524 



SEROUS AND SYNOVIAL MEMBRANES. 



similar mucous membranes ; and that the 

 cell-lining of the arteries, which becomes 

 deficient where these pass into the capillaries, 

 and thus leaves the latter vessels with a 

 simple membranous wall, seems to exhibit a 

 kind of natural analysis of a yet more similar 

 compound structure. And it must be recol- 

 lected that failure of recognition is by no 

 means a satisfactory argument against the 

 presence of such a delicate structure ; i. e. 

 that one such affirmation as those above 

 ought to outweigh many denials. Still those 

 who, after repeated and careful examination, 

 have failed to recognise it, are no doubt 

 justified in continuing to doubt its existence. 



Areolar Tissue. A stratum of areolar 

 tissue occupies the outer or inferior surface 

 of the preceding cellular structure, and in- 

 cludes in and amongst its meshes the remain- 

 ing constituents of the serous membrane. 

 The inner surface of this lamina is smooth 

 and condensed, where it immediately under- 

 lies the cells : exteriorly, it can scarcely be 

 considered as possessing a defined surface, 

 but gradually merges into the areolar tissue 

 of the neighbouring organs. The separation 

 of the two structures is, however, generally 

 indicated by an interval, in which their tex- 

 ture is somewhat looser. This is called the 

 "" subserous cellular tissue" 



As this layer constitutes the chief thick- 

 ness of the membrane, and is the constituent 

 on which its physical properties are mainly 

 dependent ; so its varieties of constitution 

 and arrangement, correlatively with the re- 

 quisite differences of these properties, are 

 both numerous and important. 



One of the most common of these altera- 

 tions is an augmented quantity of the yeliow 

 fibrous element ; indeed, in many portions of 

 the serous membranes, this increase is so 

 considerable as to constitute a continuous 

 special layer of the elastic fibre, which occu- 

 pies a horizontal plane immediately beneath 

 the epithelium. The fibres of this layer are 

 delicate, of a smaller diameter, and somewhat 

 paler colour, than those which are found in 

 the ordinary areolar tissue : they branch at 

 acute angles in every direction, and unite with 

 those in the immediate neighbourhood ; while 

 beneath, and partly amongst them, are seen 

 the white fibrous bundles, with their ordinary 

 arrangement. The advantage of such a pre- 

 dominance of the yellow element is obvious : 

 it confers an increased elasticity on the mem- 

 brane, and better adapts it for distention, or 

 for a return to its original bulk after this 

 force is removed. The situations in which it 

 is found are in exact conformity with this 

 view : in the peritoneum, which lines the 

 anterior abdominal wall and covers the blad- 

 der, it attains its maximum ; in the detached 

 folds of the mesentery, in the costal pleura, 

 and in the so-called suspensory ligaments 

 of the liver, it is still very prominent ; but 

 on the posterior wall of the belly, and in the 

 serous membranes where they cover many 

 of the viscera, such as the heart, brain, lungs, 

 liver, &c., it is almost completely deficient. 



On the lungs, the necessity of its presence 

 is probably superseded by the large quantity, 

 both of the texture and of the property, which 

 is inherent in these organs themselves : the 

 remaining viscera are all organs of a size 

 which is either little variable, or of uniform 

 variety. 



In the areolar tissue beneath the spinal and ce- 

 rebral arachnoid, another modification occurs. 

 Between the vascular pia mater, which closely 

 envelopes the nervous centre, descending in- 

 to its sinuosities of surface, and the visceral 

 layer of the arachnoid, a considerable interval 

 exists, in which the meshes of this tissue are 

 exceedingly long and lax ; while, in many parts, 

 the distance between them is so much in- 

 creased as to form cavities, which have re- 

 ceived the name of " the subarachnoid spaces." 

 They are filled with the fluid of the same 

 name ; and by its presence the visceral and 

 parietal layers of the serous membrane are 

 maintained in contact, pressure generally be- 

 comes equalised, and large portions of the 

 nervous centre hang suspended in fluid. The 

 chief interruption to this arrangement obtains 

 at the summit of the cerebral convolutions, 

 where the arachnoid and pia mater are strongly 

 adherent to each other : but the more minute 

 description of these spaces or cavities belongs 

 to the special anatomy of these membranes. 



The fat cells which are so often deposited 

 in the intervals of areolar tissue frequently 

 occupy its meshes in the serous membranes. 

 In most of these instances, however, it would 

 be more correct to regard the subserous or 

 connecting areolar tissue as the seat of the 

 deposit, than that more condensed portion of 

 it, to which an artificial separation would limit 

 the term " serous membrane." It is plentifully 

 found in connection with both layers of the 

 peritoneum, while it is comparatively* absent 

 from the arachnoid. In the case of the other 

 serous membranes, the parietal layer is that 

 which is most liable to its presence ; indeed, 

 on the lungs, it appears to be completely and 

 invariably absent. This latter circumstance 

 has been ascribed to a supposed local antago- 

 nism of respiration to the deposit, analogous 

 to that which is known to be exerted by this 

 process generallv. But this supposition seems 

 quite untenable, since the lungs themselves 

 are not nourished by the blood which it is 

 their function to depurate, but by the ordinary 

 arterial fluid, which exhibits the usual changes 

 in the bronchial veins ; and one can hardly 

 imagine respiration to exert an influence on 

 the tissue, apart from, or greater than that 

 which it exerts on its blood. Here, as else- 

 where, the necessities of movement seem to be 

 the circumstances which chiefly regulate the 

 locality of the deposit : excessive mobility, as 

 in the scrotum, penis, and eyelids, seeming to 

 contraindicate the formation of adipose tissue. 

 The amount present in these membranes 

 generally exhibits a direct relation with that 

 which is contained in the whole body. 



The vessels of the serous membranes ramify 

 in their areolar tissue, and by their numerous 

 anastomoses with each other constitute a 



