l»;l 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ished ; the one may be increased, while the other is diminished. The vagus 

 effect quickly reaches a maximum and then slowly decreases. The interval 

 between the contractions of different parts of the sinus is sometimes increased 

 by vagus excitation, so that the different parts are dissociated and heat at 

 measurably different times. Attempts have been made to explain the sev- 

 eral actions of the vagus nerve, together with the various forms of intermit- 

 tent ami irregular pulse, by variations in the transmission of the cardiac ex- 

 citation ; ' hut it is probable that alterations in the condition of the muscle-cells 

 in the sinus, auricle, and ventricle are of equal or greater importance. 2 



The action of the vagus is accompanied by an electrical variation. This 

 has been shown in the muscular tissue of the resting auricle of the tortoise 

 (see Fig. 32). The auricle is cut away from the sinus without injuring the 

 coronary nerve, which in the tortoise passes from the sinus to the auricle and 

 contains the cardiac fibres of the vagus. After this operation the auricle and 

 ventricle remain motionless for a time, and this quiescent period is utilized for 

 the experiment. The tip of the auricle is injured by immersion in hot water, 

 and the demarcation current (the injured tissue being negative toward the unin- 

 jured) is led off to a galvanometer. On exciting the vagus in the neck, the 

 demarcation current is markedly increased. No visible change of form is seen 

 in the auricular strip. 



Fig. 32.— The tortoise heart prepared for the demonstration of the electrical change in the cardiac 

 muscle accompanying the excitation of the vagus nerve: t', vagus nerve; C, coronary nerve; S, sinus 

 and part of auricle in connection with it ; (7, galvanometer, in the circuit formed by two uon-polarizable 

 electrodes and the part of the auricle between them ; /■-', induction coil (Gaskell, 1887). 



Changes in the Sinus and Auricle. — There is little probability that the 

 action of the vagus on the sinus and auricle, or greal veins, 3 differs essentially 



from the action on the ventricle. The force of the contraction is diminished. 



1 Muskens: American Journal of Phygiolor/t/, 1 898, i. p. 486. 

 - Bofmann : Archivfur 'lit <\,~<nini,i, Vhy/iuhiijie, 1898, lxxii. p. 409. 



3 Knoll : Archiv fur die gesammL Physiologie, 1897, lxviii. p. 839; Engelmann: Ibid., 1896, 

 lxv. p. 109. 



