60S THE MICROSCOPE. 



determines the way in which the stream should pass ; and 

 we find in general that the current is from the rarer to the 

 denser fluid. If we immerse a porous tube half filled with 

 a strong solution of common salt in a jar of water, we notice 

 that the level of the fluid inside the tube rapidly rises 

 above the outside, while the water becomes slightly salt 

 to the taste. It is not a constant circumstance that the 

 stream is from the rarer to the denser fluid ; with alcohol 

 and water, for instance, the stream is from the latter to 

 the former. Mineral substances, even pipeclay and chalk, 

 permit of endosmosis in a low degree. 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 cells, differing, in different 

 situations, in regard to form, colour, 

 and composition. These laminated ele- 

 mentary cells, found on the surfaces, 

 have generally nuclei. The form of 

 the nucleus is rounded or oval, and is 

 the 1 -3000th to l-5000th of an inch 

 in diameter. Each nucleus has two or 

 three nucleoli, with outlines more or 

 less irregular. The epithelium cells 

 may be divided into three kinds : the 

 1st is termed the tesselated or pave- 

 ment ; 2d, the columnar or basaltic ; 

 3d, the ciliated or vibratile epithelium. 

 Kig. 30i.— ^f section of the Some make a 4th, combining the tesse- 

 lated and the columnar : this may be 

 considered as transition epithelium, and is found only in 

 certain mucous passages. These various cells are repre- 

 sented in fig. 303, a, b, c, and d. 



