VASOMOTOR REFLEXES 365 



From a reference in Ergebnisse der Physiologie by Asher/ it is 

 not clear whether the stimulation was directly upon the nerve 

 centers or reflexly through afferent nerves. 



But the credit of pointing out clearly that the frequency of 

 stimulation has an effect upon vasomotor reflexes must be 

 ascribed to Gruber.^ This writer remarks: ''That summation 

 takes place with rapid rates of stimulation is undisputable, but 

 it does not seem probable where the strength is more than 400 

 times threshold that the phenomenon of summation can explain 

 the different effect obtained with these rates of 1 per two seconds 

 and 20 per second interruptions." The similar effect of fre- 

 quency of stimulation was afterward proved incidentally by 

 Reid Hunt,^^ who considers it convenient to use the infrequent 

 rate of stimulus to obtain a reflex fall of blood-pressure. 



Our experiments on this subject have been carried out on 

 dogs with rates of stimuli of 1, 2, 5, 10, 20, 40, and 80 per sec- 

 ond upon various nerves, under chloroform and curare or under 

 brain compression. Though our results were not so conclusive 

 as those obtained by Gruber (in fifteen out of forty stimulations 

 similar results to his were obtained), still we do not hesitate to 

 ascribe an important role to the frequency of stimulation. Ac- 

 cording to Martin's^^ investigations, the intensity of stimula- 

 tion in Z-units is directly proportional to that of the current 

 in the primary circuit. We arranged the apparatus in such a 

 way as to get a current of a certain strength and one ten times 

 stronger as we desired. With the former current we obtained a 

 fall by stimulating five times per second, and a distinct rise by 

 stimulating ten times per second, while with the latter current 

 we observed a fall with the rate of stimulus one per second and 

 a rise with five per second stimulations. A selected record is 

 shown in figure 9, where one and the same nerve was stimulated 

 with the same intensity but with different frequency. 



Much more remarkable were the rates of stimulation at which 

 the maximum pressor response was reached. 



