L66 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Comparative Inhibitory Power. — One vagus often possesses more inhibi- 

 tory power than the other. 1 



Septal Nerves in Frog. — 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 i< then an anatomical division of the septal branches of the frog's vagus, 

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

 uaodifying the force of contraction and thetonusof the ventricle run within them. 2 



Nature of Vagns Influence on Heart. — The nature of the terminal 

 apparatus by which the vagus inhibits the heart is unknown. It is probable 

 that the same intracardiac apparatus serves for both nerves, for Hurler finds 

 that when the heart escapes from the inhibition caused by continued stimula- 

 tion of one vagus, the prolonged diastole growing shorter again, the immediate 

 stimulation of the second vagus has no effect upon the heart. 3 Dogiel and 

 Grahe have recently observed that the lengthening of diastole which follows 

 stimulation of the peripheral stump of the vagus, the other vagus being intact, 

 is less marked than when both vagi are cut. 4 



The earlier attempts to form a satisfactory theory for the inhibitory power 

 of the vagus met with little success. The statement of the Webers' that the vagus 

 inhibits the movements of the heart gave to nerves a new attribute, but is 

 hardly an explanation. The view of Budge and Schiff, that the vagus is the 

 motor nerve of the heart and that inhibition is the expression of its exhaustion, 

 is now of only historical iutere.-t. Nor has a better fate overtaken the theory 

 of Brown-Sequard, who saw in the vagus the vaso-motor nerve of the heart, 

 the stimulation of which, by narrowing the coronary arteries, deprived the 

 heart of the blood that, according to Brown-Sequard, is the exciting cause of the 

 contraction. 



Of recent years, the explanation that has commanded most attention is the 

 one advanced by Stefan i '' andGaskell, namely, that the vagus is the trophic nerve 

 of the heart, producing a dis-assimilation or katabolism in systole and an 

 as-imilation or anabolism in diastole. Gaskell supports this theory by the 

 observation that the after-effect of vagus excitation is to strengthen the force 

 of the cardiac contraction and to increase the speed with which the excitation 



1 Ilofinann: Archie fiir die gegammte Physiologic, 1895, lx. p. 169. 



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

 Arloing: Archives <{■• I'fui.tioloijie, 18U4, p. lti:;; and Knoll: Archil) fiir die gesammte Physiologic, 

 1897, Ixvii. p. 587. 



3 Hough Journal of Physiology, 1895, xviii. p. 198. 



* Dogiel and Grahe: Archiv fur Physiologie, 1895, p. 393. Changes in the peripheral effi- 

 ciency of the vagi are discussed by McWilliams : Proceedings Royal Society, 1893, liii. p. 475. 



5 Stefani : Archives italiennesde Biologie, 1895, xxiii. p. 17G. 



