296 PRACTICAL PHYSIOLOGY 



during which to overcome the inertia of the load. It is possible in 

 several ways to prolong the period of active pull ; only two will be 

 mentioned. (1) By cooling the muscle. Record on a rapidly-revolving 

 drum the contraction of a gastrocnemius preparation, stimulated directly 

 by a single maximal shock, and weighted with 20 grms. near the axis 

 of the lever, first at 15 C., and then after ice has been in contact with 



PIG. 170. Comparison of the height of contraction of hyoglossus during tetanus, 

 upper curve, and of the same muscle in response to a single maximal stimulus after 

 thorough poisoning with veratrine. Time marking in seconds. (A.P.B.) 



the muscle for some minutes. (2) By veratrine. On a slowly-revolving 

 drum record first a tetanus curve of a hyoglossus preparation, and then 

 the contraction of the same preparation in response to a single maximal 

 stimulus, after thorough poisoning with veratrine (see p. 30). It will 

 be found that the two curves are practically of the same form and 

 height (Fig. 176). 



CHAPTER Y. 

 SUMMATION OF STIMULI. 



IN a previous chapter the subject of summation of contractions has been 

 dealt with. This summation of ' effect ' must be distinguished from 

 the summation of stimuli, by which an inadequate stimulus, if repeated 

 sufficiently often, becomes first adequate and then for a time increas- 

 ingly effective. This is a summation of 'cause,' and probably plays 

 an important part in the life of all living matter. 



In order to demonstrate the summation of stimuli, arrange the 

 apparatus for stimulating a gastrocnemius muscle directly with single 



