26 GENERAL STRUCTURE OF THE BODY. 



amount of work done is calculated in foot-pounds or 

 gram-meters, that is, the energy required to raise one 

 pound one foot or one gram one meter. As a rule the 

 muscles with the longest fibers, as the biceps, do the 

 most work and those with a large number of fibers do 

 more than those with less. It has been calculated that 

 whereas an engine gives back one-twelfth of the energy 

 of the coal consumed, muscle liberates one-fourth of the 

 energy brought to it in the form of food. During ac- 

 tivity the glycogen or sugar in the muscle is used up and 

 the muscle becomes more acid, owing to the lactic acid 

 that is formed. The carbon is taken in and carbon di- 

 oxide given off. Nitrogen puts the muscle in condition 

 to do its work but is not so much used up in the work as 

 is the carbohydrate material. So it is the non-nitrogen- 

 ous matter that does the work and any increase in urea, 

 the end-product of protein metabolism, is mere wear and 

 tear. 



Sudden heat or cold causes muscular contraction and 

 moderate heat favors both muscular and nervous irri- 

 tability. Moderate cold, however, lessens the force of 

 contraction and below zero muscle very largely loses 

 its irritability without necessarily becoming rigid. 



While well supplied with blood, muscle will contract 

 without fatigue, but if the blood supply is shut off, it 

 soon loses its irritability and becomes rigid. The more 

 a muscle is used in moderation the more it develops, but 

 after it has done a certain amount of work it becomes 

 exhausted, losing its irritability or power to respond to 

 stimuli and later becoming rigid. Such fatigue is due 

 to the production of certain poisonous waste products 

 which have a paralyzing effect on the nerves and which 

 are ordinarily gradually carried away in the blood, but 

 which sometimes, if produced to excess, accumulate 

 too fast for the blood wholly to remove them. Usually 

 the nerve becomes exhausted first and the muscle sub- 

 stance later. So long as it is connected with the nervous 

 system a muscle will respond to stimuli, but when the 



