282 THE CIRCULATION. 



the arterial system originates. As the blood, consequently, in its 

 passage from the heart outward, flows successively through larger 

 and larger spaces, the rapidity of its circulation must necessarily 

 be diminished, in the same proportion as it recedes from the heart. 

 It is driven rapidly through the larger trunks, more slowly through 

 those of medium size, and more slowly still as it approaches the 

 termination of the arterial system and the commencement of the 

 capillaries. 



The movement of the Hood through the arteries is primarily caused 

 by the contractions of the heart ; but is, at the same time, regulated 

 and modified by the elasticity of the vessels. The mode in which 

 the arterial circulation takes place is as follows. The arterial sys- 

 tem is, as we have seen, a vast and connected ramification of tubular 

 canals, which may be regarded as a great vascular cavity, divided 

 and subdivided from within outward by the successive branching 

 of its vessels, but communicating freely with the heart and aorta 

 at one extremity, and with the capillary plexus at the other; 

 and this vascular system is filled everywhere with the circulating 

 fluid. At the time of the heart's contraction, the muscular walls of 

 the ventricle act powerfully upon its fluid contents. The auriculo- 

 ventricular valves at the same time shutting back and preventing 

 the blood from regurgitating into the ventricle, it is forced out 

 through the aortic orifice. A charge of blood is therefore driven 

 into the arterial ramifications, distending their walls by the addi- 

 tional quantity of fluid forced into their cavities. When the ven- 

 tricle immediately afterward relaxes, the active distending force is 

 removed ; and the elastic arterial walls, reacting upon their contents, 

 would force the blood back again into the heart, were it not for the 

 semilunar valves, which shut together and close the aortic orifice. 

 The blood is therefore urged onward, under the pressure of the 

 arterial elasticity, into the capillary system. When the arteries 

 have thus again partially emptied themselves, and returned to their 

 original dimensions, they are again distended by another contraction 

 of the heart. In this manner a succession of impulses or distensions 

 is produced, which alternates with the reaction or subsidence of the 

 vessels, and which can be felt throughout the body, wherever the 

 arterial ramifications penetrate. This phenomenon is known by 

 the name of the arterial pulse. 



When the blood is thus driven by the cardiac pulsations into the 

 arteries, the vessels are not only distended laterally, but are elongated 

 as well as widened, and enlarged in every direction. Particularly 



