CH. III.] CHEMISTRY OF EPITHELIUM 27 



nerves as a rule. But in stratified epithelium, particularly that 

 covering the cornea at the front of the eye and in the deeper layers 

 of the epidermis, a plexus of nerve- fibrils is found. 



Chemistry of Epithelium. 



There is not much to add to what has been already stated con- 

 cerning cells ; protoplasm and nucleus have the same chemical com- 

 position as has been already mentioned in Chapter II. Two new 

 substances have, however, been mentioned in the foregoing chapter 

 namely, mucin and keratin. 



Mucin. This is a widely distributed substance occurring in 

 epithelial cells or shed out by them (see goblet cells, fig. 30). It also 

 forms the chief constituent of the cementing substance between 

 epithelial cells. We shall again meet with it in the intercellular 

 substance of the connective tissues. The mucins obtained from 

 different sources varies somewhat in composition and reactions, but 

 they all agree in the following points : 



(a) Physical character : viscid and tenacious. 



(b) Precipitability from solutions by acetic acid. They all dis- 



solve in dilute alkalis, such as lime-water. 



(c) They are all compounds of protein, with a carbohydrate 



material; by treatment with mineral acid this is hydro! ysed 

 into a reducing but non-fermentable sugar-like substance. 

 The substance mucin, when it is formed within cells (goblet cells, 

 cells of mucous glands), is preceded in the cells by granules of a sub- 

 stance which is not mucin, but is readily changed into mucin. This 

 precursor, or mother-substance of mucin, is called mucigen or mucinogen. 

 Keratin, or horny material, is the substance found in the surface 

 layers of the epidermis, in hairs, nails, hoofs, and horns. It is very 

 insoluble, and chiefly differs from other proteins in its high per- 

 centage of sulphur. 



The silver nitrate reaction of cementing substance. The principal 

 chemical reaction which is employed by histologists for demonstrat- 

 ing the cement or intercellular substance which binds epithelial cells 

 together was formerly supposed to be due to the formation of a 

 silver-protein compound -which was reduced by sunlight. Macallum 

 has conclusively shown that this is not the case, but that it is an 

 inorganic reaction. Cementing material is specially rich in chlorides 

 (mainly sodium chloride) ; the addition of silver nitrate leads to the 

 formation of silver chloride, and it is this which is reduced by light. 

 The silver reaction obtained in other tissues is similarly explained : 

 in fact silver nitrate is a micro-chemical reagent for detecting the 

 localities in the body where chlorides occur. According to Mac- 

 allum the nuclei of all cells are entirely free from chlorides. 



