176 GENERAL PROPERTIES OF LIVING TISSUES 



nerve or muscle into the active state, and the 

 energy that the stimulus sets free. It is as if a 

 spark fell into powder ; the active process is to 

 be regarded, with some reservations, as an explo- 

 sion. But only a part of the latent energy of 

 muscle can be set free by any one stimulus. 



Threshold Value Independent of Load. Re- 

 peat the preceding experiment, and load the 

 muscle with 50 grams instead of 10. 



The threshold value will not be changed. 



Summation of Inadequate Single Stimuli. 

 Place the secondary coil of the inductorium at 

 such a distance from the primary that a break 

 current shall be nearly, but not quite sufficient 

 to cause a contraction. Let the muscle rest 

 without stimulation for about a minute. Repeat 

 the inadequate single stimulation at intervals of 

 five seconds. No curve need be written. 



After a time, contraction will be secured. 



The excitation outlasts the stimulus, and rein- 

 forces subsequent stimuli: finally, the summed 

 excitations call forth a contraction. Summation 

 is of frequent occurrence probably in all living 

 tissues. ^ 



Relative Excitability of Flexor and Extensor 

 Nerve Fibres ; Ritter-Rollett Phenomenon. Ex- 

 pose the sciatic nerve in a brainless frog in 

 the pelvic region. Set the hammer of the in- 



