XX 



No scientific career, however, can be fruitful which is devoted 

 exclusively to the cultivation of wide generalisations. The success 

 of his essay did not tempt Helmholtz to stray into the easy path 

 of speculation. Profoundly conscious of the importance of general 

 principles, he nevertheless felt that the surest way to gain true 

 knowledge of the relations between mechanics and life was to investi- 

 gate the mechanism of living bodies in detail. He had already 

 (1845) studied the chemical changes which accompany muscular 

 activity by showing that both the aqueous and alcoholic extracts of 

 the muscles of a frog's leg were different, according as it had, or had 

 not, been stimulated by a long series of electrical discharges. To 

 this was added, in 1847, a memoir on the production of heat during 

 muscular action. It had been held that the increase of temperature 

 in an irritated muscle is due to the more rapid supply of arterial 

 blood. Experiments to test this view had been made on warm- 

 blooded animals, but Helmholtz pointed out that less elevated tem- 

 perature and persistent irritability were qualities which made the 

 muscles of the frog more fitted to be the subjects of a crucial experi- 

 ment. He succeeded in proving that the rise in temperature is due 

 to work within the muscle itself, and that the amount of heat thus 

 evolved is far greater than any which may be generated in the nerves 

 through which the stimulation is produced. 



From these investigations Helmholtz passed to another difficult 

 problem namely that of the rate of propagation of nervous action. It 

 had been thought that this was practically instantaneous, but in a paper 

 published in 1850 he proved that if two different points of a motor 

 nerve are equally stimulated, the interval of time that elapses before 

 any given stage of the resulting muscular contraction is reached is 

 greater as the point of stimulation is more distant from the muscle. 

 The lag thus detected was very small, being less than two thou- 

 sandths of a second on the short lengths of nerve employed, but the 

 velocity of propagation, instead of being infinitely great, was found 

 to be less than a tenth of the velocity of sound, the most probable 

 value being 26'4 metre-seconds. 



Various problems connected with this research continued to 

 occupy the mind of Helmholtz for some time. In 1850 he discussed 

 the " Methods of measuring very small intervals of time, and their 

 application to physiological purposes." This was followed in 1851 

 by a memoir on the duration of induced electrical currents, and in 

 1852 by a further investigation, conducted with improved apparatus, 

 into the velocity of the propagation of nervous action. The result 

 was 27'25 metre-seconds, thus confirming the substantial accuracy of 

 his earlier work. 



Helmholtz had thus opened up an entirely new field of research, which 

 alone would have been sufficient to absorb all the energies of a 



