470 Dr. J. A. Mac William. Influence of the Central [May 1 8, 



these nerves, or by destruction or death of the medulla and cervical 

 spinal cord (cat and rabbit). 



The general results obtained were these : — a. In this condition it 

 was found that the existence of an extremely low pressure continuing 

 for some time (minutes) leads to a very pronounced slowing of the 

 pulse rate, and a subsequent elevation of the pressure may cause a 

 more or less extensive acceleration of the beat. 



b. An exceedingly high blood pressure may canse marked slow- 

 ing of the heart through its influence on the intra-cardiac mechanism. 



c. When the blood pressure is at a fair height, the occurrence of 

 a great fall of pressure lasting for short periods (e.g., 30 seconds) 

 causes no change in the cardiac rhythm. 



~Nor does a rise of pressure from this low level — established as it 

 has been for only a brief space of time — to the preceding level, or 

 even a good deal higher, involve any appreciable change in the pulse 

 rate. 



A similar negative result is present when, instead of undergoing a 

 fall from the original level, the pressure is made to rise ; there is no 

 change in the pulse rate unless the rise attains to such proportions as 

 to induce the slowing mentioned in (b). 



There can be no doubt that extensive variations in the blood 

 pressure (e.g., between 30 — 40 mm. and 150 — 160 mm.) if of brief 

 duration do not as a rule involve any important or constant change 

 in the rate of the heart's beat. 



B. When all the Cardiac Nerves were Intact. — The results here ob- 

 tained were in complete accordance with those usually described — a 

 raised blood pressure causing well-marked slowing of the heart, and 

 a fall of blood pressure carrying with it a notable acceleration — in 

 the absence of disturbing or complicating causes. 



It may be remarked that similar changes are seen after the cardiac 

 augrnentor nerves have been divided ; they are evidently essentially 

 due to changes in the activity of the vagi. 



It may also be noted that the continuance of an excessively low 

 pressure will ultimately lead to a great slowing of the pulse rate — 

 following on the primary acceleration — in virtue of the influence of 

 such a condition of blood pressure on the cardiac rhythm already 

 mentioned (A, a), so that an extreme and prolonged lowering of the 

 blood pressure occurring when the heart is beating at an approxi- 

 mately normal rate causes first a remarkable acceleration of the pulse, 

 which ultimately gives place to a slow rate of beat. The latter 

 change may be counteracted by an abnormally high temperature if 

 such is present. 



