THERMAL PHENOMENA OF MUSCULAR CONTRACTION 763 



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

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

 ture is much less ; and thermo-electrical methods, which are the 

 most delicate at present known, have generally been used for its 

 detection and measurement. 



They depend upon the fundamental fact of thermo-electricity, that 

 in a circuit composed of two metals a current is set up if the junctions 

 of the metals are at different tempera- 

 tures. 



Where no very fine differences of 

 temperature are to be measured, a single 

 thermo-j unction of German silver and 

 iron, or copper and iron, is inserted into 

 a muscle or between two muscles. But 

 the electromotive force, and therefore 

 the strength of the thermo-electric cur- 

 rent, 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 cir- 

 cuit is such that it passes through the 

 heated junction from bismuth to anti- 

 mony 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. In the 

 thermopiles employed in the recent ex- 

 tensive investigations of Hill the alloy 

 constantan is coupled with iron, the 

 electromotive force of this combination 

 being exceptionally great. 



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. 

 The image will now come to rest on the 

 scale; and excitation of the muscle will 

 cause a movement indicating an increase 

 of temperature in it, the amount of which 

 can be calculated from the deflection. In 

 one form (Fig. 270) the thermopile con- 

 stitutes a hollow cone, in which a muscle can be arranged so as to eliminate 

 largely the errors due to differences of temperature of the muscle, or to 

 the " slip " of the contracting muscle over the junctions. 



In this way Helmholtz observed a rise of temperature of 0-14 to 

 o -18 C. in excised frogs' muscles when tetanized for a couple of minutes . 



S 



Fig. 270. Conical Thermopile 

 containing Gastrocnemius Mus- 

 cle Reversed. C, copper leads 

 to galvanometer; S, stimulating 

 wire. The straight lines indicate 

 iron, the crossed lines constan- 

 tan, the external junctions em- 

 bedded in the ebonite frame 

 being at a, the internal junc- 

 tions, b, in contact with the 

 muscle. 



