CIRCULATION. 



453 



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. 1 



The eifect 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 2 (see Fig. 1 1 5). 



FIG. 115. Pulsations of frog's heart, inhibited by the excitation of the left vagus nerve (Tarchanoff, 

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

 lating current, one vibration for each induction shock. 



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. 116). A 

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

 lengthens both systole and diastole. 3 The difficulty of producing a continued 

 arrest in diastole is much greater in some animals than in others. Even when 

 easily produced, the arrest soon gives away in the manner described by E. H. 

 and E. Weber, the heart beginning to beat in spite of the vagus excitation. 4 



FIG. 116. 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 

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

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

 municated with a Hiirthle 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 of stimulation ; C, time in seconds. 



The force of the contraction, measured by the height of the up-stroke of the 

 intra-ventricular pressure curve, or by placing a recording lever on the heart, 



1 For literature see Tigerstedt, Physiologic des Kreislaufes, 1893. 



2 Schiff, 1849, p. 192; Pfliiger, 1865, p. 30 ; Czermak, 1868, p. 644; 1868, p. 32; Bonders, 

 1868, p. 339; 1872, p. 6; Tarchanoff, 1876, p. 300; Pruszynski, 1889, p. 569. 



3 Arloing, 1894, p. 88 ; Meyer, 1894, p. 698. 



4 Hough, 1895, p. 161. The terrapin heart is said not to escape, as a rule, from vagus inhibi- 

 tion. Compare Mills, 1885, p. 255 ; see also Laulanie", 1889, p. 409. 



