 on such a question. By the first of these ana- 

 tomists it is regarded as " a continuous trans- 

 parent membrane of excessive tenuity," and 

 "homogenous, or nearly so."* The latter 

 describes it in much the same terms, but 

 considers it sometimes, or generally, separable 

 into component cells, which are of a rhom- 

 boidal and extremely flattened shape ; and 

 it has been named by him as the Germinal 

 Membrane.-\- As somewhat corroborative of 

 these statements, it may be urged, that such 

 a structure is easily seen to exist in the very 



* Op. cit. p. 130. 

 t Ibid. p. 41. 



