THE PRIMARY CAUSE OF FATIGUE. 883 



and voluntary muscles, and thus creates the ne- 

 cessity for repose or sleep, the object of which is 

 to repair what has been lost. Nor is it unworthy 

 of remark, that after exposure to intense cold for 

 a few hours, or until the body is chilled and be- 

 numbed, there is generally a feeling of drowsiness 

 and tendency to sleep, which are also induced by 

 exhaustion from over exertion. It is therefore 

 evident, that an expenditure of the vital heat and 

 substance of the body, whether by exercise or ex- 

 posure to cold, faster than they are renewed by re- 

 spiration and nutrition, is the proximate cause of 

 fatigue, hunger, and sleep. 



When the lungs are large and sound, and men 

 are supplied with an abundance of nourishing 

 food, they can endure protracted exercise and 

 intense cold without serious injury.* But when 

 exhausted by loss of blood, or when its vital pro- 

 perties are greatly impaired, as in phthisis, disease 

 of the heart, the advanced stages of fever, scurvy, 

 and some other maladies, fainting is often in- 

 duced by comparatively slight exertion, and 



* Edward Johnson maintains, that " it is possible by very 

 rapid exertion, to fill almost every vein in the body with arterial 

 blood." But as if not aware, that although arterial blood is thus 

 rapidly formed and united with the solids, it is still more rapidly 

 expended by exertion, he recommends walking four or five miles 

 every morning, the same distance before dinner, and the same 

 again in the evening. (Life, Health, and Disease, pp. 278-85.) 

 This would, doubtless, be the way to obtain a full developement 

 of the lungs and muscular system ; but it would certainly be at 



