THE TISSUES 



but the skin, protecting the body from the buffeting of 

 the outer world, requires, as it were, several lines of 

 defense. The outer layers are contin- 

 ually being worn away, but others are 

 beneath, ready to take their place. 

 Epithelial cells are of two principal 

 types, squamous or scale-like and col- 

 umnar or column-like. Squamous 

 cells are easily obtained for examina- 

 tion by gently scraping the roof of the FIG. 2. 

 mouth. The surface layers of the skin, 

 or epidermis (epi, upon, dermis, the 

 skin), are also flat, the outermost cells 



upward and sweep the 

 mucus, dust, and germs 



From Ritchie's "Hu- 

 man Physiology" 



Cells from the 

 lining of the trachea, 

 a is a cell that manu- 

 factures sticky mucus 

 (b) in which dust and 

 . germs from the air are 



becoming horny as they lose their vi- caug ht. The cilia (c) 



tality. In the intestine the cells are on the other cells beat 

 columnar, and also in the trachea or 



windpipe, where they have in addition U P ut f the air pas- 



r i ,1 j sages and lungs. 



a covering 01 cilia on the exposed sur- 

 face. These cilia, moving somewhat like the oars of a Ciliated 

 boat, are very fine protoplasmic threads of no great "achea the 

 length. Their activity depends upon the life of the 

 cell, not upon that of the whole organism, and they may 

 be seen in motion under the microscope after removal 

 from the body. Their function is to convey upward to 

 the mouth the innumerable fine particles of dust which 

 exist in the air we breathe, and adhere to the moist sur- 

 faces of the air passages. 



Horns are special developments of epidermal tissue, Horns and 

 characteristic of certain hoofed animals. The horn core antlers 

 of the ox, which is a bony extension of the skull, is 

 covered with true horn of epidermal origin ; and hence 

 the horn when removed is hollow, and may be used by 

 Little Boy Blue and others to produce a sound. The 

 antlers of deer are outgrowths of bone, and are thus 



