THE CIRCULATION OF THE BLOOD. 311 



taneously, there is no necessary relation or connection between the two as 

 they can and do occur independently of each other. For this reason it is 

 generally assumed that the sympathetic nerves contain two groups of fibers, 

 viz., accelerators and augmentors, the functions of which are respectively 

 to accelerate the rate and augment the force of the heart-beat. From the fact 

 that both auricles and ventricles exhibit these changes it is assumed that 

 the nerve impulses stimulate both chambers. This is rendered probable 

 also from the experiments of Erlanger, who found that after complete heart- 

 block, stimulation of the sympathetic caused independent acceleration of 

 both auricles and ventricles. 



The Physiologic Action of the Vagus Nerve in Mammals. In the 

 mammal the same or similar effects result from stimulation of the vagus as 

 in the frog. If the thorax of the dog is opened and artificial respiration 

 maintained the heart will continue to beat in a practically normal manner for 

 a long time. Under such conditions if the vagus nerve on one side be divided 

 and its peripheral end stimulated with induced electric currents of moderate 

 strength, the heart will be seen to come to a standstill almost immediately in 

 the condition of diastole, and may be so kept for a variable period, from 

 fifteen to thirty seconds or more, during which its walls are re axed and its 

 cavities filled with blood. On cessation of the stimulation the contractions 

 return and in a very short time the former rate and force of the beat are 

 regained. If the electric currents are of feeble strength, the heart will 

 come to rest gradually, through a gradual diminution in the rate and force 

 of the contraction. During the period of the inhibition the heart presents 

 an appearance similar to that presented by the heart of the cold-blooded 

 animal. When the heart of an animal is thus exposed, the auricle and the 

 ventricle of one side may be attached by threads to writing levers and their 

 contractions registered on a moving recording surface. The effects on both 

 auricles and ventricles which follow vagus stimulation will then become 

 more apparent. Fig. 146 is a tracing thus obtained. The animal employed 

 for the experiment was a rabbit. 



The inhibitor effect of the vagus varies in degree and duration in 

 different animals. In the dog the effect of vagus stimulation is usually 

 pronounced, lasting from 15 to 30 seconds; in the rabbit it is perhaps equally 

 well pronounced but somewhat less in duration; in the cat it is almost want- 

 ing. In this latter animal a complete standstill, even for a few seconds, is 

 very rarely seen; usually there is produced merely a slight diminution in the rate 

 of the beat even though the stimulus employed is quite strong. In all these 

 animals, however, after a very short time the nerve impulses lose their 

 inhibitor influence on the heart-muscle, and notwithstanding continued 

 stimulation of the vagus, the heart returns to its former rate and vigor. 

 This result is in marked contrast to that observed during stimulation of the 

 vagus in the cold-blooded animals, in which the heart may be kept at rest 

 for relatively very long periods of time. No satisfactory explanation for 

 this loss of vagus control or escape of the heart from the vagus control has 

 as yet been offered. 



Seat of Action of the Vagus Impulses. In a foregoing experiment of 

 which Fig. 146 is a graphic result, stimulation of the left vagus with a fairly 

 strong current was followed by a diminution in both the rate and force of the 



