HORSE-BACK RIDING. 37 



important and most easily produced by experiments. 

 It is evidently formed in the muscle, for the blood 

 contains before entering it much less than is found 

 after its exit, and it is formed during contraction, 

 since after contraction the proportion is increased. 



It has been found that the venous blood contains 

 an average of 6-75 parts more of carbonic acid than 

 the arterial, when the muscle is in a state of repose, 

 and 10-79 when it is in a state of contraction. 



If the action of the muscle is too long continued, 

 the increased circulation in its substance is no longer 

 sufficient to carry away the products of combustion, 

 which goes on incessantly, so they accumulate in the 

 muscle and a new product is formed there — lactic 

 acid. Then the muscle loses its elasticity, its energy 

 and precision ; movement becomes painful, combus- 

 tion is less active, there is a decrease in power, and 

 we have an exhibition of the phenomenon termed 

 fatigue. Repose, by allowing the venous blood to 

 carry the products injurious to the economy, by 

 lessening the intensity of the combustion, which is 

 the cause of the trouble, removes all these symptoms, 

 which might be produced, on the other hand, in an 

 animal in a state of repose, by a simple injection of 

 lactic acid into the substance of the muscle. 



This activity of the circulation has another object 

 and effect, that of carrying to the organ a still 

 larger quantity of nutritive matter, which it assimi- 

 lates. It is shown by experiment that a muscle ex- 



