PRODUCTION OF HEAT IN MUSCLE 



221 



Ctv 



: 



strength of the constant external field in which the magnet lies, and leave it chiefly 

 supported in any position by the quartz fibre. Thus all the movements set up in 

 the magnet by the thermo-electric currents are working against little more than the 

 torsion of a quartz fibre only 6//, thick. This explains the great sensitivity of the 

 instrument. 



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

 of a wire to the passage of an electric current. A current detector consists of a small 

 grid of fine platinum wire wljich is placed against the muscle between two muscles. 

 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 



ows through the galvano- 



eter. Any alteration in 



mperature 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 



FIG. 74A. Arrangement of apparatus for measuring 

 small differences of temperature. 



as the resistance in the grid 

 is increased or diminished. It 

 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 Calender'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 

 bhese lines, Heidenhain discovered that the heat production in muscle 

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

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

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

 mditions when it was not allowed to shorten at all during contraction, 

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

 iving certain elastic properties to one having other elastic properties, 

 'he 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. If it 

 >e not allowed to shorten, the state of tension passes off and the whole 

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

 ieveloped in a muscle twitch is approximately equal to ^ Tl, where T is the 



