758 THE CARDIAC INHIBITORY CENTER. 



THE CENTER FOR THE INHIBITORY NERVES (DIMINISHING 

 THE FREQUENCY AND THE STRENGTH) OF THE HEART 

 AND THE FIBERS PASSING TO THE VAGUS. 



The fibers of the vagus nerve, moderate irritation of which diminishes 

 the activity of the heart, strong irritation causing arrest of the heart, 

 and which are conveyed to the vagus through the accessory nerve, have 

 their center in the medulla oblongata far to the side of the floor of the 

 fourth ventricle near the restiform body. The center sends to all por- 

 tions of the heart, including the muscles of the superior vena cava, fibers 

 that diminish the number of beats and others that diminish the vigor 

 of the contractions. Slight irritation of the vagus occasionally 

 exerts an inhibitory effect only upon the auricles. The force-diminishing 

 fibers at the same time also prolong the diastole. If the diastole is 

 rendered difficult by increased pressure within the pericardium, irritation 

 of the vagus is believed to cause a prolongation of the diastolic distention. 



This center can be stimulated both directly and reflexly from centri- 

 petal nerves. 



Many investigators assume that this center is in a state of tonic innervatipn, 

 that is that impulses pass out from it uninterruptedly through the vagus '"exerting 

 a regulatory and inhibitory influence upon the heart-beat. According to Bern- 

 stein this tonic irritation is induced reflexly through the abdominal and cervical 

 cords of the sympathetic. Landois did not accept this view, but maintained that 

 under normal conditions of the respiration and the state of the blood, the center 

 is not irritated, but that it is placed in a state of irritation only under special 

 conditions. 



Direct Irritation of the Center. The center can be irritated locally 

 by the same influences that affect the respiratory center, i. Sudden 

 anemia of the medulla oblongata (through ligation of both carotid and 

 both subclavian arteries, or through decapitation of a rabbit, with preser- 

 vation of the vagi alone) causes slowing and even temporary arrest of 

 the heart -beat. 2. Sudden venous hyperemia, which can be brought 

 about by ligation of the veins passing from the head, has a similar effect. 

 3. Also increased venosity of the blood, either through direct interruption 

 of breathing (in the rabbit), or through insufflation into the lungs of a 

 gaseous mixture containing much carbon dioxid, acts in a similar way. 

 As, with marked uterine contractions, the circulation in the placenta, the 

 actual lung of the fetus, is interfered with, the constant enfeeblement of 

 the heart's action in association with severe uterine contractions is to be 

 looked upon as a dyspneic, central irritation of the vagus. 4. At the 

 moment when inspiration takes place as a result of irritation of the 

 respiratory center there is a fluctuation in the irritation of the cardiac 

 inhibitory center. 5. Also increased blood-pressure in the cerebral arte- 

 ries stimulates the cardiac inhibitory center. 



That the center (in rabbits) is under normal conditions not in a state of tonic 

 innervation was demonstrated in 1863 by Landois by the fact that when, after 

 exposure of the vagi, care was taken, by means of artificial respiration, that the 

 number of heart-beats remained exactly the same as in the intact rabbit, section 

 of both vagi failed to cause increase in pulse-frequency. These observations were 

 confirmed by Schiff . It is true that in dogs after division of the vagi (in adult 

 dogs and never in the newborn) sudden increase in pulse-frequency and in 

 blood-pressure has been observed occasionally, but by no means constantly. 

 The frequency of the pulse of the previously resting animal under observation 

 should, however, be carefully determined first; and it should also be noted whether 

 the preparations for the experiment did not cause slowing of the pulse. Then, 



