1893.] Nervous System on the Cardiac Rhythm, fyc. 475 



It will be seen that the evidence yielded from these three different 

 sources is in complete accord — the evidence derived from (1) a con- 

 sideration of the nature of the cardiac acceleration which may occur, 



(2) the negative results obtained after section of the vagi, and 



(3) the (complementary) positive results seen in unmistakable form 

 after exclusion of the augmentors while the vagi remain intact. 



Possibility of Changes in the Peripheral Efficiency of the Vagi. 



It remains to be seen how far a diminution of the controlling in- 

 fluence exercised by the medulla on the cardiac rhythm might be due 

 to changes in the heart itself — changes leading to an impairment of 

 the peripheral efficiency of the vagi, e.g., the influence of the pro- 

 ducts of muscular contraction, or of altered conditions of intra-cardiac 

 tension, especially in the right heart, depending on the occurrence of 

 convulsive movements, &c. 



As far as the splanchnic afferent nerves are concerned, these causes 

 may be set aside, for, as has been already stated, reflex acceleration 

 frequently occurs without any contraction of the skeletal muscles. 



In the case of the stimulation of the somatic afferent nerves (where 

 muscular contraction does occur) it is quite clear that the causes 

 mentioned above are also unessential. Reflex acceleration sometimes 

 occurs with an amount of muscular contraction far too slight to 

 cause any such sudden alterations in either of the ways suggested ; 

 and, more, it may occur even after the skeletal muscles have been 

 paralysed by curare. It has been already stated that there is no 

 constant relation between the changes in blood pressure (frequently 

 slight in amount) and the occurrences of reflex acceleration. 



Further, I have examined the relation borne towards the peripheral 

 efficiency of the vagi by (a) changes in arterial and intra-cardiac 

 pressure and (b) well marked convulsive movements excited by 

 stimulation of afferent nerves, after all the cardiac nerves have been 

 cut, or the medulla destroyed or killed (cat or rabbit). The results 

 have been as follows : — 

 I a. Great and sudden changes in arterial pressure (e.g., between 40 

 and 150 mm.) induced by compression of the descending thoracic or 

 the upper part of the abdominal aorta, closure of the vena porta, &c, 

 for short periods (e.g., 30 sees.) though frequently accompanied by 

 great changes in intra-cardiac tension had little or no appreciable 

 effect upon the maintenance of the moderate or slow rate of beat 

 maintained by slight continued excitation of the vagus in the neck. 



b. The occurrence of very marked convulsive movements in 

 response to stimulation of afferent nerves does not, even after several 

 1 seconds, cause any perceptible change in the slowing influence of 

 slight continued excitation of the vagus. 



