THE FLOW OF BLOOD THROUGH THE ARTERIES 971 



phery. The occurrence of reflected waves serves to explain why the systolic 

 pressure in the femoral artery is found higher and the diastolic pressure lower 

 than in the brachial. The femoral artery being more rigid than the brachial 

 and the peripheral resistance more definitely localised, reflected waves occur 

 in the artery at so short a time after the primary wave has passed down 

 that there is summation of the two waves, with production of a higher maxi- 

 mum and a lower minimum than was present in the waves as started in the 

 aorta. If the leg be plunged into hot water, so as to dilate all its arterioles, 

 this difference between the arm and the leg systolic pressures disappears. 

 The varying development of reflected waves on the two sides also explains 

 why the systolic pressures in the two arms are rarely found to be identical. 



The general form of the pulse curve varies with changes in the heart, 

 in the arteries, and in the peripheral resistance. Thus some curves may 

 present secondary elevations on the ascending part, and are called anacrotic, 

 while in others all secondary elevations occur on the descending part. The 

 latter type is called catacrotic, and is the tracing usually obtained from a 

 normal radial artery. By comparing these two types of curves with the 

 corresponding intra ventricular pressures, we find that in both cases blood 

 is flowing into the aorta during the whole time from the beginning of the 

 primary elevation to the notch just before the dicrotic elevation. This is 

 shown by the fact that the intraventricular pressure is all this time slightly 

 higher than the aortic pressure. So long as this is the case blood must 

 flow from ventricle into aorta. (This fact proves that there is normally 

 no part of the cardiac cycle during which the ventricle remains contracted 

 and empty, the ventricle in all cases relaxing before it has completely emptied 

 itself of blood.) 



Now it is easy to see the conditions which determine whether the systolic 

 plateau shall be ascending or descending, and therefore when the pulse 

 shall be anacrotic or catacrotic. If, after the first sudden rise of pressure 

 in the aorta, the blood can escape more rapidly through the peripheral 

 resistance than it is thrown into the beginning of the aorta, the ' systolic 

 plateau ' will sink, and a catacrotic pulse tracing is obtained. If, on the 

 other hand, the peripheral resistance is high, or an extra large amount of 

 blood be thrown into the aorta at each stroke of the heart (e.g. by pro- 

 longation of the diastole), the aortic pressure will rise so long as blood is 

 flowing in, and we get an ascending systolic plateau and an anacrotic pulse. 

 Thus we obtain an anacrotic pulse in old people with Bright 's disease, in 

 whom the peripheral resistance is very high, and also in animals when the 

 heart is slowed by vagus action. 



The production of the dicrotic elevation is favoured by any influence 

 which increases the elastic resiliency of the arteries or causes the primary 

 elevation of the pulse to be rapid and sharp. Thus it is much more pro- 

 nounced in young people than in old people, whose arteries have become 

 rigid. When the peripheral resistance is low through relaxation of the 

 arterioles,' and the heart is beating forcibly, as in many cases of fever and 

 also to some extent after a good mea 'with alcohol, the dicrotic elevation 



