THE MUSCULAR SYSTEM 413 



of the products of muscular activity on the central motor nerve- 

 cells, and some experiments on men go to show that the central 

 nervous system is readily affected by fatigue products. On the 

 other hand, there is the possibility that fatigue, if we look upon 

 it as an inability to drive the muscular machinery up to its normal 

 capacity, may be due to the deleterious influence of the products 

 of muscular contraction on the end-plates. This view receives 

 support from the fact that a fatigued muscle will contract by 

 direct stimulation when it refuses to respond to a stimulus 

 brought to it through its nerve. 



Experiments on soldiers performing muscular work show how 

 readily the intake of 2 may be increased ; such causes as diffi- 

 culties in the road, rising ground, increase in pace, change in the 

 load carried, unpractised movements, even a sore foot, may 

 increase the consumption by 18 per cent. Fatigue produces 

 wasteful metabolism, and may increase the C0 2 excreted even as 

 much as 21 per cent. The abnormal use of certain muscles, such 

 as a man with sore feet would employ in order to save himself 

 pain, produces extravagant combustion and fatigue. What 

 applies to man in these matters applies equally to the horse ; 

 ungreased axles, badly-fitting harness and saddlery, badly-made 

 roads, sore backs and lameness, all represent undue muscular 

 wear and tear. Fortunately we are indebted to the labours of 

 Zuntz* for a more exact expression of the facts. He found a 

 difference of 33 per cent, in the oxygen absorbed by two horses 

 doing the same amount of work, and this difference was attributed 

 to one of the horses being a high stepper. 



The nervous mechanism connected with the use of the skeletal 

 muscles cannot be considered here. The muscles have to produce 

 two effects — viz., posture and movement. We shall see in the 

 chapter devoted to the Nervous System that these are in essence 

 reflex acts, for the mechanisms connected with movements and 

 posture may be carried out even after removal of the brain. 

 The question of muscular movement and attitude is of great 

 practical importance, especially as affecting horses, while the 

 experimental work, mainly carried out by Sherrington, by which 

 the facts have been ascertained, have thrown remarkable light 

 on the inner working of the nervous system. 



Condition. — That remarkable state of the body described as 

 1 condition,' into which horses can be brought by care in feeding, 

 general management, and carefully regulated work, must be 

 regarded as the highest pitch of perfection to which muscles can 

 attain. In its highest degree it is not a permanent state ; no 

 horse can remain in it for any length of time, and many can never 

 be got into condition for severe work. It is easy in the training 



* Op.cit. 



