MUSCLE 663 



matical relation of this sort might be expected to appear when 

 determinations of the rate of oscillation and of accompanying 

 periodic changes are made by methods varying in principle and 

 in delicacy. For instance, an arrangement suited to record and to 

 count coarse vibrations could not be expected to give the same 

 result as an arrangement suited to record and count fine vibrations. 

 But if both the coarse and the fine vibrations were related to a 

 fundamental rhythm, a simple proportion might be expected to exist 

 between the two sets of results. 



(3) Thermal Phenomena of the Muscular Contraction. 

 When a muscle contracts its temperature rises ; the production 

 of heat in it is increased. This is most distinct when the 

 muscle is tetanized, but has also been proved for single con- 

 tractions. The change of temperature can be detected by a 

 delicate mercury or air thermometer ; and, indeed, a thermometer 

 thrust among the thigh-muscles of a dog may rise as much as 

 i to 2 C. when the muscles are thrown into tetanus. In the 

 isolated muscles of cold-blooded animals the increase of tem- 

 perature is much less ; and electrical methods, which are the 

 most delicate at present known, have generally been used for 

 its detection and measurement. 



They depend either upon the fundamental fact of thermo-elec- 

 tricity, that in a circuit composed of two metals a current is set up if 

 the junctions of the metals are at different temperatures ; or upon 

 the fact that the electrical resistance of a metallic conductor varies 

 with its temperature. On the former principle the thermopile has 

 been constructed (Fig. 250), on the latter the bolometer, or 'electrical- 

 resistance thermometer ' (Fig. 251). 



Where no very fine differences of temperature are to be measured, 

 a single thermo- junction of German silver and iron, or copper and 

 iron, is inserted into a muscle or between two muscles. But the 

 electro-motive force, and therefore the strength of the thermo- 

 electric current, is proportional for any given pair of metals to the 

 number of junctions, and for delicate measurements it may be 

 necessary to use several connected together in series. A thermopile 

 of antimony-bismuth junctions gives a stronger current for a given 

 difference of temperature than the same number of German silver- 

 iron couples, but from its brittle nature is otherwise less convenient. 



The direction of the current in the circuit is such that it passes 

 through the heated junction from bismuth to antimony and from 

 copper or German silver to iron. Knowing this direction, we are 

 aware of the changes of temperature which take place from the 

 movements of the mirror of the galvanometer with which the pile 

 is connected. The galvanometer must be of low resistance, since 

 the electromotive force of the thermo-electric currents is small, and 

 a high resistance would cut down their intensity too much. 



The muscle which is to be excited is brought into close contact 

 with one junction or set of junctions, the other set being kept at 

 constant temperature by immersing them in water, or covering them 

 with muscle that is not to be stimulated. The image will now come 

 to rest on the scale ; and excitation of the muscle will cause a move- 

 ment indicating an increase of temperature in it, the amount of 

 which can be calculated from the deflection. 



