36 HORSE-BACK RIDING. 



produces the heat ; then the fibres consume a portion 

 of this heat, converting it into mechanical force. 



A great number of facts support these propositions, 

 and brilHant experiments corroborate them. It has 

 been demonstrated that the temperature of a con- 

 tracting muscle rises, that it absorbs more oxygen 

 and exhales more carbonic acid when in action than 

 when in repose, and, lastly, that the energy of the 

 contractions is in direct proportion to the activity 

 of the internal combustion. The real agent, there- 

 fore, of muscular contraction is the heat produced by 

 the combustion, of which the muscles are the seat, 

 resulting from the conflict between the blood and the 

 nervous system. According to Mayer, *' a muscle is 

 simply an apparatus by which a conversion of forces 

 is effected, but it is not the substance by the chemical 

 change of which the mechanical effect is produced." 



The contraction of the muscle, then, causes a fresh 

 portion of the arterial blood to enter the organ in a 

 far greater portion than would flow to it in a state of 

 repose, and consequently the capillary circulation of 

 the muscle Is accelerated. The phenomena of com- 

 bustion accomplished, the venous capillaries carry 

 away this blood charged with the products of oxydi- 

 zation, while the contraction, the result of the chem- 

 ical action, aids in the disgorgement of the muscle 

 in order to give place to a fresh arterial flood which 

 will produce a fresh contraction. Of all the products 

 eliminated by this process carbonic acid is the most 



