BY DR. BURDON-SANDERSON. 285 



in reflex relation with the vagus, than in the latter. In the 

 frog, section of the sympathetic at the level of the junction of 

 the aortffi has no direct effect on the frequency of the pulse, for 

 the same reason, viz., that in this animal the heart-beat is not 

 quickened by section of the vagi. 



The influence of reflex excitation of the vagus through the 

 fifth nerve may be easily shown in the rabbit by causing the 

 animal to smefl ammonia. The effect is immediate. Accord- 

 ing to the strength of the ammonia, the heart is arrested in 

 diastole, or the diastolic intervals are lengthened. The inha- 

 lation of chloroform, which is so apt to be fatal to rabbits, stops 

 the heart in the same way. When sudden death occurs in a 

 man by a blow on the epigastrium, or by drinking a large 

 quantity of cold water, the heart is arrested in diastole by the 

 agency of the same nerves as in Goltz's experiment. 



79. Demonstration of the Influence of Increase or 

 Diminution of the Arterial Pressure on the Fre- 

 quency of the Contractions of the Heart. The pulse is 

 retarded by increase, accelerated by diminution of arterial 

 pressure. That these effects are mainly dependent on the in- 

 hibitory heart nerves, can be shown in the rabbit as follows : 

 Ligatures having been passed round the vagus nerve on each 

 side, and a needle inserted in the heart, the fingers of the right 

 hand are placed under the animal's back, while the thumb is 

 firmly pressed upon the aorta, the beats of the needle having 

 been previously counted. On making pressure, the frequency 

 of the contractions of the heart is diminished, and this effect 

 continues so long as the pressure lasts. 



Both vagi are now divided and the experiment repeated. 

 The frequency of the pulse is still slightly diminished, but the 

 degree of diminution is not to be compared with the previous 

 effect. This experiment can be made with greater exactitude 

 by applying the pressure to the aorta directly, at the same 

 time connecting the carotid artery with Pick's kymograph. 

 To accomplish the first of these objects, the abdominal cavity 

 is opened in a chloralized rabbit in exactly the same way as 

 for excitation of the left splanchnic nerve. It is then easy to 

 place the thumb directly on the aorta as it passes between the 

 crura of the diaphragm. Tracings are thus obtained which 

 show that, during obstruction of the aorta, the arterial press- 

 ure is doubled, or even trebled, and the pulse rate much di- 

 minished, the status quo being re-established when the thumb 

 is removed from the aorta. After division of the vagi, the 

 effect as regards pressure is of course as marked as before, but 

 there is scarcely any slowing of the pulse. 



The fact that the effect of aortic obstruction in diminishing 

 the frequency of the pulse is so markedly weakened by section 

 of both vagi, shows that these nerves bear a large part in its 



