SlT.rCTUEE OF SEROUS MEMBRAXES. 



197 



parietes or upon each other, and their motion is rendered smoother Iby 

 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 floceulent aspect. 

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

 the membrane can scarcely 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 wliich abdominal hernia} pass ; 

 and accordingly, when such protrusions of the viscera happen to take 

 place, tliey usually push the peritoneum 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 con- 

 nection ; and a quantity of pellucid fluid is interposed, especiall}' in the 

 vertebral canal and base of the cranium, between the arachnoid and the 

 j)ia mater, which is tlie membrane immediately investing the brain and 

 spinal cord. 



Structure and properties. — Serous membranes are thin and trans- 

 parent, so that tlie colour of subjacent parts shines through them. They 

 are tolerably strong, with a moderate degree of extensibility and elasti- 

 city. They are lined on the inner surface by a simple epithelioid layer of 

 flattened cells (fig. 132), each of which contains a clear, round or oval 

 nucleus with one or two nucleoli. The outlines of the cells may readily 



Fi^. 132. 



;^zi:;y«^<;v'/>^:;L'Av?ss^/^Si?^iy,^^:^s^sls^^ 



Fig. 132. — Portion op Epithelioid Layer op Peritoneum froji Diaphragm of Rap.bit 



(Klein.) 



«, larger cells h, smaller ones, with here and there a pseiidostoma het-n'een. 



