THE CIRCULATION OF THE BLOOD 321 



transmitting nerve impulses from the centers from which they arise to the 

 heart upon which they exert a stimulating influence on the rate and force 

 of the beat. 



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

 mammal the same or similar effects in varying degree result from stimu- 

 lation of the vagus as in the frog. These results can be readily shown in 

 the dog or rabbit in the following way. The thorax of the animal is opened 

 and artificial respiration maintained. Under these circumstances the heart 

 will continue to beat in a practically normal manner for a long time. The 

 vagus nerve is then exposed on one side and divided; its peripheral end is 

 then stimulated with induced electric currents of moderate strength, 

 whereupon the heart comes to a standstill almost immediately in the con- 

 dition of diastole, and may be so kept for a variable period, from fifteen to 

 thirty seconds or more, during which its walls are relaxed 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 con- 

 traction. During the period of the inhibition the heart presents an 

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

 animal, viz. : completely relaxed walls and the cavities filled and distended 

 with blood. 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. 144 is a tracing thus obtained. The 

 animal employed for the experiment was a rabbit. 



Division of one vagus is followed in some mammals, e.g., dog by a marked 

 increase in the rate of beat and if both vagi are divided the increase may 

 amount to from 50 to 75 per cent. The results of stimulation and division 

 of the vagus nerves indicate that they are continuously transmitting nerve 

 impulses jfrom the centers from which they arise, to the heart-muscle, on 

 the activity of which they exert a restraining or inhibitor influence. 



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 frequently 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 the foregoing experiment of 

 which Fig. 144 is a graphic record, stimulation of the left vagus with a fairly 



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