148 THERMAL AND CHEMICAL CHANGES IN MUSCLE [CH. XIII. 



On a cold day one keeps oneself warm by muscular exercise ; this 

 common fact is confirmed by more accurate experiments on isolated 

 muscles, the heat produced being sufficient to raise temporarily the 

 temperature of the muscle. This can be shown in large animals by 

 inserting a thermometer between the thigh muscles and stimulating 

 the spinal cord. The rise of temperature may amount to several 

 degrees. 



In the case of frog's muscles, Helmholtz found that, after tetanis- 

 ing them for two or three minutes, the temperature rises 0*14 to 

 018 C. ; and for each single twitch Heidenhain gives a rise of 

 temperature of from O'OOl to 0'005 C. 



For the detection of such small rises in temperature, a thermopile, 

 and not a thermometer, is employed. 



A thermopile consists of a junction of two different metals ; the 

 metals are connected by wires to a galvanometer. If the junction 

 is heated an electrical current passes round the circuit, and is 

 detected by the galvanometer. The metals usually employed are 



A-B B-A 



7 Couple. 2 Couples. 3 Couples. 



FIG. 178. Scheme of thermo-electric couples. (After Waller.) 



iron and German silver, or antimony and bismuth. If the number 

 of couples in the circuit is increased, each is affected in the same, 

 way, and thus the electrical current is increased through the galvano- 

 meter. The arrangement is shown in the fig. 178, which also indicates 

 the direction of the currents produced, the metals employed being 

 antimony and bismuth. By using 16 couples of this kind Helmholtz 

 was able to detect a change of ^Q of a degree Centigrade. 



Within certain limits, the strength of the current is directly 

 proportional to the rise of temperature at the junction. 



If two couples are in circuit, as shown in the second diagram, and 

 they are heated equally, no current will pass through the galvano- 

 meter, the current through one couple being opposed by the current 

 through the other. But if the two couples are heated unequally, the 

 direction of swing of the galvanometer needle indicates which is 

 the warmer. To apply this to the frog's gastrocnemius, plunge several 

 needle-shaped couples (diagram 3) into a frog's gastrocnemius of one 

 side and the same number of couples into the gastrocnemius of the 

 other side, and then excite first one then the other sciatic nerve ; 

 a deflection of the galvanometer will be observed first in one, then in 



