120 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



that there is a support afforded by the first contraction to the second, must 

 also play an important part, and we must turn to this for the completion of the 

 explanation of the great height acquired by the tetanus curve. 



Effect of Support on the Height of Contractions. Von Kries 1 and Von 

 Frey 2 found that, in general, the shorter the distance the muscle has. to raise 

 a weight, the higher it can contract, and that if a muscle be excited at a regu- 

 lar rate, and the support for the weight be raised between each of the succeed- 

 ing contractions, at a certain height of the support the contractions may be 

 as high as during tetanus (see Fig. 52). This effect can be got with a fresh 

 muscle when the interval between the excitations is such that there can be no 

 summation in Helmholtz' sense. 



The importance of this discovery to our understanding of tetanus is very 

 great, for it has been found that if an unsupported muscle be rapidly excited, 

 effects are observed which closely resemble those obtained by the aid of a sup- 



FIG. 53. Effect of a gradually increasing rate of excitation. Excitation of a gastrocnemius muscle 

 of a frog with breaking induction shocks of medium strength. The time was recorded directly, by a 

 tuning-fork making 100 vibrations per second. The rate of excitation was gradually increased, and 

 then gradually decreased. The ascending curve, a-6, shows the effect of increasing, and the descending 

 curve, c-d, of decreasing the rate of stimulation. Excitation was given by means of a special mechanism 

 for interrupting the primary circuit of an induction apparatus and at the same time short-circuiting the 

 making shocks. This interrupter was run by an electric motor which was allowed to speed up slowly, 

 and was slowed down gradually. 



port; this we have seen in the experiments recorded in Figures 47, 48, p. 116. 

 After a certain amount of excitation, a change occurs in the condition of a 

 muscle, owing to which it acts as if it had received an upward push, and as 

 if a new force had been developed within it, which aids the ordinary con- 



1 Archivfur Anatomie und Physiologic, 1886. 2 Ibid., 1887. 



