SCALY EPITHELIUM. 



liii 



Fig. XX. 



applied, following accurately all its eminences, depressions and inequalities. 

 Epithelium when destroyed or cast off, is, for the most part, very readily 

 regenerated. 



In accordance with the varied purposes which the epithelium is destined 

 to fulfil, the cells of which it is composed come to differ in different situa- 

 tions, in figure and size, in their position in respect of each other, 

 their degree of mutual cohesion, and in the nature of the matter they 

 contain, as well as in the vital endowments which they manifest ; and, 

 founded on these modifications of its constituent cells, or, at any rate, those 

 forming the superficial layer, four principal varieties of epithelium have been 

 recognised, namely, the scaly, the columnar, the spheroidal, and the ciliated, 

 each of which will now be described in particular. 



It may first be remarked, however, that amidst these changes the nucleus 

 of the cell undergoes little alteration, and its characters are accordingly 

 remarkably uniform throughout. It is round or oval, and more or less 

 flattened ; its diameter measures from ffjpnpth to ^y 1 ^ 0th of an inch, or 

 more. Its substance is insoluble in acetic acid, and colourless, or but 

 slightly tinted. It usually contains one or two nucleoli, distinguished by 

 their strong dark outline ; and a variable number of more faintly marked 

 granules irregularly scattered. For the most part, the nucleus is persistent, 

 but in some cases it disappears from the cell. 



Scaly Epithelium. The scaly, lamellar, 

 tabular, or flattened epithelium (comprehend- 

 ing, in part, the pavement or tesselated 

 epithelium of the German anatomists). In 

 this variety the epithelium particles have the 

 form of small angular plates, or thin scales ; 

 in some situations forming a single thin layer, 

 in others accumulating in many superimposed 

 strata, so as to afford to the parts they cover 

 a defensive coating of considerable strength and 

 thickness. 



As a simple layer, it is found on the serous, 

 and some synovia! membranes, the inner surface 

 of the heart, blood-vessels and absorbents ; 

 also partly lining the cerebral ventricles and 

 covering the choroid plexuses ; on certain parts 

 within the eye and ear, and in some gland-ducts. 



If the surface of the peritoneum, pleura, 

 pericardium, or other serous membrane be 

 gently scraped with the edge of a knife, a 

 small quantity of soft matter will be brought away, which, when examined 

 with the microscope, will be found to contain little shred-like fragments of 

 epithelium, in which a few of its constituent particles still hold together, like 

 the pieces composing a mosaic work (fig. xx.). These particles, which are 

 flattened cells, have for the most part a polygonal figure, and are united to 

 each other by their edges. Each has a nucleus, apparently in or near the 

 centre. The addition of weak acetic acid renders the angular outline of the 

 cells as well as the nucleus more distinct. The cells differ somewhat in size 

 on different parts of the serous membrane ; and those which cover the 

 plexus choroides send downward short, pointed, transparent processes to- 

 wards the subjacent tissue. 



The epithelium of the vascular system resembles in many parts that of 



Fig. XX. FRAGMENT OF En 



THELIUM FROM A SEROUS 



MEMBRANE (PERITONEUM) ; 

 MAGNIFIED 410 DIAMETERS. 



a, cell ; b, 

 cleoli (Henle). 



nucleus ; c, nu- 



