PRODUCTION OF HEAT IN MUSCLE 



supported in any position by the quartz fibre. 

 the magnet by the thermo-electric Lrents are 



a q bre only 6 * thick - Thia 



an 



>(> t uj in 



A second method depends on the fact that rise of temperature increases the resistan 

 of a wire to the passage of an electric current. A currVnt detector comilts oU 

 grid of fine platmum w,re which is placed against the muscle between two 

 This grid is then made one 

 limb of a Wheatstone's bridge 

 (Fig. 74A). A small current 

 is passed through the circuit, 

 and the resistances are so 

 adjusted that no current 

 flows through the galvano- 

 meter. Any alteration in 

 temperature of the grid will 

 alter the balance of the re- 

 sistance and will cause a 

 current to flow through the 

 galvanometer in a direction 

 which will vary according 



as the resistance in the grid FIG. 74A. Arrangement of apparatus for measuring small 

 is increased or diminished. It differences of temperature, 

 is possible to calibrate the 



arrangement so that a deflection of the galvanometer over one degree will correspond 

 to a certain fraction of a degree of difference in temperature of the grid. This method 

 is employed in Callender's recording thermometers, and has been made by Gamgee 

 the basis of an arrangement for the continuous record of the temperature of the 

 human body. 



Most of the earlier work on the development of heat in muscle had as 

 its leading motive the discovery of the relation between the heat produced 

 and the work performed by a muscle under varying conditions of load. 

 When a loaded muscle contracts, however, it is not easy to analyse its 

 mechanical conditions, since part of the shortening of the muscle during 

 contraction can be regarded merely as a recovery from the condition of 

 extension induced by the weight, and the amplitude of the excursion may 

 be largely conditioned by the inertia of the weight moved. Working on 

 these lines, Heidenhain discovered that the heat production in muscle 

 during contraction is not an invariable quantity, but varies according to 

 the condition of the muscle and especially according to the tension developed 

 in it during contraction. It was therefore at its maximum under isometric 

 conditions when it was not allowed to shorten at all during contraction. 

 As we have seen, the muscle changes, as the result of excitation, from a body 

 having certain elastic properties to one having other elastic proper! i-s. 

 The whole energy of the contraction is converted for a short period into a 

 state of tension which can be used to do work by raising a weight, 

 be not allowed to shorten, the state of tension passes off and th> wh 

 energy which has been set free must appear as heat. The potential en< 

 developed in a muscle twitch is approximately equal to /. T/. where 1 

 tension developed and I the length of the muscle, and it 

 which must be compared with the heat production measured in the 



