12 Life and Health 



to form fibers, such as those of tendon, muscle, and nerve. 

 Later on we shall see that the white blood corpuscles 

 exhibit all the characters of the amoeba (Fig. 2). Even 

 such dense structures as bone, cartilage, and the teeth 

 contain cells which have for their duty in life the formation 

 and repair of the hard substances of these tissues. 



In short, cells may be regarded as the anatomical units 

 of animal tissues, by the combination of which the body is 

 built up. 



In addition to the living cells, every tissue contains more 

 or less of a lifeless substance between the cells called inter- 

 cellular substance, which is produced at some time by 

 the cells. 



19. Kinds of Cells. Cells vary greatly in size, some of 

 the smallest being only -g-gVo of an inch or less in diam- 

 eter. They also vary greatly in form, as may be seen in 

 Figs. 4 and 6. 



The typical free cell is usually globular in form, other 

 shapes being the result of pressure or of similar modifying 

 influences. The globular as well as the large flat cells 

 are well shown under the microscope in a drop of saliva 

 scraped from the tongue. 



Then there are the columnar cells, found in various parts 

 of the intestines and the respiratory passages. The latter 

 cells sometimes have on the free surface delicate prolonga- 

 tions called cilia. Under the microscope they present a 

 wavy appearance, like that of a field of grain when the 

 wind blows over it (Fig. 6). There are, besides, cells 

 known as spindle, stellate, and pavement cells, and others 

 having various names suggested by their shapes. 



Cells are also described as to their contents. Thus, fat 

 cells and pigment cells are alluded to in succeeding 

 sections. 



