STllUCTUUE OF SEROUS MEMBRANES. 303 



organic independence ; it is immediately derived from the general 

 continuity of the connective tissue throughout the body. A 

 serous membrane is merely a layer of this connective tissue 

 whose continuity does not happen to be interrupted by the im- 

 plantation of structural elements of a different order. The 

 serous sac is an interstice — a fissure in the general connective 



o 



tissue of the organism. 



§ 266, This view is hard to grasp ; but its value in enabling 

 U3 to judge rightly concerning all the morbid states of serous 

 membranes can hardly be over-estimated. In the first place, it 

 defines the problem which w^e have to solve. It implies that the 

 connective tissue of the serous membranes is uninterruptedly 

 continuous with the interstitial connective tissue of those organs 

 which they invest. The inference from this is plain : that the 

 serous membranes may, and do in fact take part in all the 

 morbid changes to which the interstitial connective tissue of 

 those organs is liable. We are compelled for convenience sake 

 to put asunder things wdiich are naturally connected, and to 

 postpone our consideration of a large number of morbid changes 

 in which the serous membranes are involved — viz. chronic inflam- 

 mations and formative processes — till we come to speak of the 

 several organs. The present chapter will be devoted to those 

 superficial disorders which really are confined to the serous 

 membrane ; abnormal secretions, excrescences and parenchyma- 

 tous alterations — if a serous membrane can be said to have a 

 parenchyma. 



§ 267. Let us begin by reviewing briefly the normal anatomy 

 of serous membranes. Here too we shall find our definition of 

 great use, more particularly as regards the epithelium. Every- 

 body knows that the serous membranes are lined by a single 

 layer of pavement-epithelium. Flat, polygonal, nucleated cells 

 form a mosaic; by treatment with silver nitrate their outlines 

 may be readily defined (fig. 96). Is there any tissue in the his- 

 tological series whose structure seems at first sight to be simpler 

 than that of pavement-epithelium ? But this simplicity is only 

 apparent. The question as to the position of the nucleus plunges 

 us at once into difficulties. Is the nucleus lodged in an interstice 

 of the plate, as in the flattened cells of the epidermis ? or is the 

 plate furnished with a hole into which the nucleus is fitted like 

 a pane of glass into its frame ? Neither of these views is the 



