162 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY 



gata between the corpora quadrigernina and the lower end of the calamus scrip- 

 torius was stimulated did the arrest take place. Cutting away the spinal cord 

 and the remainder of the brain did not alter the result. 



Having determined that the inhibitory power had its seat in the medulla 

 oblongata, the question arose through what nerve the inhibitory influence is 

 transmitted to the heart. In a frog in which the stimulation of the medulla 

 had stopped the heart, the vagus nerves were cut and the ends in connection 

 with the heart stimulated. The heart was arrested as before. 



Thus the fundamental fact of the inhibition of a peripheral motor mechan- 

 ism by the central nervous system through the agency of special inhibitory 



Fig. 30.— Pulsations of frog's heart, inhibited by the excitation of the left vagus nerve (Tarchanoff, 

 1876, p. 296): C, pulsations of heart ; 6, electric signal which vibrated during the passage of the stimu- 

 lating current, one vibration for each induction shock. 



nerves was firmly established. A great number of investigations have demon- 

 strated that this inhibitory power is found in many if not all vertebrates and 

 not a few invertebrates. 



The effect of vagus stimulation on the heart is not immediate ; a latent 

 period is seen extending over one beat and sometimes two, according to the 

 moment of stimulation (see Fig. 30). 



Fig. 31.— Showing the lengthened diastole and diminished force of ventricular contraction during 

 weak stimulation of the peripheral end of the cut vagus nerve. The heart (cat) was isolated from both 

 Bystemic and pulmonary vessels, and was kept beating by circulating defibrinated blood through the 

 coronary arteries : A, Pressure in lefl ventricle, which was filled with normal saline solution, and com- 

 municated with a Bilrthle membrane manometer by means of a cannula which was passed through the 

 auricular appendix and the mitral orifice; B, line drawn by the armature of an electro-magnet in the 

 primary circuit ; the heavy line indicates the duration <>f stimulation ; C, time in seconds. 



Changes in the Ventricle. — The periodicity of the ventricular contraction 

 is altered by vagus excitation, a weak excitation lengthening the duration of dias- 

 tole, while leaving the duration of systole unchanged (see Fig. 31). A 

 stronger excitation, capably of modifying largely the force of the contraction, 

 Lengthens both Bystole and diastole. 1 The difficulty of producing a continued 



'Meyer: Archives de Phy$iologie, 1894, p. 698; Arloinij: Ibid., p. 88. 



