SIGNS OF ACTIVITY IN MUSCLE AND NERVE 439 



contractions of human muscles it amounts, according to various authors, to 

 10 kg. per square centimeter of cross section. , 



If an experiment be so arranged that the muscle lifts its load after it has 

 contracted to different heights, and the absolute power for these successive heights 

 be determined, we find that it grows steadily less (Schwann's experiment). 



4. The Work of Tetanized Muscles. In tetanus it is evident that the 

 work done after the tetanus has reached its full height is from a mechanical 

 point of view nothing at all. Since, however, the contracted state always 

 calls for an expenditure of energy, tetanus is accompanied by a relatively 

 great consumption of substance, which in its turn leads to rapid fatigue. 



The work of tetanus as related to its shortening is in general similar to 

 that of a single contraction, only it is more extensive, so that under favorable 

 circumstances the shortening may amount to as much as sixty-five to eighty- 

 five per cent of the muscle's length. Moreover the ratio of shortening in 

 tetanus to shortening in simple contraction is very different for different kinds 

 of muscles; thus it is stated that the maximum shortening in tetanus of the 

 white muscles of the frog is two to three times the maximum shortening in 

 a simple contraction; of the red muscles eight to nine times. 



The height of tetanus with a constant load depends on the strength of 

 stimulus, but not upon the frequency of stimulation. 



E. HEAT FORMATION IN MUSCLE 



By employing the thermo-electrical method, Helmholtz (1847) demon- 

 strated the formation of heat in the tetanus of the exsected frog's muscle. 

 Later the production of heat in a simple contraction was demonstrated by 

 Heidenhain. And Blix has shown that heat is formed even in resting muscle. 

 Even with the most delicate methods no heat production can be demonstrated 

 in nerves. 



Since the performance of mechanical work and the production of heat 

 are the two chief functions of muscle, and since, as we have seen above, the 

 mechanical work done under a constant stimulus increases up to a certain 

 limit with the load, it might be supposed that the heat production going on 

 at the same time would be in inverse relation to the load, so that the dissimi- 

 latory process evoked in a muscle by a given stimulus would be independent 

 of the load, and the latter therefore would influence only the apportionment 

 of the total output of energy by the muscle to the two functions. But this 

 is not the case. Since the investigation of Heidenhain, we know that the 

 total output of energy in an exsected frog's muscle under a constant stimulus, 

 increases up to a certain limit with the load. 



This property of the muscle appears to be of very great importance. For 

 if the total performance of the muscle were independent of the load and were 

 dependent only on the strength of stimulus, the development of energy in the 

 muscle might often be out of all proportion to the work to be done. The rela- 

 tionship discovered by Heidenhain is to be looked upon as a regulatory mechanism 

 which, independently of the nervous impulses, controls the metabolism in the 

 muscle according to its momentary needs (cf. also page 441). 



