THE CIRCULATION OF THE BLOOD. 227 



consequence of so many angles, be a source of much fric- 

 tion. 



The action of the corpuscles on one another may be com- 

 pared to a crowd of people hurrying along a narrow passage — 

 the obstruction comes from interaction of a variety of forces, 

 owing to the crowd itself rather than the nature of the thor- 

 oughfare. We must set down a great deal to the influence of 

 the corpuscles on one another, as they are carried along accord- 

 ing to mechanical principles ; but, as we shall see later, other 

 and more subtle factors play a part in the capillary circulation. 

 Owing to the peripheral resistance and the pumping force of 

 the heart, the arteries become distended, so that, during cardiac 

 diastole, their recoil, owing to the closure of the semilunar 

 valves, forces on the blood in a steady stream. It follows, then, 

 that the main force of the heart is spent in distending the 

 arteries, and that the immediate propelling force of the circu- 

 lation is the elasticity of the arteries in which the heart stores 

 up the energy of its systole for the moment. 



BLOOD-PRESSURE. 



Keeping in mind our schematic representation of the circu- 

 lation, we should expect that the blood must exercise a certain 

 pressure everywhere throughout the vascular system ; that this 

 blood-pressure would be highest in the heart itself ; considera- 

 ble in the whole arterial system, though gradually diminishing 

 toward the capillaries, in which it would be feeble ; lower still 

 in the smaller veins ; and at its minimum where the great veins 

 enter the heart. Actual experiments confirm the truth of these 

 views ; and, as the subject is one of considerable importance, we 

 shall direct attention to the methods of estimating and record- 

 ing an animal's blood-pressure. 



First of all, the well-known fact that, when an artery is cut, 

 the issuing stream spurts a certain distance, as when a water- 

 main, fed from an elevated reservoir, bursts, or a hydrant is 

 opened, is itself a proof of the existence of blood-pressure, and 

 is a crude measure of the amount of the pressure. 



One of the simplest and most impressive ways of demon- 

 strating blood-pressure is to connect the carotid, femoral, or 

 other large artery of an animal by means of a small glass tube 

 (drawn out in a peculiar manner to favor insertion and reten- 

 tion by ligature in the vessel), known as a cannula, by rubber 



