THE LIVING MATERIAL OF THE BODY 



FIG. 12. FAT CELLS 



a, representing young cells 

 just beginning to store fat; 

 6, fully developed cells 

 filled with fat. 



We may, therefore, define a cell in two ways. We may 

 say that it is one of the unit masses of which the tissues are 

 formed, or we may say that it is a 

 bit of protoplasm containing a nucleus 

 and generally surrounded by a cell wall. 

 In either case, the cell is the unit of 

 life. 



The Life of Cells. There are cer- 

 tain great differences between this 

 unit of living matter and a non-living 

 thing. Three distinguishing qualities 

 belong to the living cell: (1) growth, 

 (2) self-repair, (3) increase in num- 

 bers through self-division. These powers are possessed by 

 no other material in the world save protoplasm. 



The growth of a cell is in all cases brought about by material 

 taken in from the outside. In the human body this material 

 is food, which after di- 

 gestion passes into the 

 blood and is then taken 

 in by the cells. This 

 process will be described 

 more fully later. In 

 some of the very lowest 

 organisms, where the 



FIG. 13. SHOWING THE RELATIVE SIZE 

 OF A PIN HEAD AND A LARGE SIZED CELL 



The small dot in the center represents the cell. 



whole animal is a single 

 cell, solid particles may be taken into the cell through 

 definite openings or "mouths"; Fig. 15. In others, the 

 cell may change its shape so as to wrap itself about ths 

 particle to be taken in. But even in these instances the parti- 

 cles must be dissolved or digested before they can be built 

 up into the protoplasm of the cells. 



A machine in motion wears out, and the worn out parts 

 must be replaced. Cells, too, wear out and new cells must be 

 formed or new protoplasm is required to repair the old ones. 



