GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 133 



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. 1 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. 



The 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, 2 Fick and his pupils, 3 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 

 activity, greatly increases the heat-production. As the weight is increased, 

 both the amount of heat developed and the work are increased, but the libera- 

 tion of heat reaches its maximum and begins to decline sooner than the amount 

 of work, i. e. with large weights the muscle works more economically ; similarly, 

 as the muscle is weakened by fatigue the heat-production lessens sooner than 

 the work. 



Muscle-torms and Chemical Tonus. During waking hours, the cells 

 of the central nervous system are continually under the influence of a shower 

 of weak nervous impulses, coming from the sensory organs all over the body ; 4 

 moreover, activity of brain-cells, especially emotional forms of activity, leads 



1 Fick : Pfluger's Archiv, 1878, xvi. p. 89. 



2 Mechanische Leistung, Wdrmeentwicklung und Sto/umsatz bei der Muskelthatigkeit, Leipzig, 

 1864. 



3 Myothermische Untersuchungen aus den physiologischen Laboratorium zu Zurich und Wurzburg, 

 Wiesbaden, 1889. 



4 Brondgeest: Archiv fur Anatomic und Physiologic, 18^0, p. 703; Hermann, Ibid., 1861, 

 p. 350. 



