244 



HUMAN PHYSIOLOGY 



CHAP. 



ments and makes them more accurate. It makes many 

 matters of daily routine, as dressing and undressing, 

 entirely mechanical, and leaves the mind free to do 

 other things. 



167. Fatigue and Overwork. Whenever brain cells 

 work they consume foods, produce wastes, and become 

 fatigued. Up to a certain point brain activity is condu- 

 cive to health, but if too long continued or too severe, 

 the cells may be so injured that ordi- 

 nary rest and sleep cannot restore 

 them. Fatigue is due (i) to the con- 

 sumption of the foods stored in the 

 cells and lymph, which causes the 

 cells themselves to shrink in size and 

 become rather irregular in outline, 

 and (2) to the formation of poisonous 

 wastes that accumulate in the cells 

 and in the blood. The brain centers 

 may be so poisoned by the waste 

 products that one does not realize how 

 fatigued he really is. Unless these 

 wastes are removed, the cells repaired, 

 and new foods stored up for future 

 use, the brain may become perma- 

 Food, oxygen, rest, and sleep must 

 alternate with periods of activity if we would keep our 

 bodies in a healthy condition. 



Mental overwork is possible. Frequently, however, 

 the real cause is not the amount of work actually accom- 

 plished, but the conditions under which it is done. If a 



FIG. 132. Showing 

 changes in a nerve 

 cell due to fatigue. 

 A, resting ; B, stim- 

 ulated one hour; 

 C, stimulated six 

 and one half hours. 

 (From Hodge.) 



nently injured. 



