THE ARTERIES AND THE ARTERIAL CIRCULATION. 287 



centre of the circulation outward. The pulse of the radial artery 

 at the wrist is perceptibly later than that of the heart ; and the 

 pulse of the posterior tibial at the ankle, again, perceptibly later 

 than that at the wrist. The arterial circulation, accordingly, is not 

 an entirely simple phenomenon ; but is made up of the combined 

 effects of two different physical forces. In the first place, there is 

 the elasticity of the entire arterial system, by which the blood is 

 subjected to a constant and uniform pressure, quite independent of 

 the action of the heart. Secondly, there is the alternating contrac- 

 tion and relaxation of the heart, by which the blood is driven in 

 rapid and successive impulses from the centre of the circulation, to 

 be thence distributed throughout the body. 



The passage of the blood through the arterial system takes place 

 under a certain degree of constant pressure. For these vessels being 

 everywhere elastic, and filled with blood, they constantly tend to 

 react, more or less vigorously, and to compress the circulating fluid 

 which they contain. If any one of the arteries, accordingly, be 

 opened in the living animal, and a glass tube inserted, the blood 

 will immediately be seen to rise in the tube to a height of about 

 five and a half or six feet, and will remain at that level ; thus indi- 

 cating the pressure to which it was subjected in the interior of the 

 vessels. This constant pressure, which is thus due to the reaction 

 of the entire arterial system, is known as the arterial pressure. 



The degree of arterial pressure may be easily measured by con- 

 necting the open artery, by a flexible tube, with a small reservoir 

 of mercury, which is provided with a narrow upright glass tube, 

 open at its upper extremity. "When the blood, therefore, urged by 

 the reaction of the arterial walls, presses upon the surface of the 

 mercury in the receiver, the mercury rises in the upright tube, to 

 a corresponding height. By the use of this instrument it is seen, 

 in the first place, that the arterial pressure is nearly the same all 

 over the body. Since the cavity of the arterial system is every- 

 where continuous, the pressure must necessarily be communicated, 

 by the blood in its interior, equally in all directions. Accordingly, 

 the constant pressure is the same, or nearly so, in the larger and the 

 smaller arteries, in those nearest the heart, and those at a distance. 

 This constant pressure averages, in the higher quadrupeds, six 

 inches of mercury, which is equivalent to from five and a half to 

 six feet of blood. 



It is also seen, however, in employing such an instrument, that 

 the level of the mercury, in the upright tube, is not perfectly steady, 



