SEROUS and SYNOVIAL MEM BRANES.—(Illembranes sereuses, Fr.; Serihe Haiite, Seriise Ueberziige, Wasserhaide, Germ. Membranes synoviales, Fr. ; Synovial-Kapseln, Synovial Haiite,Germ.) —The names by which these structures are designated seem to have been originally derived from the appearances presented by fluids which are frequently found after death in the so-called cavities formed by their continuous interior surface.
Thus, for instance, rejecting those cases where marked symptoms of disease of these tissues precede death, the structures first named, where they offer any contents at all, present a fluid the colour and composition of which greatly approximate to that of the serum of' the blood ; and thence the fluid so found names the tissues yielding it as the " serous " membranes : while the interior of the joints constantly affords a small quantity of a fluid, the viscid consistence of which, resembling that of the white of an egg, gives rise to the application of the name " s3novial " membrane (am, wen.) to the tissue which immediately lines the articulation, and is presumed to yield it.
But neither do these circumstances, nor that of their rnembranous form, by which the terms at the head of this article are completed, sufficiently express their most important characteristics. A serous membrane essen tially consists of an endogenous cell-growth, covering a thin expansion of areolar tissue. The compound structure which results from this arrangement of these two tissues is thrown around the more inoveable organs of the body, and also lines the cavities which they fill. By thus affording to these two opposed surfaces uniformity of texture and smoothness of surface, it greatly diminishes their mutual friction ; or, in other words, facilitates their movements upon each other.
It will, I think, be advantageous to defer for the present all consideration of the pos sible or probablefienction of these membranes, as implying by that word an immediate organic operancy in virtue of their intimate structure; and to fix our attention chiefly on their mechanical use in reference to motion.
In the living man, there are many processes which necessitate changes in the relations to space of the different parts of the body. The actions of locomotion, digestion, circu lation, and respiration, for instance, all imply some degree of movement in the organs which are their immediate agents, often in the more important parts to which they immediately minister ; and, in many cases the protection of delicate organs appears to be partly ac complished by an increase of their mobility upon neighbouring structures. The neces
sity of movement thus comes to be more or less participated in by almost all the tissues, organs, and segments of the body ; and as suming, what is above stated, that it is the most obvious want for which serous mem branes are destined to provide, we might naturally imagine, either that these structures would pervade as universally as this require ment, or that those of similar import which should be substituted for them would suffi ciently approximate in their nature and com position to be referrible to the same class of tissues : a class, in which the degrees of re semblance afforded by the different members should somewhat accord with the varying mechanical requirements of those different parts of the body, to the movements of which they were subservient.
An appeal to facts abundantly confirms tuch an inference. Observation shows that in the human body a variety of structures exist, which are united by the characteristics not only of considerable analogy of office, but also of' similarity of structure, almost complete identity of chemical composition, and intimacy of pathological relations.
Adopting the possession of these common properties as a natural and safe basis of classification, we form a group in which are included all those tissues which serve to limit, define, or facilitate movement. The class of structures thus constituted was formerly termed the Cellular System ; " but the cel lularity which the name connotes, as it was never supposed to be predicable of all its members, so it is now known to be erro neously used of that part of them to which it was originally applied ; and they have there fore been preferably arranged under the head of " Passive Organs of Locomotion." And if any should consider this term open to the lesser objection of specifying a general, but not essential fact, that of " Passive Organs of Movement " might be again substituted.