THE CARDIAC INHIBITORY CENTER. 759 



the section itself may cause irritation the accelerator fibers in the vagi or the 

 pressor fibers, which likewise accelerate the heart -beat. In the dog whose vagi 

 are paralyzed after injection of curare into the veins, with maintenance of artificial 

 respiration, the heart -beat is not accelerated, and in the frog section of both vagi 

 is invariably unattended with acceleration of pulse. Also, the increase in blood- 

 pressure after division of both vagi is not solely dependent upon the associated 

 increase in the pulse-rate that occurs. 



The cardiac inhibitory center can be stimulated reflexly: i. 

 Through the irritation of sensory nerves. 2. Through irritation of the vagus 

 itself, as by irritation of the central stump of one vagus, with preserva- 

 tion of the other. 3. Irritation of the sensory nerves of the ab- 

 dominal viscera, by percussion of the abdomen of the frog (Goltz's 

 percussion-experiment), has a cardiac inhibitory effect; as does also 

 irritation of the splanchnic directly or of the abdominal and cervical 

 cords of the sympathetic. Severe irritation of sensory nerves, however, 

 inhibits the reflexes affecting the vagus described, and has a reflex 

 inhibiting effect generally. 



The experiment of Goltz succeeds at once if the irritation is permitted to 

 act upon the exposed intestines (of the frog) , which become inflamed on protracted 

 exposure to the air. Also in dogs irritation of the stomach causes slowing of the 

 pulse. 



The irritation of the cardiac inhibitory center can be diminished 

 reflexly, according to Hering, by vigorous distention of the lungs with 

 atmospheric air. Under such circumstances there is marked reduction 

 in blood-pressure. 



In man forcible expiratory effort causes acceleration of the heart-beat in conse- 

 quence of the increased intrapulmonary pressure, and this has been attributed by 

 Sommerbrodt to reduction in the activity of the cardiac branches of the vagi, 

 which are in a state of tonic innervation. At the same time a depressant effect 

 is exerted upon the vasomotor center. 



In the entire course from the center downward through the trunk of the 

 vagus and further on through its cardiac branches, irritation causes slowing and 

 enfeeblement and finally cessation of the activity of the heart. In the frog this 

 result can be brought about even by irritation of the fibers of the vagus at the 

 venous sinus of the heart. Feeble irritants slow the heart-beat, while stronger 

 irritants cause diastolic arrest. If irritants of considerable intensity affect either 

 the center or the course of the nerve for a considerable period of time, the irritated 

 area becomes exhausted and the heart again pulsates more rapidly in spite of 

 the persistent irritation. If, however, the site of irritation is displaced nearer to 

 the heart, renewed inhibition takes place, as the irritation now affects a new 

 nerve segment. 



With reference to the irritation of the inhibitory fibers the following points 

 are w r orthy of note: i. It is probable that the fibers diminish the number of 

 heart-beats and those diminishing the strength of the heart are distinct, both 

 with reference to their anatomic arrangement, as well as with respect to their 

 susceptibility to various poisons. The experiments of Heidenhain on frogs, con- 

 firmed by Lowit, have shown that electrical and chemical stimulation of the vagus 

 have varying results with reference to the size and the number of the heart -beats. 

 Either the contractions become only smaller, or they become only less frequent, 

 or they become smaller and at the same time less frequent. Those branches of 

 the vagus that in the frog are situated in the nerves of the septum exert an influ- 

 ence alone upon the strength and the tone. Those fibers, however, that enter 

 the frog's heart outside of the nerves of the septum have an influence alone upon 

 the number of heart-beats. In the turtle also, both sets of fibers are anatomically 

 distinct. 2. To obtain the inhibitory effect persistent irritation is not necessary, 

 but a moderately rapid rhythmic, interrupted irritation will suffice : from eighteen 

 to twenty irritations in a second in warm-blooded and two or three in cold-blooded 

 animals. 3. Bonders observed in association with Prahl and Nuel that the inhi- 

 bition manifested itself not immediately at the moment of irritation, but that 

 from one-sixth to two-fifths of a second elapsed before the onset of the action. 



