126 HERMANN VON HELMHOLTZ 



the mechanical properties of the muscle show no alteration, the 

 period of rising energy that in which the tension of the muscle 

 increases till it reaches a maximum, and, lastly, the period of 

 falling energy that in which the tension falls rapidly at first, 

 and afterwards very gradually, until finally the initial state 

 of rest is re-established he deduces a series of important 

 theorems, by means of the myograph, by simple inspection of 

 the fully or partially coinciding curves of contraction. In 

 these he states that the negative variation of the muscle 

 current which induces secondary contraction appears before 

 the contraction of the muscle, while the electrotonus of the 

 nerve on the contrary coincides with the electrical current 

 that excites it. The most important result, however, was the 

 proof that two instantaneous excitations produce the strongest 

 contraction of the muscle when the interval between them is 

 * equal in length to the period of rising energy, while, on the 

 contrary, two stimuli are not stronger than a single stimulus 

 -when the interval between them is so small that the first 

 contraction has not reached any perceptible height before 

 the second begins. He notes provisionally a result to which 

 he came back later, pointing out its importance for the 

 mechanics of the spinal cord, since it is a means of dis- 

 tinguishing between direct and reflex twitches, viz. that the 

 tracing of the contraction of the thigh-muscle in strychninized 

 frogs excited from the sensory nerve shows that as com- 

 pared with rate of propagation in the nerve, the reflex 

 twitch is evoked after a comparatively long interval, and that 

 in reflexes the passage of the excitation in the cord takes 

 more than twelve times as long as its transmission in the 

 afferent and efferent nerves. 



Helmholtz now became more and more immersed in optical 

 problems, and was hoping, after a number of publications on 

 this subject in the journals during the summer of 1854, to 

 finish his great work on Accommodation, when on October i 

 he received the news of his mother's sudden death on 

 September 30: the long distance made it impossible for him 

 to arrive in time for her funeral. 



' For the departed such a swift decease can only be regarded 

 as a blessing/ he writes to his father; ' she had suffered enough, 

 and more, in her lifetime for the readiness with which she 



