222 IRRITABILITY 



great number of tracks, for example, the sensory neurons of the 

 spinal cord at different levels, the neurons of the cerebellum, the 

 pyramidal tracks from the motor areas of the cerebral cortex, 

 etc. On the contrary, for example, the sensory neurons of the 

 spinal cord are strictly "private paths" in the sense of Sherring- 

 ton, for excitation can enter by this means only from the special 

 paths of the spinal ganglia and, therefore, from the periphery. 

 The motor neurons of the anterior horns offer, therefore, excel- 

 lent opportunities for the experimental investigation of the inter- 

 ference of two series of excitations which enter by different 

 paths. The spinal cord consequently has become a much-used 

 object of investigation for this purpose. In fact, we can observe 

 and produce all types of interference in the spinal cord. These 

 conditions have been quite thoroughly investigated by Sherring- 

 ton 1 and his coworkers on the dog, and Frohlich,- Veszi? Tiede- 

 mann 4 and Satake 5 on the frog. 



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 suffi- 

 cient to bring about perceptible effect. If both stimuli were 

 simultaneously operative, an increase in the response was ob- 

 served. 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 



\Sherrington: "The integrative action of the nervous system." New York 1906. 



2 Fr. W. Frohlich: "Der Mechanismus der nervosen Hemmungsvorgange." Med. 

 Natur. Arch. Bd. I, 1907. The same: "Beitrage zur Analyse der Reflexfunction des 

 Riickenmarks, etc." Zeitschr. f. allgem. Physiologic Bd. IX, 1909. The same: "Das 

 Princip der scheinbaren Erregbarkeitssteigerung." Ibid. 



3 Julius Veszi: "Der einfachste Reflexbogen im Ruckenmark." Zeitschr. fur allgem. 

 Physiol. Bd. IX, 1910. 



4 Tiedemann : "Untersuchungen iiber das absolute Ref ractarstadium und die 

 Hemmungsvorgange im Ruckenmark des Strychninfroscb.es." Zeitschr. f. allgem. 

 Physiologic Bd. X, 1910. 



5 Satake: The researches are not yet published. 



