44 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



the fingers of your right hand firmly upon the key. When the lever 

 has again assumed the horizontal position, turn the drum with your 

 left hand about 2 mm. to the left, and record the break contraction. 

 Move the secondary, coil outward for a distance of 1 cm. Turn the 

 drum 5 mm. to- the left. Again make and break the current as described. 

 Repeat this experiment, constantly increasing the distance between the 

 coils, 1 cm. at a time, until the stimuli cease being effective. In closing 

 and opening the key, hold the handle firmly, so that a steady contact 

 is obtained each time. Determine the threshold values of the make 

 and break shocks in terms of "distance of coils" and "deviation of the 

 secondary coil." Note that the maximal reactions are obtained at 

 about 1 or 2 cm. distance of coils, and not when the coils are fully 

 approximated. Beginning at this point the height of the contractions 

 decreases gradually to zero, the make contractions disappearing sooner 

 than the break contractions. Explain this difference upon the basis 

 of the results of Lesson II. 



ll .1 m , . . . ? 



I 2, 3 t S" 6 7 8 9 JO i| ia 15 Ifh 15 



FIG. 23. SUCCESSIVE MAKE AND BREAK CONTRACTIONS. 



The strength of the current is gradually diminished by more widely separating the 

 secondary from the primary coil. The figures indicate this separation in centimeters 

 of distance. M, Threshold of make; B, threshold of break. 



6. Summation of Subminimal Stimuli. With induction shocks 

 which just fail to evoke a visible contraction, stimulate the muscle at 

 intervals of one second. Repeat the stimulation at a much faster rate. 

 Do the subminimal stimuli eventually become supraminimal? 



Annotation. Two views may be held, namely: (a) that the individual electric 

 potentials are added to one another and finally produce a potential of sufficient 

 magnitude to stimulate, and (b) that the successive subminimal stimuli progressively 

 increase the irritability of the muscle tissue until it eventually reacts to a stimulus 

 which, when applied singly, does not activate it. The first view, therefore, advo- 

 cates a summation of stimuli, and the second, a summation of protoplasmic exci- 

 tability. 



7. Relation of the Amount of the Load to the Height of Contrac- 

 tion. Prepare a fresh muscle and arrange the apparatus for stimulation 

 with moderate break shocks. Attach a scale pan to the writing lever and 

 record the abscissa. Keep the lever in this line by means of the after- 

 loading device. Record a make and break contraction upon the stationary 

 drum. In this case it is permissible to record both in the same ordi- 

 nate, because the break contraction is higher than the make contrac- 

 tion, but do not break the circuit too soon after the make, otherwise 



