182 tiLttMKMIH OF HIPPOLOGY. 



control the organs of respiration, digestion, and circulation are, 

 almost entirely, involuntary, but their actions are under control 

 of nerve impulses, coming, as a rule, from nerve-centers other 

 than the brain. No muscle contracts unless a nerve animates it, 

 and there must be a nerve impulse for every contraction. 



" Prolonged exercise is always accompanied by an expend- 

 iture of food, of blood, and of nerve-tissue, by an increased cir- 

 culation, by an acceleration of respiration, and, finally, by a 

 greater activity' '* of the organs of excretion. Admitting this, it 

 is plain that the tired horse owes his condition indirectly to one 

 of two causes: either to nervous exhaustion, or to an excessive 

 clogging of the muscles with waste tissues. In either case, rest 

 and food are the only restoratives. 



The ordinary gasoline motor illustrates the situation ex- 

 actly. A drop of gasoline is exploded by an electric spark in a 

 cylinder. The resultant gases, by their expansion, move a piston 

 that is geared to the wheels, producing locomotion. As long as 

 the supply of gasoline is kept up, the electric batteries remain 

 " alive," and the spent gases can escape and not clog the cyl- 

 inders, the efficiency of the machine is unimpaired. But let 

 any one of these conditions lapse and the machine stops. So 

 with the horse. As long as the digestive apparatus enriches the 

 blood, the lungs oxidize it, the veins carry away the waste 

 tissues, the skin, lungs, kidneys, and other organs of excretion 

 remove the debris from the system, and the vigor of the nervous 

 system (the horse's electric battery) is unimpaired, so long will 

 the horse do his work. The failure of any one of these functions 

 and the limit of his endurance is reached. He may stumble 

 along a little longer, but it is with apparent labor and to little 

 purpose. He needs rest. 



From the exterior of the horse a great deal can be predicted 

 of his powers of endurance. The competent horseman can tell 

 exactly to what sort of work he is best suited — whether he is a 



*"The Exterior of the Horse," Goubaux and Barriere, p. 415. 



