30 STUDIES IN PHYSIOLOGY 



the protoplasm is oxidized ; every time we think or exert 

 our will power, some of the living tissue of the brain is 

 probably changed into dead waste material. But, in contrast 

 to lifeless machines, our bodies are self-repairing. The food 

 we eat not only goes to increase the size of the body ; it also 

 furnishes material to make good the wear and tear of every- 

 day life. 



In the human body, then, a given kind of food substance 

 may at one time be used for the growth of the body ; at another 

 time it may serve for the repair of the tissues; or still again, 

 as in the engine, it may be burned to keep us warm and 

 to give us power to work. The processes of growth, repair, 

 and the production of energy in the human body are un- 

 doubtedly extremely complex. Oxidation is probably only 

 one of the processes involved. To the whole series of 

 changes by which food is transformed into protoplasm or 

 is made to yield energy is given the name me-tab'o-lism 

 (Greek metabole = a change). 



Cell Division Since cells are the units of which the 

 body is composed, it is evident that when the tissues grow, 

 cells must increase either in size or in number. Biologists 

 know that cells remain nearly constant in size, however 

 large the body may grow. The number of cells, however, 

 increases enormously as one grows from childhood to adult 

 life. 



The formation of new cells can be watched in animals like 

 the amoeba. Some little time before division is to take place 

 the cell ceases to move about, pulls in its false feet, and be- 

 comes more or less spherical in form. Important changes 

 appear first in the nucleus. It gradually assumes the form of 

 a dumb-bell ; the knobs at the end of the dumb-bell then 

 draw away from each other, until finally the connection 

 is broken between the two halves of the nucleus. The 

 amoeba has now two nuclei, one at each end of the cell. 

 Meanwhile the cell body has been assuming a more or less 

 oval form, and its protoplasm begins to divide into halves. At 



