SEROUS AND SYNOVIAL MEMBRANES. 



527 



excision ; while in other situations of the 

 same kind, but in which they are not usually 

 present, the application of long-continued 

 pressure and friction gives rise to their pro- 

 duction. If we add to these phenomena the 

 development of diarthrodial false joints, it 

 will appear that a certain amount of pressure 

 is capable of determining the formation of a 

 cavity, and the growth of a cell-covered 

 membrane, which secretes a synovial fluid ; 

 and that, exerted in a higher degree upon 

 the more resisting bones, it clothes their 

 extremities with a substance which presents 

 all the appearances of cartilage. The presence 

 of blood plasma is no doubt a necessary con- 

 dition of both processes, but in neither are 

 the subsequent minute changes known. In 

 the case of the areolar tissue which is con- 

 verted into a bursa, we may indeed infer, 

 that mechanical violence exerted upon it 

 would produce an increase of vascularity or 

 an active congestion ; but we can scarcely 

 conjecture how this alone should result in an 

 increased vital activity, in the removal of some 

 of the partitions of its net-work, the com- 

 pression of others, the formation of a cavity, 

 and the regular cell-covering of its inner 

 surface. 



But while the production of this structure 

 in answer to pressure is exceedingly inte- 

 resting, and offers a remarkable analogy to its 

 general development in the foetus and animal 

 kingdom, there is perhaps a danger of our 

 exaggerating the resemblance, and becoming 

 too mechanical in our views. On closer 

 consideration, their ordinary and extraor- 

 dinary formation will be found to exhibit a 

 difference, which may teach us caution in our 

 conclusions as to the method in which me- 

 chanical force acts. It is this : that while, in 

 the latter case, their development appears to 

 provide for a want already experienced ; in the 

 former instance, in the young embryo, we 

 may observe a very similar development oc- 

 curring, which is a provision for a necessity 

 that has never yet existed, and cannot there- 

 fore be the immediate cause. 



Physiology of the serous and synovial mem- 

 branes. At present, the physiological import 

 of the preceding minute structural details is 

 so little recognised or understood, as to leave 

 scarcely anything to be said under this divi- 

 sion of the subject. Bat, in this respect, 

 these textures present so close a parallel to 

 many others in the human body, that to con- 

 sider this imperfect knowledge as demanding 

 a complete silence, would be to interpose an 

 insuperable barrier to almost all conclusions 

 on any physiological subject. In truth, the 

 question of the abstract truth or falsity of 

 physiological conjectures by no means involves 

 the question of their usefulness ; and the 

 difficulty of retaining mere details, the danger 

 of considering them as essentially knowledge, 

 the possibility of allowing a philosophic sus- 

 pension of judgment to merge into sloth- 

 fulness, all these circumstances taken to- 

 gether perhaps claim that such anatomical 

 minutiae should at least be considered with a 



view to their explanation ; even while they 

 demand that the various shades of probability 

 possessed by these conjectural explanations 

 should, as far as possible, receive their due 

 estimation ; and that their adoption should 

 never interrupt the collection of fresh facts. 



Some attempt has already been made to 

 discriminate between the physical and vital 

 properties of these membranes ; and the 

 mechanical advantages conferred by some of 

 the former were enumerated as constituting 

 their most prominent use. Their secretory 

 function has next to be considered, together 

 with any relations which this process may 

 possibly bear to the organism generally, as the 

 further use of these structures. 



A most important feature, and one which 

 belongs to all these membranes, is their pe- 

 culiar arrangement. The general statement, 

 that they are so disposed as to form shut 

 sacs, has been already alluded to, and was at 

 no late period considered their characteristic 

 definition. But as mere form could scarcely 

 be thought of such essential importance, 

 various attempts have been made to explain 

 this morphological character, by referring it 

 to some other term, which should either ex- 

 press a real cause, or should approximate to 

 this by enunciating some physiological purpose 

 itself implying the fact. Many of these, 

 however, such as its being the result of the 

 universal presence of epithelium, &c., are 

 little more than re- statements of the fact in 

 another form. But to the exception of the 

 female peritoneum, long known, must now be 

 added (unless the definition of these structures 

 be arbitrarily extended so as. to include many 

 varieties of cartilage) all the articular synovial 

 membranes, and many of the bursae ; in which 

 the interruption to the continuity of the 

 membrane constitutes the phrase " shut sac," 

 an inaccurate expression as applied to them. 



In short, all that the term really implies is, 

 that there is no visible outlet by which the 

 cavities these tissues form, or assist to form, 

 can communicate with the exterior of the 

 body. And even in the case of the apparent 

 exception at the extremity of the Fallopian 

 tube, it is exceedingly probable that the small 

 size of the aperture of communication, the 

 ciliated lining which it possesses, and the 

 direction in which the current of ciliary 

 motion sets, constitute it, in effect, a closure. 

 The internal position of the serous membranes 

 is followed by this important physiological 

 consequence, that the contents of their 

 cavities are never directly eliminated from 

 the body ; but that such portions of the 

 substance of the membranes, or of its sepa- 

 rated products, as may become effete in the 

 course of the vital changes, can only be dis- 

 charged from the system after a "previous 

 reception into the general mass of the circu- 

 lating fluid. This fact at once establishes a 

 broad line of distinction between these tissues 

 and the mucous membranes, or true glands ; 

 while at the same time it tends to prove that 

 their secretion, whatever it may be, possesses 

 little of the deleterious quality, or excretory 



