142 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



even during violent muscular exercise five times more energy may be ex- 

 pended as heat than as mechanical energy, and the disproportion may be 

 even oreater than this. Rosenthal says that at the most not more than 30 

 per cent, of the energy developed in the muscle by oxidation and splitting 

 processes is to be got as available mechanical energy. So great is the pro- 

 duction of heat during exercise that, in spite of the great amount leaving the 

 body, the temperature of an oarsman has been found to be increased, during 

 a race of 2000 meters, from 37.5° C. to 39° or 40° C. 1 



It is exceedingly difficult to ascertain with accuracy on the warm-blooded 

 animal the exact relation of heat-produceion to muscular contraction. The 

 best results have been obtained by experiments on isolated muscles of cold- 

 blooded animals. Helmholtz observed the temperature of a muscle of a 

 frog to be increased by tetanus lasting a couple of minutes 0.14° to 0.18° 

 C. ; Heidenhain saw a change of 0.005° C. result from a single contraction; 

 and Fick ascertained that a fresh, isolated muscle of a frog can by a single 

 contraction produce per gram of muscle-substance enough heat to raise 

 3 milligrams of water 1° C. 2 To obtain evidence of the slight changes 

 of temperature which occur in such small masses of muscle-tissue it is 

 necessary to employ a very delicate instrument, such as a thermopile or a 

 bolometer. 



TJie thermopile consists of strips of two dissimilar metals, united at their extremities, 

 so as to form a series of thermo-electric junctions. If there be a difference of temperature 

 at two such junctions, a difference of electric potential is developed, which causes the 

 flow of an electric current. If the current be passed through the coils of wire of a 

 galvanometer its amount can be measured, and the extent of the change in tempera- 

 ture at one of the junctions, the other remaining constant, can be estimated. In the 

 more sensitive instruments, several thermo-electric junctions are used. The amount of 

 current depends largely on the metals employed, antimony and bismuth being a very 

 sensitive combination. 



The action of the bolometer is based on the fact that the resistance of a wire to the 

 passage of an electric current changes with its temperature. 



The amount of heat developed within the muscle by direct conversion of 

 potential to thermal energy, and the amount formed indirectly, through con- 

 version of mechanical to thermal energy, has been made a subject of careful 

 study by Heidenhain, 3 Fick and his pupils, 4 and others, the experiments being 

 made chiefly with isolated muscles of frogs. 



In general, the stronger the stimulus and the greater the irritability of the 

 muscle — in other words, the more extensive the chemical changes excited in 

 the muscle — the greater the amount, not only of mechanical, but of thermal 

 energy liberated. Increase of tension, which is very favorable to muscular 



1 George Kolb : Physiology of Sport, translated from the German, second edition, London. 

 1892. 



J Fick : Pfliiger'a Archiv, 1878, xvi. S. 89. 



3 Michanische Leistung, Warmeentwicklung und Stoffumsatz bei der Muskdthatigkeit, Leipzig, 

 1864. 



4 Mnothermische Untersuchungen aus den physiologischen Laboratorium zu Zurich and Wurzburg, 

 "Wiesbaden, 1889. 



