A _summation of two excitations_ was observed already by _Exner_. This investigator connected the abductor pollicis of the rabbit with an apparatus for making graphic records. He then stimulated first the paw and then the motor areas of the cerebral cortex with faradic shocks, the intensity of which was just sufficient to bring about perceptible effect. If both stimuli were simultaneously operative, an increase in the response was observed. Even when the stimuli were sub-threshold in type, as a result of summation there was a perceptible muscle contraction. (Figure 56.) _Exner_ had at that time referred to this increase of the response as “Bahnung” (reinforcement). However, the word “Bahnung” has more than one meaning, for processes of various types are involved in this term. Thus writers have differentiated real and apparent “Bahnungen.” On account of this lack of clearness 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 _summation of excitation_, for here it is simply a question of summation of two excitations of the motor cells of the spinal cord. [Illustration: Fig. 56. Summation of two excitations in the rabbit. The one proceeds from the paw, the other from the motor sphere of the cerebral cortex. _S_--Time in seconds. _Pf_--Stimulation of the paw. _H_--Stimulation of the motor sphere. _M_--Contractions of the abductor pollicis. (After _Exner_.) ] _Fröhlich_ 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. [Illustration: A B Fig. 57. Summation of two excitations in the spinal cord produced by stimulation of the ninth and tenth posterior root. Lower line indicates faradic stimulation of the tenth, upper line of the ninth root. ] [Illustration: A B Fig. 58.] [Illustration: Fig. 59.]