48 IIORSE-BACK RIDING. 



different emunctories, where it is purified, and to the 

 lungs, where it is endowed with a new life. But by 

 this act of muscular exercise the proportion of hurt- 

 ful principles has increased, the need of active ele- 

 ments is still greater, that which was before sufficient 

 to meet the demand is now too little, and the activity 

 of the respiratory phenomena should correspond with 

 the respiratory. The surface of contact between the 

 blood and the air has enlarged, the number of respir- 

 atory movements has increased, the air expired con- 

 tains more carbonic acid than when in a state of 

 repose, and it has at the same time lost a greater 

 quantity of oxygen, and it retains a larger amount of 

 azote which is derived from the combustion of the 

 quaternary substances, and this exaggeration of the 

 respiratory phenomena has a limit. If the internal 

 combustion is always effected with the same inten- 

 sity, the moment comes when the circulation is no 

 longer able to carry away the products of combus- 

 tion which accumulate in the muscle, and end by 

 hindering the chemical action ; at the same time 

 bringing with it that peculiar sensation known as 

 fatigue. The phenomena do not cease here, for if 

 the exercise is prolonged beyond reason, the carbonic 

 acid does not all escape through the lungs ; a certain 

 quantity remains in the blood which returns to the 

 heart, and mingling with the arterial blood, produces 

 the accidents which we call cephalalgia, dyspnoea, 

 etc. 



