THE TISSUES 23 



changes are interfered with, and the protoplasm undergoes 

 a change into a body closely allied in composition to the 

 proteids, keratin. This substance is a hard, horny material, 

 which is well seen in the nails and hair, and in the horns and 

 hoofs of certain animals. It first makes its appearance as a 

 number of little masses or granules in the cells, and these 

 run together to fill the cells, which from pressure become 

 flattened out into thin scales. 



FIG. 4. Stratified Squamous Epithelium from the cornea. 



Keratin forms an admirable protective covering to the 

 body, not only on account of its hardness and toughness, but 

 because poisons cannot readily pass through it, and also 

 because it is not easily acted on by chemicals. Like the 

 proteids, it contains carbon, hydrogen, oxygen, nitrogen, and 

 sulphur; and the first four of these elements are in about 

 the same proportion as in the proteids. But the sulphur is 

 in greater proportions (3 to 5 per cent.), and readily enters 

 into combination with various substances. Hence, lead solu- 

 tions, which give the black sulphide of lead, colour keratin 

 black, and are largely used to dye the hair (see Chemical 

 Physiology, p. 6). A slightly modified stratified squamous 

 epithelium lines the urinary passages. 



2. Columnar Epithelium (Fig. 5, a). The inmost set of 

 cells in the embryo, lining the stomach and intestine, elongate 



