INTERFERENCE OF EXCITATIONS 



323 



real and apparent "Bahnungen." On account of this lack of clear- 

 ness in the meaning of the term "Bahnung," I wish to discard its 

 use as it is not at all essential. We will speak simply of a summa- 

 tion of excitation, for here it is simply a question of summation 

 of two excitations of the motor cells of the spinal cord. 



AT 



TV 



If 



JL 



r\rAr\ririrAr\rir\r-\i 



s 



-« 



ji i_ 



-V >y 



_» L. 



\^^^^M^^^^\'^^^^^'mi\i\x\i\i\^ 



■M 



-H 

 -Pf 



S 



Fig. 56. 



B 



Summation of two excitations in tlie rabbit. The one proceeds from the paw, the other from 

 the motor sphere of the cerebral cortex. S— Time in seconds. /¥— Stimulation of the paw. 

 tf— Stimulation of the motor sphere, itf— Contractions of the abductor pollicis. (After 

 Exner.) 



Frohlich has shown that summation of two excitations upon a 

 motor cell of the anterior horn coming by way of different paths 

 is more readily obtained when the stimuli are somewhat strong, 

 or when the duration of the excitation processes in the ganglion 

 cells are somewhat prolonged by fatigue. 



On the other hand, the conditions for the production of inhibi- 

 tion are favored when the intensity of the series of stimuli is weak. 

 Here it is a question of the development of a relative refractory 

 period for the weak stimuli by increase in their frequency. A 

 relative fatigue of the motor ganglion cells for weak stimuli 

 rapidly occurs, and there develops a state of equilibrium beneath 

 that of the threshold of perceptible effect throughout the con- 

 tinuation of stimulation. Vessi succeeded in isolating these types 

 of summation and inhibition in the spinal cord. His method con- 

 sisted in cutting the posterior roots of the spinal cord of the frog 

 and stimulating faradically the central ends, and at the same time 

 graphically recording the response of the gastrocnemius muscle. 

 Upon faradic stimulation of the ninth posterior root, one obtains 

 tetanic reflex contraction of this muscle. When the tenth poste- 



