THE TISSUES 63 



portion than the increased work just as to increase the 

 speed of a ship or an engine requires an increase of coal 

 consumption in a proportion roughly corresponding to the 

 square of the increased speed. 



Y. Heat Production. 



In muscle, as in other machines, by no means the whole 

 of the energy rendered kinetic is used for the production 

 of mechanical work. In a steam-engine much of the 

 energy is dissipated as heat, and the same loss occurs in 

 muscle. 



If heat is given off when a muscle contracts, either the 

 muscle itself, or the blood coming from it, will become 

 warmer. Hence to detect such a change some delicate 

 method of measuring changes of temperature must be 

 employed. The mercurial thermometer is hardly sufficiently 

 sensitive, and, therefore, the thermo-electrical method is most 

 generally employed. Various forms of thermopile may be 

 used. 



The rise of temperature in a muscle after a single con- 

 traction is extremely small, but after a tetanic contraction, 

 lasting for two or three minutes, it is very much greater. 



The amount of heat produced may be calculated if 

 (a) the weight of the muscle; (6) its temperature before 

 and after contraction; and (c) the specific heat of muscle, 

 are known. 



The specific heat of muscle is slightly greater than that 

 of water, but the difference is so slight that it may 

 be disregarded. If, then, ten grammes of muscle had a 

 temperature of 15 C. before it was made to contract, and 

 a temperature of 15*05 C. after a period of contraction, then 

 0*5 gramme-degrees of heat have been produced ; i.e. heat 

 sufficient to raise the temperature of 0*5 gramme of water 

 through 1 C. 



The amount of heat produced by muscle in different 

 conditions varies so greatly that it is unnecessary to consider 

 it further. 



Relationship of Heat Production to Work Production. 

 Since it is possible to measure both the mechanical work 



