528 



SEROUS AND SYNOVIAL MEMBRANES. 



composition, which marks many of the pro- 

 ducts of the mucous system. 



Contrast of serous and synovial membranes. 

 The shape of the cell of serous membrane 

 may afford some indications of its history. 

 In flatness, it occupies a position about mid- 

 way between the squamous outer epithelial 

 particles of the skin, and the columnar in- 

 testinal cells. The conditions which lead to the 

 excessive horizontal extension of the former 

 appear to be, a vertical pressure acting upon 

 them during their growth, and aided by an eva- 

 poration which diminishes the cell-contents, 

 themselves originally small in quantity. On 

 the other hand, the immediate cause of the 

 prismatic or columnar shape is, no doubt, a 

 horizontal pressure mutually exerted by the 

 growing cells themselves. This pressure 

 appears generally to limit their diameter to 

 that of the contained nucleus; a smaller 

 diameter, which implies the existence of a 

 greater number of cells in a given space. 

 Their longitudinal extension similarly involves 

 a greater amount of contents ; so that, on the 

 whole, this might be termed the highest form 

 of cell-growth, the development and filling 

 of a large number of cells simultaneously.* 

 Comparing the serous epithelium with these 

 two extremes, we may recognise in its flattened 

 shape the effect of vertical pressure on a cell 

 containing but little in its cavity ; while the 

 comparatively small number of cells in a given 

 space, and the oneness of the layer, are 

 further indications of the moderate activity of 

 the cell-growth. The uniform size and poly- 

 gonal shape of the constituent cells, together 

 with their great mutual adhesion by their 

 edges, or in the horizontal plane; these are 

 circumstances which seem to point to the 

 simultaneous development of the whole 

 layer, and to the previous causes of flatness 

 determining its growth almost exclusively in 

 this direction. 



The little aid afforded by the composition 

 of these cells is derived from observations 

 which are chiefly of a negative kind : since 

 they show that the cells do not offer any con- 

 siderable chemical differences from the liquor 

 sanguinis, but consist chiefly of albuminous 

 and fibrinous materials. 



The nature of the serous secretion seems 

 little understood. In health, the quantity of 

 fluid present in the interior of the membranes 

 is only sufficient to moisten their free surface ; 

 while where its amount is enough for the 

 purposes of analysis, the accompanying dis- 

 eased conditions would prohibit our assuming 

 its identity with the normal fluid, even if the 

 supposition were not rendered untenable by 

 the varying composition of the fluids them- 

 selves. But, on the whole, the very small 

 quantity of fluid naturally present, its compa- 

 ratively limpid consistence and transparent 

 appearance, together with the absence of the 

 cell-form in which secretions are involved, 

 probably refer it immediately to the simple 



* For some further remarks on the subject of 

 cell-growth, the reader is referred to a future article, 

 " STOMACH AND INTESTINAL, CANAL." 



physical process of transudation ; a process 

 which is present everywhere in the body, but 

 is favoured by the thin parietes of these struc- 

 tures, while their position prevents the re- 

 moval of the fluid by evaporation. 



But the fluid yielded by this supposed 

 process appears to be chiefly aqueous ; and the 

 question therefore readily suggests itself, 

 whether any mere transudation could filter off 

 the dissolved constituents from a perfect so- 

 lution, such as the liquor sanguinis is known 

 to be; and whether the elective affinities of 

 the tissue itself may not constitute the main 

 agents of the process, by retaining certain 

 materials, and allowing others to obey this 

 physical law. Valentin * mentions some ex- 

 periments in which dried serous membrane 

 was used as the filter, and albumen, so far 

 suspended in water as to constitute a homo- 

 geneous fluid under the microscope, was passed 

 through it. The result was, that it retained 

 a thicker portion, while only a thinner or 

 more dilute part passed through. But saline 

 solutions transuded entire, and perhaps the 

 doubtful state of solution of the organic con- 

 stituent will not permit much reliance to be 

 placed on these experiments. In connection 

 with this subject, Mr. Paget f has pointed 

 out that the different serous membranes seem 

 to effect this " filtration " with different de- 

 grees of fineness. And, possibly, the dimi- 

 nution of albumen noticed in the liquor amnii 

 of advanced pregnancy may be ascribed to a 

 similar subtraction from this fluid by the 

 serous membrane in the cavity of which it is 

 situated. The share which the cells as such 

 take in this process can scarcely be conjec- 

 tured ; but that their disposition in such a 

 form is not absolutely essential to the fluid, is 

 shown by its occurrence in the areolar tissue, 

 where such a stratum is absent. And while 

 we know next to nothing of the process itself, 

 and have no name by which it may be exclu- 

 sively indicated, it is important to recollect 

 that the words used above, " elective affinity," 

 " subtraction," " filtration," are probably alike 

 inaccurate ; that the first seems to imply 

 chemical combination, the second represents 

 the subtracted materials as too passive, the 

 third is the name of a physical process by 

 which solid objects are left behind after the 

 removal, by capillary attraction, of the fluid 

 in which they were suspended. The pro- 

 cesses to which it seems most analogous, and 

 to which it may best be compared, are those 

 curious varieties of heterogeneous adhesion 

 existing between bodies of different cohesive 

 forms, of which the action of charcoal or 

 platinum upon certain gases are familiar in- 

 stances 



The period of duration of the cell-growth, 

 and the manner of its renewal, can only be 

 conjectured. But from the constant absence 

 of shed epithelium from the interior of the 

 membrane, and the uniform shape and mutual 



* Lehrbuch der Physiologie des Menschen, Band 

 1., S. 601. 



f Report on the Progress of Human Anatomy 

 (Brit, and For. Review, year 1843-4, p. 10.) 



