12 APPLIED PHYSIOLOGY 



separate cell there are only enough fibers to keep the cell 

 in place. Even these connective tissue fibers are the 

 threadlike arms of very small cells set apart for the 

 menial work of supporting other cells. 



6. Three tests of life. We know that each cell is alive ; 

 for it moves, it takes in food, and it multiplies. 



(1) Motion. Although each cell is held in place by the 

 connective tissue, the tiny particles of its body are in con- 

 stant motion, just as a boy's eyes and mouth and hands 

 and feet may move, even if he sits still in a chair. Besides 

 this continual motion of the particles within the cell, some 

 cells show a greater motion, in which the cell as a whole 

 takes part. Thus, a muscle cell becomes thicker and 

 shorter when the muscle bends a joint. A white blood 

 cell can force its way through the wall of the blood tube, 

 and can wander about among the cells of the body. 



(2) Nutrition and Grozvth. The blood bathes the cells 

 with food which does not resemble their protoplasm. Each 

 cell takes in the food through any portion of its body, and, 

 endowing it with life, makes it a part of itself. Thus each 

 cell increases in size. 



(3) Reproduction. During the period of growth of the 

 body, there is a constant production of the cells ; for the 

 man is composed of more cells than the child. Even in a 

 full-grown man certain cells, as those of the skin, are con- 

 stantly being shed and new ones formed. When a cell 

 reaches mature life, the nucleus first divides into two parts, 

 which separate from each other ; then the body of the cell 

 divides between the two nuclei. Thus each cell becomes 

 two cells, and each of the two exactly resembles the origi- 

 nal cell, except that it is smaller at first ; but it soon grows 

 to be as large as the original cell. All the peculiarities of 

 the life of the first cell go on in each of the two cells into 



