SKELETAL MUSCLE 75 



supply will be drawn upon and exhaustion will steadily 

 come nearer. The situation is like that of an automobile 

 on the road. Its possible travel is determined by the 

 quantity of gasoline carried. The isolated muscle can- 

 not be furnished with fresh fuel as the motor car can be. 

 The comparison suggests that a fatigued muscle is one 

 in which the store of fuel is running short. We shall find, 

 however, that this is not the whole story. 



A fire may go out from lack of draught as well as from 

 want of fuel. The chemical processes in a muscle which 

 are necessary to cause contraction may be retarded in 

 an analogous way. A deficient oxygen supply will 

 certainly hasten fatigue. Another condition to be given 

 due weight is the possible gathering in the tissue of 

 products generated in the course of its own activity. 

 The draught provided for a fire has a double purpose. 

 It introduces oxygen and the next instant it sweeps 

 away the gases formed in combustion. The blood which 

 is normally flowing through a muscle has a like service. 

 Interrupt the circulation and the oxygen supply is cut 

 off at the same time that provision for removing carbon 

 dioxid is abolished. 



Without detailing many experiments that have thrown 

 light upon this matter we may say that muscular fatigue, ' 

 as observed in laboratory conditions, is much more 

 likely to be due to accumulating waste-products than 

 to imminent exhaustion of fuel. Carbon dioxid is net 

 the only one concerned; another which receives much 

 attention by writers on the subject is a variety of acid 

 (sarcolactic acid) closely resembling that which is formed 

 in the familiar souring of milk. This is certainly pro- 

 duced in active muscles whenever their working is out 

 of proportion to the available oxygen and it definitely 

 lowers their capacity for contraction. Still other 

 "fatigue substances" beside carbon dioxid and lactic 

 acid have been mentioned by different investigators. 



If a muscle is in the body instead of isolated from it, 

 the resistance to fatigue will naturally be increased. 



