ORGANIZATION 



231 



very delicate skin, formed of flat or cylindrical cells, 

 and in some parts (as in the windpipe of air-breathing 

 animals, and along the gills of the oyster) is covered 

 with cilia, or minute hairlike portions of protoplasm, 

 about -$-^0-$ of an inch long, which are incessantly mov- 

 ing. Continuous with this inner lining of the body (as 

 seen on the lip), and covering the outside, is the epi- 

 dermis or ctiticle. It is the outer layer of the " skin," 

 which we can remove by a blister, and in man varies in 

 thickness from -g~J 7 of an inch on the cheek to $ on 

 the sole of the foot. 

 It is constantly 

 wearing off at the 

 surface, and as con- 

 stantly being re- 

 plenished from the 

 deeper portion; and 

 in the process of 

 growth and pas- 

 sage OUtward, the FlG I99> _ Various kinds of Epithelium Cells magnified; 

 Cells change from a ' c l umnar from small intestine; 3, a single cell, 



the spherical form 

 to dead horny 

 scales (seen in scurf 

 and dandruff). In the lower layer of the cuticle we find 

 the pigment cells, characteristic of colored races. Nei- 

 ther the epidermis nor the corresponding internal tissue 

 (epithelium) has any blood vessels or nerves. The epi- 

 thelial tissue, then, is simply a superficial covering, 

 bloodless and insensible, protecting the more delicate 

 parts underneath, or, as in the alimentary canal, pro- 

 ducing mucus and digestive juices. Hairs, horns, hoofs, 

 nails, claws, corns, beaks, scales, tortoise shell, the wings 

 of insects, etc., are modifications of the epidermis. 



The next three sorts of tissue are .characterized by a 



showing nucleus; b, ciliated, from one of the small air- 

 tubes ; d, the same, from the windpipe, with single cell 

 magnified, about 200 times; c, squamous, from eyelid 

 of a calf, showing changes of form, from the deep to 

 superficial cells, i being the scurf. 



