THE LIVING MATERIAL OF THE BODY 



17 



Although there is much diversity in the forms of cells in 

 the body, each tissue is made of only a single kind which 

 does a single kind of work. The size of cells varies widely. 

 Some of the smaller cells of animals are not more than 



0.003 mm. (^Vo in ) in 

 diameter, while the egg, 

 which is essentially a cell, 

 is sometimes several 

 inches in diameter. The 

 majority of cells in most 

 animals vary from 0.008 

 mm. to 0.01mm. (-g-yVir to 

 -S-SIHF i* 1 -) m diameter; 

 Fig. 13. 



Structure of Cells. In 

 recent years much atten- 

 tion has been paid to the 

 more minute details of 

 cell structure. As a re- 

 sult we now know that 

 all cells, while differing 

 in outward shape, are very much alike in their internal 

 organization. Each one is filled with a semifluid substance 

 called protoplasm, and this protoplasm is the part 'of the 

 cell which is really alive. Protoplasm should not be thought 

 of as life itself but it is the only material in which life is 

 known to occur. When highly magnified, protoplasmic fluid 

 always seems to contain fine granules, or it may look like 

 a foam of extremely small bubbles or show a network 

 of excessively minute lines; Fig. 14. At all times, if the 

 cells under observation are alive and are not too much 

 disturbed, movement of the protoplasm is evident. No 

 one knows the cause of this motion; it can only be 

 accounted for by saying that protoplasm is alive. 



Within each cell is a specially dense bit of protoplasm called 



FIG. 10. BLOOD 



As it appears when put on a glass slide and 

 highly magnified. At a are shown red cor- 

 puscles and at b white corpuscles or leuco- 

 cytes. 



