154 CHANGES IN MUSCLE. [CH. xni. 



contractions; that is, more chemical decomposition occurs, and 

 fatigue ensues more rapidly in the first case. This fact is within 

 the personal experience of everyone. If one ascends a tower, the 

 work done is the raising of the weight of one's body to the top of 

 the tower. If the staircase in the tower has a gentle slope, each 

 stair being low, far less fatigue is experienced than if one ascended 

 to the same height by a smaller number of steeper steps. 



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 tempo- 

 rarily 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 

 tetanising them for two or three minutes, the temperature rises 



Ir---- J 



A<-B B--A 



1 Couple. 2 Couples. 3 Couples. 



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



0-14 to-o'i8 C. ; and for each single twitch Heidenhain gives a 

 rise of temperature of from o'ooi to 0-005 ^ 



For the detection of such small rises in temperature a thermo- 

 pile, 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 

 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 galvanometer. 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 

 4 ^ of a degree Centigrade. 



Within certain limits, the strength of the current is directly 

 proportional to the rise of temperature at the junction. 



