486 



APPENDIX 



The existence of the threshold is universal wherever protoplasm is 

 concerned. If a sound-stimulus be too faint our ears, as mechanisms, 

 are not set in action owing to their inevitable inertia, and we hear no 

 sound; if a light be too faint, our eyes fail to see it. The threshold is a 

 necessary accompaniment of the inertia of matter, organic and inorganic. 



The maximum stimulation of a tissue is of a similar nature logically. 

 A mechanism can work only so hard, a string can vibrate only so far. 

 When a muscle has received a stimulation which corresponds with the 

 maximum of its action, any increase in strength of the stimulus up to a 

 certain degree will produce only the same maximal effect. Beyond that 

 certain degree of strength of stimulus the organism will be injured and 

 its activity impaired in all directions. 



Expt. 42. Threshold Independent of Load. Repeat the latter half 

 of Experiment 41, using no load instead of 10 gm., which is a fair load. 

 Repeat again, using 50 gm. 



FIG. 271 



The relation of cross-striated muscle's contraction to the increasing strength of its stimulation 

 by induced electricity. The strength of the induced break-shocks increased by degrees, each 

 represented on the inductorium by one notch of the vertical arc-index, then by 3 mm. on the 

 horizontal scale. To be read from left to right. Intervals, ten seconds. (The greater contraction 

 seen near the end is unexplained.) Observe that even ten seconds apart, the electric shocks- 

 fatigue the muscle, lessening both its contraction and its relaxation. 



It is not obvious why the threshold should be in any way dependent 

 on the muscle's load, for, unless the load be so excessive as to tear apart 

 the muscular fibers, it does not affect in any way the action-conditions 

 of the muscle, either in pulling apart the sarcomeres, or in disturbing 

 the relations of the nerves to the muscle. As a matter, however, of 

 precise fact, load does influence in one respect the threshold, for a muscle 

 works more normally in all respects when it has a moderate load 

 than when it has too little or too much. This normal load apparently 

 keeps up the normal tonus of the organ (muscle) and this in all sorts 

 of tissues is a pre-requisite of optimum action. It cannot be doubted, 

 then, that the threshold would be found slightly lower in a muscle with 

 a normal load than in one quite unloaded. 



Expt. 43. Contraction of Smooth Muscle. (Apparatus: Myograph r 

 signal.) Cut four or more rings out of the frog's stomach, tie them 

 together in a line with fine copper wire, and place this long line of rings 

 as the gastrocnemius is usually placed. Use two cells, and put the signal 

 in the circuit. Let the lever write carefully a fine abscissa line. Stimu- 

 late this stomach-muscle with one pair of maximal make-and-break 



