208 



Prof. J. A. McWilliam. 



[May 17, 



simultaneous motor discharge from a nerve centre to all the muscular 

 fibres ; the contraction is, on the other hand, a progressive process 

 passing over the auricular walls in a wave-like fashion. 



9. A reversal of the normal sequence of the heart's contraction can 

 be induced and kept up for a considerable time by applying to the 

 ventricles a series of single stimulations (e.g., induction shocks) at a 

 rate somewhat more rapid than that of the spontaneous rhythm of the 

 organ. 



V. b " Inhibition of the Mammalian Heart." By John A. 

 McWilliam, M.D., Professor of the Institutes of Medicine 

 in the University of Aberdeen. Communicated by Pro- 

 fessor- M. Foster, Sec. R.S. Received May 3, 1888. 



The following conclusions are based upon a long series of experi- 

 ments performed upon the cat, dog, rabbit, rat, hedgehog, and guinea- 

 pig, the cat being the animal commonly used. The animals were 

 anaesthetized, usually with chloroform ; artificial respiration was kept 

 up ; the thorax and often the pericardial sac were laid open, and the 

 action of the heart was examined with the aid of the graphic 

 method. 



Section- of the Vagi. 



The results of section of both vagi vary according to the conditions 

 obtaining at the time the nerves are cut — according to the amount of 

 controlling influence exercised by the medullary cardio-inhibitory 

 centre upon the heart. When the cardio-inhibitory centre is inactive, 

 section of the vagi causes no appreciable change in the heart's action. 

 On the other hand,, section of the nerves at a time when the con- 

 trolling influence of the medullary centre is acting to a decided extent, 

 is followed by very pronounced results — by an increase not only in 

 the rate of the cardiac beat, but also in the contraction force of both 

 the auricles and the ventricles. There is a marked augmentation in 

 the strength of the beats ; the change in the energy of the auricular 

 contractions is usually more extensive than that occurring in the case 

 of the ventricles. 



Stimulation of the Vagus Nerve. 



The latent period of vagus stimulation varies remarkably in different 

 conditions ; there is often a period of many seconds before the heart 

 stands still. 



When the vagus nerve is stimulated so as to slow the heart, it is 

 usually seen that the inhibitory influence is not of maximal intensity 

 at its first manifestation, but goes on increasing for some time. 



