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 activity 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 dioxide 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-nitrogenous 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 irritability. 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 substance later. So long as it is connected with the nervous system a muscle will respond to stimuli, but when the
