456 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



duration, number, and frequency on the tortoise heart. 1 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 with 

 the strength of the stimulus. 2 



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. 3 



The frequency can be kept comparatively small by continued moderate 

 stimulation. 4 



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

 middle cervical region is sometimes followed, according to Rouget, 6 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 6 

 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 striated muscle. 7 



Comparative Inhibitory Power. One vagus often possesses more inhibi- 

 tory power than the other. 8 



Septal Nerves in Frog 1 . The electrical stimulation of the peripheral 

 stump of either of two large nerves of the inter-auricular septum in the frog 

 alters the tonus and the force of contraction of the ventricle, but not the fre- 

 quency. After section of these nerves, the excitation of the vagus has very 

 little effect on the tonus, and almost none on the force of the ventricular beat, 

 while the frequency is diminished in the characteristic manner. Evidently, 

 therefore, the two large septal nerves take no part in the regulation of fre- 

 quency, but leave this to the nerves diffusely distributed through the auricles. 

 There is then an anatomical division of the septal branches of the frog's vagus, 

 the fibres affecting periodicity running outside the septal nerves, while those 

 modifying the force of contraction and the tonus of the ventricle run within them. 9 



1 Morat, 1894, p. 10 ; Legros and Onimus, 1872, p. 565. 



2 v. Bezold, 1863, p. 50; Pfliiger, 1859, p. 19 ; Bonders, 1868, p. 356. 



3 Johansson and Tigerstedt, 1889; Hoy and Adami, 1892, p. 237; Bayliss and Starling, 

 1892, p. 411. 4 Laulanie", 1889, p. 408. 



6 Rouget, 1894, p. 398. 6 Arloing, 1893, p. 112. 



7 For other unusual alterations in the heart-beat in consequence of vagus excitation see 

 Arloing, 1893, p. 163. 



8 Cold-blooded Animals: Meyer, 1869, p. 61 ; Tarchanoff, 1876, p. 293; Gaskell, 1882, p. 82; 

 McWilliam, 1885, xvi. ; Mills, 1885, p. 259; 1887, p. 11 ; 1888, p. 2. 



Mammals: Masoin, 1872, p. 410; Legros and Onimus, 1872, p, 575; Arloing and Tripier, 

 1872, p. 420; Langendorff, 1878, p. 68; compare Brown-Se'quard, 1880, p. 211. 



9 Hofmann, 1895, p. 169; examine Eckhard, 1876, p. 192; and Dogiel, 1890, p. 258. 



