cxcii SEROUS MEMBRANES. 



cardium adheres to its outer or 6brous part. Such a combination is often 

 named a fibro-serous membrane. 



The inner surface of a serous membrane is free, smooth, and polished ; 

 and, as would occur with an empty bladder, the inner surface of one part 

 of the sac is applied to the corresponding surface of some other part ; a 

 small quantity of fluid, usually not more than merely moistens the con- 

 tiguous surfaces, being interposed. The parts situated in a cavity lined by 

 serous membrane can thus glide easily against its parietes or upon each 

 other, and their motion is rendered smoother by the lubricating fluid. 



The outer surface most commonly adheres to the parts which it lines or 

 covers, the connection being effected by means of areolar tissue, named 

 therefore " subserous," which, when the membrane is detached, gives to its 

 outer and previously adherent surface a flocculent aspect. The degree of 

 firmness of the connection is very various : in some parts, the membrane 

 can scarce be separated ; in others, its attachment is so lax as to permit 

 easy displacement. The latter is the case in the neighbourhood of the 

 openings through which abdominal herniae pass ; and accordingly, when such 

 protrusions of the viscera happen to take place, they usually push the peri- 

 toneum before them in form of a hernial sac. 



The visceral portion of the arachnoid membrane is in some measure an 

 exception to the rule of the outer surface being everywhere adherent; for, in 

 the greater part of its extent, it is thrown loosely round the parts which it 

 covers, a few fine fibrous bands being the sole bond of connection ; and a 

 quantity of pellucid fluid is interposed, especially in the vertebral canal and 

 base of the cranium, between the arachnoid and the pia mater, which is the 

 membrane immediately investing the brain and spinal cord. 



Structure and properties. Serous membranes are thin and transparent, so 

 that the colour of subjacent parts shines through them. They are tolerably 

 strong, with a moderate degree of extensibility and elasticity. They con- 

 sist of, 1st, a simple layer of scaly epithelium already described and figured 

 (fig. xx.), which, however, is in part ciliated on the serous membrano 

 lining the ventricles of the brain and central canal of the spinal cord ; 2ndly, 

 the fibrous layer. This consists of fine but dense areolar connective tissue, 

 which is, as usual, made up of bundles of white filaments mixed with fine elastic 

 fibres ; the former, when there are two or more strata, take a different direc- 

 tion in the different planes ; the latter unite into a network, and, in many 

 serous membranes, as remarked by Henle, are principally collected into a 

 reticular layer at the surface, immediately beneath the epithelium. Tho 

 constituent connective tissue of the serous membrane is of course continuous 

 with the usually more lax subserous areolar tissue connecting the mem- 

 brane to the subjacent parts. Where the arachnoid membrane lines the 

 dura mater, and possibly al?o in some other cases, the fibrous layer 

 usually belonging to the serous membrane is wanting, its place being sup- 

 plied by the fibrous membrane beneath, on which the epithelium is imme- 

 diately applied. 



Blood-vessels ending in a capillary network with comparatively wide meshes 

 pervade the subserous tissue and the tissue of the serous membrane. Plex- 

 uses of lymphatics also exist in the subserous tissue, but not under every 

 part of the membrane ; in the costal pleura, for example, the lymphatics are 

 confined to the parts which cover the intercostal and stemo-co: tal muscles. 

 When present, the lymphatics extend in form of fine superficial plexuses 

 through the fibrous layer of the membrane to its surface, immediately 

 beneath the epithelium (Dybkowski), and may then open into the serous 



