456 MANUAL OF PHYSIOLOGY. 



the continued (tetanic) contraction produced by repeated stimu- 

 lation is associated with repeated negative variations. Because 

 the contraction of the stimulated muscle whose nerve lies on the 

 stimulating muscle follows exactly all the variations of the stim- 

 ulator, and is kept contracted as long as the other is contracted, 

 and, as we shall see presently, the continued contraction can only 

 be brought about by a rapidly repeated series of stimulations, so 

 that the electric condition of the stimulating muscle must undergo 

 a series of variations. 



If an isolated part of a muscle be stimulated the contraction 

 passes from that point as a wave to the remainder of the muscle. 

 This contraction wave is preceded by a wave of negative varia- 

 tion, which passes along the muscle at the rate of 3 metres per 

 second (the same rate as the contraction wave, see under), lasting 

 at any one point .003 of a second, so that the negative variation 

 is over before the contraction begins, for the muscle requires a 

 certain time, called the latent period, before it commences to 

 contract. 



The origin of the electric currents of muscle will be discussed 

 with nerve- currents, to which the reader is referred (p. 504). 



Temperature Change. Long since it was observed in the human 

 subject that the temperature of muscles rose during their activity. 

 In frogs' muscle a contraction lasting three minutes caused an 

 elevation of .18 C. And a single contraction is said to produce 

 a rise varying from .001 to .005 C., according to circumstances. 



The production of heat is in proportion to the tension of the 

 muscle. When the muscles are prevented from shortening a 

 greater amount of heat is said to be produced. 



The amount of heat has alsd a definite relation to the work per- 

 formed. Up to a certain point the greater the load a muscle has 

 to move, the greater the heat produced ; when this maximum is 

 reached any further increase of the weight causes a falling off 

 in the heat production. Repeated single contractions are said 

 to produce more heat than tetanus kept up for a corresponding 

 time. 



The fatigue which follows prolonged activity is accompanied by 

 a diminution in the temperature elevation. 



