An American text-book of physiology . th 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 si)ecial inhibitory 1 Bever and von Bezold, 1867, pp. 236, 247. Ludwig and Thiry, 1864, p. 429; Bever, 1867, p. 249. ^ It is probable tliat the fibres of spinal origin end in the sympathetic ganglia, making con-tacts there with sympathetic ganglion-cells, the axis-cylinder processes of which pass up thecervical chain and descend to the heart in company with the vagus. * G
An American text-book of physiology . th 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 si)ecial inhibitory 1 Bever and von Bezold, 1867, pp. 236, 247. Ludwig and Thiry, 1864, p. 429; Bever, 1867, p. 249. ^ It is probable tliat the fibres of spinal origin end in the sympathetic ganglia, making con-tacts there with sympathetic ganglion-cells, the axis-cylinder processes of which pass up thecervical chain and descend to the heart in company with the vagus. * Gaskell and Gadow, 1884, p. 369. * E. Weber, 1846, p. 42. CIRCULATION. 453 nerves was firmly established. A great imniher of investigations have demon-strated that this inhibitory power is found in many if not all vertebrates andnot a few invertebrates. The effect of vagus stinmlation on the heart is not immediate; a. latentperiod is seen extending over one beat and sometimes two, according to themoment of stimulation- (see Fig. 115).. Fig. 115.—Pulsations of frogs heart, inhibited by the excitation of the left vagus nerve (, 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 contractionis altered by vagus excitation, a weak excitation lengthening the duration of dias-tole, while leaving the duration of systole unchanged (see Fig. 116). Astronger excitation, capably of modifying largely the force of the contraction,lengthens both systole and diastole.^ The difficulty of producing a continuedarrest in diastole is much greater in some animals than in others. Even wheneasily produced, the arrest soon gives away in the manner described by E. E. Weber, the heart beginning to beat in spite of the vagus excitation.*
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