FATIGUE. 55 



the same time, the system requires rest proportionate in 

 length to the effect which the exertion has had on it/so^as to 

 allow time for the assimilation of the necessary potential 

 energy and for the removal of poisonous products (p. 56). 

 Although the heart, which is a hollow muscle, appears to be 

 in constant work, it has a succession of brief periods of rest 

 and of labour, the former being about twice as long as the 

 latter. 



FATIGUE. 



We may define fatigue as exhaustion from too much exercise 

 or from too much work. Bunge aptly states that " the 

 sensation of fatigue is one of the safety valves of our nature," 

 which is a remark we may also apply to horses. If an extract 

 of fatigued muscle be injected into the muscles of a healthy 

 and unfatigued animal, it will produce symptoms of fatigue, 

 which fact tends to prove that the feeling of fatigue is due to 

 the formation, during exercise or work, of a poison or poisons 

 in the system. Bodily exertion carried to the extent of 

 fatiguing an animal, interferes with the healthy performance of 

 excretion ; the blood becomes loaded with carbonic acid, on 

 account of the lungs not being able to give it off into the air 

 with sufficient quickness for the requirements of health ; 

 breathlessness ensues ; and congestion of the lungs may 

 supervene in extreme cases. Ordinary breathlessness is there- 

 fore the poisoning of the body with carbonic acid which the 

 action of the muscles causes to be produced at a faster rate 

 than the lungs can remove from the system. In less violent 

 work, fatigue may take place without breathlessness 

 occurring. By practice, the organs of breathing acquire the 

 ability to regulate their action, so that breathlessness may be 

 prevented as far as possible. 



The nitrogen of the broken-up nitrogenous matter which is 

 taken into the blood, is normally found almost entirely in the 



