CII. III.] CHEMISTRY OF EPITHELIUM 33 



tongue, mouth, pharynx, and oesophagus; (3) As the conjunctiva! 

 epithelium, covering the cornea; (4) Lining the vagina and the 

 vaginal part of the cervix uteri. 



Nutrition of Epithelium. 



Epithelium has no blood-vessels; it is nourished by lymph. 

 When the blood is circulating 'through the thin-walled small blood- 

 vessels in the tissues beneath the epithelium, some of its fluid con- 

 stituents escape. This fluid is called lymph; it penetrates to all 

 parts of the cellular elements of tissues and nourishes them. In the 

 thicker varieties of epithelium, the presence of the irregular minute 

 channels between the prickle cells (fig. 48) enables the lymph to soak 

 more readily between the cells than it would otherwise be able to do. 

 Epithelium is also destitute of 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 described 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. 39). 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 mucin 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, like lime-water. 



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



material; by treatment with mineral acid this is hydrated 

 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 

 substance which is not mucin, but is readily changed into mucin. 

 This precursor, or mother-substance of mucin, is called mucigen or 

 mucinoyen. 



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

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



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