526 



SEROUS AND SYNOVIAL MEMBRANES. 



have never seen such an appearance except ' 

 under these circumstances ; and at the edge 

 of the fringe, which is usually more or less 

 shielded from pressure by the prominence of 

 the neighbouring surface receiving the weight 

 of the upper lamina of glass, this perfect glo- 

 bularity is readily verified. The cell-wall is 

 extremely delicate and thin 5 its contents are 



Fig. 40-k 



Cells of the Choroid Plexus. From the adult Cat- 

 The upper figure represents their arrangement in 

 situ; in the lower, a, nuclei of ruptured cells; 

 b, cells detached. (Magnified 320 diameters.) 



fluid, and usually nearly transparent, and of a 

 refractility not much different from that of 

 water. So great is the delicacy of the mem- 

 brane, and so little aid to observation is 

 given by its colour or refractility, that at 

 first it requires careful scrutiny before its pre- 

 sence is verified ; and its recognition is often 

 retarded by the excess of light which the 

 modern achromatic condenser affords. From 

 this, which is the ordinary size, they pass by 

 few gradations to a plentiful blastema, which 

 fills up the interstices, and more or less com- 

 pletes the covering of the vessels. On tear- 

 ing up such a fringe, most of these cells dis- 

 appear, and their collapsed membranous walls 

 may be found here and there, flattened and 

 folded upon themselves, or burst at one ex- 

 tremity, and giving vent at the rupture to a 

 faintly granular mass and their nucleus. The 

 nuclei found in multitudes in such a specimen 

 are round and pale, and contain granular 

 matter and a single small bright yellow nu- 

 cleolus ; or, rarely, there are two such spots. 

 Many of these free nuclei exhibit a flattened 

 or truncated surface, which indicates the ex- 

 tremity previously seated on the inner surface 

 of the cell. 



Whatever may be the import of this pecu- 

 liar structure, it is interesting to observe how 

 closely, both in the arrangement of the vessels 

 and the structure of the cell-covering, it re- 

 sembles the synovial fringes previously de- 

 scribed. The view of Mr. Rainey, that these 

 spherical cells are nerve vesicles, seems to 

 rest at present on the very insufficient basis of 

 a slight external resemblance. But it appeal's 



' difficult to infer such complicated functions 

 as are sustained by the nervous matter from 

 such simple physical properties as sphericity, 

 faint granularity, and the like, unaided by 

 other structural analogies. 



Development of serous membranes. The 

 steps of this process are little known, a circum- 

 stance which seems partly to depend on the 

 extreme readiness with which it occurs, partly 

 on the comparative simplicity of its nature : 

 the cell being retained as the permanent form 

 of the tissue, the mere apposition of a 

 number of these in connection with a surface 

 of areolar tissue is all that is required to 

 complete the visible phenomena of its deve- 

 lopment. 



In the animal kingdom, serous membranes 

 are almost invariably present. They mostly 

 appear in immediate connection with some 

 higher development of the several viscera 

 around which they are grouped. In this 

 manner, first the peritoneum, and next the 

 pericardium and arachnoid, appear. The first 

 indication of the pericardium is in the mol- 

 lusca, and its appearance seems to be imme- 

 diately preceded by a mechanical provision of 

 a very different kind, although perhaps of 

 similar import : the heart is suspended in the 

 centre of a muscular cord, which is attached 

 by its two extremities, and thus fixes the 

 viscus and steadies its movements. The 

 peritoneum and pleurae are united in one in 

 the reptiles ; afterwards, the latter membranes 

 are shut off by the formation of a diaphragm. 

 The tunica vaginalis is absent in those animals, 

 in whom the testicles occupy a position within 

 the belly. 



Generally, there is the same obvious relation 

 of their presence to mechanical uses which is 

 seen in the human subject. But the ciliated 

 serous membranes of many reptiles, and the 

 urinating pericardium of cephalopods, offer, 

 at present, such great and inexplicable dif- 

 ferences from the human serous membranes, 

 that one might almost doubt, especially in 

 the latter of these instances, how far textures 

 contrasted by such manifest differences of 

 structure, and probably of function, can justi- 

 fiably be called by the same name. 



In the human foetus, their development is 

 also little understood. This period of life, 

 however, adds the amnion to the list of 

 serous membranes, the structure of which it 

 closely resembles. Its cavity is occupied 

 by a saline and albuminous fluid in large 

 quantity. It is subject to fluctuations in 

 amount, and one or two analyses appear to 

 show'that the proportion of albumen which 

 it contains is considerably diminished in ad- 

 vanced pregnancy. 



Development by friction. Besides these 

 two forms, the development of some of these 

 structures is witnessed in another condition, 

 perhaps more peculiar than either; viz., in 

 answer to the application of mechanical force. 

 The subcutaneous bursae which are ordinarily 

 found over some of the various prominences 

 of bone indicate the nature of their relation 

 to these localities by their reproduction after 



