ARRANGEMENT OF THE NERVOUS SYSTEM 573 



produce a reaction. This phenomenon constitutes the summation of 

 subminimal stimuli. It should be remembered, however, that we are 

 not dealing in this case with a storage or ordinary addition of individual 

 stimuli, but with a state of increased sensitiveness of the living sub- 

 stance. In other words, the first stimulus, although subminimal 

 in character, gives rise to certain changes in the cell which render 

 it more susceptible to the succeeding stimulus. This is really a general 

 experience, because certain reactions may be elicited with much greater 

 promptness by a succession of moderate stimuli than by a single 

 stimulus of great intensity. This is especially true of the stimulation 

 of the cerebral cortex and other complexes of nerve cells mediating 

 reflex actions. 



Setschenow 1 has proved that nerve fibers and ganglion cells behave 

 very differently toward stimuli. Thus, it is conceded that the state 

 of excitation in nerve fibers does not outlast the stimulus for any con- 

 siderable length of time, while nerve cells retain a state of greater 

 irritability even after slight stimuli and show, therefore, a greater 

 responsiveness to succeeding stimuli. We make use of this fact in a 

 practical way in eliciting reactions in the realm of the sympathetic 

 system and in testing the different reflexes for purposes of diagnosis. 

 Thus, a number of light taps upon the patellar ligament often result 

 in a positive reaction when a single strong one does not. It 'has also 

 been observed that long-continued pressure is at times more effective 

 than a single mechanical stimulus of much greater intensity. The 

 same is true of the stimuli elicited by stroking the surface of the body 

 (tickling) and of the light, sound and chemical impacts imparted 

 respectively to our retinae, organs of Corti, taste-buds and olfactory 

 cells. 



Facilitation or "Bahnung." Most closely allied to this phenom- 

 enon is the so-called stair-case contraction or "Treppe" of striated 

 and cardiac muscle tissue. It will be remembered that if these tissues 

 are rendered more sensitive either by exposing them to subminimal 

 stimuli or by the administration of fatigue substances, their contrac- 

 tions gradually increase and remain large until this state of hyper- 

 susceptibility has been terminated. It should be noticed, therefore, 

 that the "Treppe" is not caused by an increased intensity of stimula- 

 tion but by an augmentation of the contractile power of the muscle 

 substance. A similar change takes place in nerve tissue when made to 

 perform the same task a number of times in succession. An impulse 

 which is made to pass through a certain set of neurons a great many 

 times, gradually breaks down the resistance in this path so that the 

 latter becomes more particularly adapted to it. This "Bahnung" 

 is largely a matter of the cell-bodies, 2 because, as we have just seen, 

 the resistance in the centers is infinitely greater than that met with 



1 Uber die elektr. Reizung der sens. Riickenmarksnerven des Frosches, Graz, 

 1868; also: Biedermann, Pfliiger's Archiv, Ixxx, 1900, 451. 



2 Exner, Pfliiger's Archiv, xxviii, 1882, 487. 



