HORSE-BACK RIDING. 47 



manent contraction, keeping the ribs constantly 

 raised and scarcely yielding to the weight of the 

 chest, which at every shock, acted upon by gravity, 

 slightly falls. This condition causes an effort which 

 cannot be kept up long, and symptoms of intoxica- 

 tion caused by the carbonic acid gas soon make their 

 appearance ; for in this case the change of air which 

 should be effected in the lungs is much less complete 

 than when the inspirations are large and deep, and 

 fatigue soon supervenes. It was to soften the shock 

 and avoid these inconveniences that the English way 

 of rising in the saddle was introduced, which makes 

 the riding a hard trotting horse less tiresome. 



In a rapid gallop, another phenomenon presents 

 itself — the difficulty of respiration. This is explained 

 by the greater pressure on the air contained in the 

 thoracic cavity, thereby requiring greater efforts for 

 expiration ; and it is to break this external pressure, 

 which, confining the air in the chest, hinders its exit, 

 that jockeys lean over their horse's neck and wear 

 large visors on their caps. 



In studying muscular contraction, we have seen the 

 blood which brings the fuel for combustion load itself 

 with the carbonic acid and the vapor of water pro- 

 duced by the oxydization of the ternary substances. 

 These gases, if left to accumulate in the blood, imme- 

 diately become an obstacle to the performance of its 

 functions ; the veins collect this blood which has lost 

 its virtue and become useless, and carry it to the 



