CIR CULA TION. 165 



The diastole is lengthened. The change in force appears earlier than the 

 change in periodicity, and sometimes without it. On the whole, the sinus and 

 auricle are more easily affected bv vastus excitation than the ventricle. 



Action on Bulbus Arteriosus. — If the bulbus arteriosus of the frog's 

 heart is extirpated in such a way as to leave untouched the nerve-fibres that 

 connect it with the auricular septum, the contractions of the isolated bulbus 

 will be arrested when the peripheral end of the vagus is excited. 1 



Diminished Irritability of Heart. — During vagus excitation with cur- 

 rents of moderate strength, the arrested heart will respond to direct stimula- 

 tion by a single contraction. With strong vagus excitation, however, the 

 directly stimulated heart contracts not at all or less readily than before. 



Effects of Varying the Stimulus. — A single excitation of the vagus does 

 not stop the heart. Morat has investigated the effect of excitations of varied 

 duration, number, and frequency on the tortoise heart. 2 With excitations of 

 the same duration, the effect was minimal at 2 per second, maximal at 7 

 per second, diminishing thereafter as the frequency increased. The longer the 

 stimulation, the longer (within limits) was the inhibition. An excitation that 

 is too feeble or too slow, or, on the contrary, is over-strong or over-frequent, 

 has no effect. Within limits, however, the degree of inhibition increases witli 

 the strength of the stimulus. 



Weak stimuli affect primarily the auricles, diminishing frequency and force 

 of contraction, and secondarily lower the frequency of the ventricle. Stronger 

 stimuli arrest the auricle, the ventricles continuing to beat with almost undi- 

 minished force but with altered rhythm. Still stronger stimuli inhibit the 

 ventricles also. 



The frequency can be kept comparatively small by continued moderate 

 stimulation. 



Arrest in Systole. — The excitation of the tortoise vagus in the upper or 

 middle cervical region is sometimes followed, according to Rouget, 3 by a state 

 of continued, prolonged contraction — in short, an arrest in systole. The same 

 effect is observed in rabbits strongly curarized and in curarized frogs. Arloing ' 

 noticed that the mechanical irritation produced by raising on a thread the left 

 vagus nerve of a horse caused the right ventricle to remain contracted during 

 seven seconds. The ventricular curve during this time presented the characters 

 of the tetanus curve of a skeletal muscle. Recent observations by Frank, 8 

 Hunt, 6 Walther, 7 and others make it probable that a kind of summation and 

 superposition of contractions may at times take place in the heart as in 

 ordinary striated muscular tissue. 



1 Dogiel: Centralblatt fur die medieinischen Wissenschaften, L894, p. 227. 



2 Morat: Archives </>• Physiologie, L894, p. 10. 

 3 Kouget: I hid., p. 398. 



* Arloing: Ibid., L893, y. 112. 



5 Frank : Zeitschrift fur Biologie, 1899, xxxviii. 



6 Hunt, Bookman, ami Tierney: Centralblatt fur Physiologie, 1897, xi. p. 274. 



7 Walther : Archivfur die gesammte Physiologie, 1900, lxxviii. p. 597. 



