GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 119 



ing from the second excitation ; and Von Frey 2 has ascertained that Helm- 

 holtz's rule of summation applies only to weighted muscles. In the case of 

 unweighted muscles the summation effect is greatest when the second contrac- 

 tion starts during the period of developing energy caused by the first excita- 

 tion, I. e. during the rise of the first contraction. If the second contraction 



FIG. 51. A schema of the effect of double excitations upon the gracilis muscle of a frog, by differ- 

 ent intervals of excitation. To obtain this figure, the results of different experiments were super- 

 imposed (after Von Frey). 



starts during the period of relaxation of the first, the second may be not even 

 as high as when occurring alone (see Fig. 51). 



The fact that the second contraction is higher if it starts during the ascent 

 of .the first, may be explained as due to a summation of the condition of ex- 



FIG. 52. Effect of support on height of contractions (after Von Frey) : a, gastrocnemius muscle of a 

 frog, separate contractions, tetanus, separate contractions, and group of supported contractions ; weight 

 10.5 grams ; b, the same, by weight of 0.5 grams. 



citation awakened by the two irritants, and hence the liberation of a greater 

 amount of energy. Nevertheless, the increased irritability, indicated by stair- 

 case contractions, and the summation of excitation effects which occur by rapidly 

 repeated excitations, shown by the above experiment, do not suffice to wholly 

 explain the great shortening of the muscle seen in tetanus. Helmholtz' idea, 



1 Archiv fiir Analomie und Physiologie, 1888, p. 213. 



