OF THE SEROUS MEMBRANES IN GENERAL. 157 



and even bony plates, are frequently observed in their thick- 

 ness, particularly in the pleura, which sometimes forms a sort 

 of shield in consequence of chronic pleurisy. These plates, 

 are generally, it is true, subjacent to, or laid over them. 



Free or pediculated concretions have their seat in the inte- 

 rior of these membranes. They are found, more particularly, 

 in the articulating serous membranes, sometimes, however, in 

 those of the tendons, and even in the splanchnic cavities. They 

 are at first external to the membrane, and afterwards pushing 

 it gradually before them, they project into its interior, where 

 they present a wide and short base, and at a later period, a 

 pedicle which becomes longer and longer, until finally break- 

 ing, they become perfectly free in the cavity of the mem- 

 brane. Such is the true mechanism of the formation of these 

 bodies, that have been taken for true concretions, when they 

 had not been observed in their different stages of development. 

 The consistence of these bodies varies: sometimes they are 

 very soft and albuminous,but most commonly they are fibrous, 

 cartilaginous or bony. 



The serous membranes participate in the degenerations 

 common to all the tissues; they appear, also, to be subject to 

 some that are peculiar to them. 



199. Vices of conformation are to be observed in some of 

 these membranes, as in the arachnoid membrane of the anen- 

 cephalous foetus, in the peritoneum and vaginal tunick, when 

 the canal of communication between these two membranous 

 sacs subsists after birth. A kind of supernumerary sacs have 

 been found in the peritoneum: Neubauer gives some examples. 

 Acquired vices of conformation are also peculiar to a small 

 number of these membranes, and belong to special anatomy. 

 Hernia is one of these vices. 



200. Cysts may with propriety be described along with 

 the serous membranes: it is in fact with this genus of organs 

 that they have the greatest resemblance. They resemble, in 

 general, like all the parts comprised in the serous system, a 

 pocket or membranous cavity, closed on all sides, adherent on 

 one surface, free on the other, and in contact with a liquid 

 which fills it: they are commonly of a globular form; their 



