THE CIKCULATION 247 



depends to so large an extent upon the amount of pressure 

 exerted by the spring, and the amount of pressure must be 

 adapted to the vascular tone in every case. Some of the most 

 interesting tracings are obtained from old people affected with 

 atheroma of the arteries. This is a condition in which, owing to 

 old-standing inflammation of the subepithelial coat of the 

 vessels, the arteries have lost their suppleness. They are hard 

 and inelastic. Instead of showing the normal steep face of the 

 pulse-wave rising abruptly to its highest point, the tracing rises 

 vertically for a short distance, and then slopes upwards. The 

 wave is flat-topped or hog-backed. 



All pulses are dicrotic, although the dicrotism may not be 

 sufficiently pronounced to be felt with the finger. The notch 

 which divides the primary from the secondary wave is pro- 

 duced by the closure that is to say, by the falling down of the 

 aortic valve. The wave from the commencement of its ascent 

 to the dicrotic notch corresponds to the period during which 

 blood is passing from the heart into the aorta. This part of 

 the tracing represents systole of the ventricle after the semi- 

 lunar valve has been forced. It is the push given to the bottom 

 of the column by the additional 3 ounces of blood thrust into 

 the aorta. The effort of the ventricle then comes to an end. 

 The pressure beneath the semilunar valve is less than that 

 above it. The valve closes. If the blood were contained in an 

 open tube, the wave would now end, save for secondary 

 oscillations, due to inertia of the fluid. But the arterial 

 system is practically closed owing to the fineness of the tubes 

 into which it ultimately divides. Its Avails are elastic. They 

 distend, taking up the pressure and returning it again in the 

 second half of the wave. In fever, after the consumption of 

 alcohol, and in other conditions in which the finest blood- 

 vessels are dilated, the division between the two parts of the 

 wave is very marked. Dicrotism is plainly felt. We have 

 used the expression " finest vessels " rather than " capillaries," 

 because the ascription to the capillary vessels of all peripheral 

 resistance has led to misunderstanding. Resistance is offered 

 throughout the whole vascular system, with the exception of 

 the largest veins. It is greatest in the small arteries, capil- 

 laries, and small veins. It is so adjusted as to fall to zero just 

 before the blood reaches the heart. 



