THE TISSUES 21 



body closely allied in composition to the proteins, 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 ap- 

 pearance as a number of little 

 masses or granules in the cells, 

 and these ruii together to fill 

 the cells which from pressure 

 become flattened out into thin 

 scales. 



Keratin forms an admivable 

 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 

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

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

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

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

 into combination with various substances. Hence, lead solu- 

 tions colour keratin black by forming the black sulphide 

 of lead, and are largely used as hair dyes (see Chemical 

 Physiology.} 



(c) Transitional Epithelium. A slightly modified stratified 

 squamous epithelium lines the urinary passages. It is charac- 

 terised by the more columnar or pear-like shape of the deeper 

 layers of cells. 



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

 cells in the embryo lining the stomach and intestine, elongate 

 at right angles to their plane of attachment, and become 

 columnar in shape. The free border of the cells looks like 

 a hem, an appearance which is due to a series of short rods 

 placed side by side. Probably this is a special development 

 of the reticulum of the protoplasm. The great function of 

 this form of epithelium is to take up the digested matter 

 from the stomach and intestine, and to pass it on to the 

 blood. 



Among these columnar cells a certain number of peculiarly 



