56 THEORY OF EXERCISE. 



form of urea (CH. t N c O), which is very soluble in water 

 (dissolving in its own weight of even cold water), and is 

 consequently a harmless product which the kidneys can 

 readily remove from the system, supposing that these organs 

 are in good condition. From the investigations of Roussin 

 (see Colin's Physiologic Compare) it appears probable that 

 during fatiguing exertion, a portion of the urea in the blood 

 becomes replaced by hippuric acid (C 9 H 9 NO 3 ), which is 

 sparingly soluble in water. Uric acid (C 5 H 4 N 4 O 3 ), which 

 is another form of nitrogenous waste, is regarded by Haig as 

 the poison in the blood which gives rise to the feeling of 

 fatigue in mankind. Colin tells us that it is present in 

 abnormally large quantities in the blood of animals which 

 are suffering from inflammatory diseases. Uric acid when 

 comparatively abundant in the blood of man, produces great 

 depression, and also gives rise to rheumatic affections, chiefly 

 in the joints, by becoming deposited in these parts, or by 

 uniting with the sodium of soda salts that are in the blood, 

 and forming " chalk-stones " composed of hydrosodium urate. 

 This salt and uric acid dissolve only to a small extent in 

 water, and their comparative insolubility increases according 

 as the temperature of the blood becomes lowered. Halliburton 

 considers that fatigue is caused by the presence of sarco-lactic 

 acid (C 3 H 6 O 3 ). Whatever may be the poison in question, 

 the fact remains that the imperfect elaboration of nitrogenous 

 waste is an injurious result of fatigue, and that a plentiful 

 supply of drinking water is necessary for the removal of 

 products thus formed. 



The degree of thirst brought on by fatigue is a reliable 

 measure of the need which the system of a tired horse has 

 for water. We find, in training either a man or horse, that 

 the thirst after hard exercise or hard work gradually 

 diminishes as " condition " is attained, which fact is a proof 

 that exercise purifies the system. 



