EPITHELIUM. 



537 



cesses and coalesced to form vessels with similar offsets from 

 other cells: ultimately these are seen to join the intercellular 

 passages, and become continuous with them. In glands, 

 the cells being filled with their peculiar fluid, are conveyed 

 to the wall of the intercellular passage, and through this 

 the secretion arrives at the surface of the body. 



The Epithelium. If we cut very thin slices from the 

 superficial portions of the skin, we can raise from it a 

 delicate membrane ; or, what is better, by using chemical 

 or mechanical irritation, we obtain what is ordinarily 

 called a blister : to it we give the name of epidermis. The 

 microscope has shown this to be a tissue of high and 

 remarkable organisation ; being, in point of fact, an aggre- 

 gation of laminated cells, differing, in 

 different situations, in regard to form, 

 colour, and composition. This invest- 

 ment serves to protect the delicate 

 structures beneath, and is likewise a 

 bad conductor of heat, thus tending 

 to maintain the temperature of the 

 body ; besides these uses, it answers 

 the purposes of excretion, and is 

 sometimes an agent of motion, fig. 272. 

 These laminated elementary cells, 

 found on the surfaces, have generally 

 nuclei. The form of the nucleus is 

 rounded or oval, and is the 1-3 000th 

 to 1 -5000th of an inch in diameter. Fig.272.-./ section of the 



-T-, , , , Epidermis- 



Each nucleus has two or three nu- 

 cleoli, with outlines more or less irregular ; a cell sur- 

 rounds the whole, which has transparent walls. The cell 

 varies in this latter arrangement : it may be flattened, 

 and the nucleus may be attached to one side of it ; or, 

 again, the nucleus may be in the centre, and the cell pro- 

 longed at either end. The cells of the epithelium may be 

 divided into three kinds : the 1st is termed the tesselated 

 or pavement; 2d, the columnar or basaltic; 3d, the ciliated 

 or vibratile epithelium. Some make a 4th, combining the 

 tesselated and the columnar : this may be considered as 

 transition epithelium, and is found only in certain mucous 

 passages. These cells are represented in Fig. 274, a, b, c, 

 and d. 



