GENERAL ANATOMY OF THE SEROUS MEMBRANES. 21 



ture, yet since they exist, as Bicbat has shown us, as distinct 

 membranes in the early periods of foetal life, and as their dis- 

 eases exhibit peculiar phenomena, they deserve to rank among 

 the elementary tissues of the body. Some of the serous mem- 

 branes consist merely of a simple bag of greater or less size 

 some line cavities which are very irregular in surface and 

 shape and others, like the pleura and peritoneum, are reflect- 

 ed over the inner face of the viscera, like the internal fold of 

 the double night-cap over the head, and presenting a very 

 complicated arrangement, so that the viscera which the mem- 

 brane lines are necessarily left upon its outer side. All the 

 vessels which enter these viscera, pass between the two layers 

 that form the roots of the folds by which they are embraced. 

 (See fig. 133, p. 24.) 



In the healthy state the serous membranes are perfectly white 

 and insensible ; and when in a state of inflammation, the appa- 

 rent redness of the membrane is frequently owing to the injection 

 of the sub-serous cellular tissue. Nevertheless they contain a 

 vast number of serous capillary vessels, which Ruysch suc- 

 ceeded in injecting in such abundance, as to be led to think 

 they were composed chiefly of these vessels. Mascagni was 

 equally successful in filling their lymphatics, and considered 

 these vessels the principal elements in the composition of serous 

 tissues. Both, therefore, exist in great numbers, and woven 

 together with some intermediate connecting fibres like those of 

 the cellular tissue, constitute these membranes. This, too, is 

 the ultimate composition of cellular tissue as proved by the 

 recent researches of Miiller and Treviranus. (See vol. i. p. 406.) 

 The serous membranes are said to yield gelatine by boiling, 

 and in this respect also they agree with the cellular membrane. 

 The serous capillaries pour out a lubricating serous fluid, (called 

 halitus from its vapory condition during life, and which con- 

 denses as the body cools after death,) by a process called 

 exhalation, which we do not fully understand, and which the 

 absorbents gradually remove during health, so as to prevent 

 its accumulation. But when the usual healthy balance of action 

 between them is disturbed, either by too copious exhalation, 



