316 PHYSIOLOGY. 



General Facts in the Velocity of the Circulation. Marey, from a 

 study of the rapidity of the flow of blood, has arrived at the- following 

 conclusions : 



If the resistance increases and the output of the heart remains 

 constant, then the actual tension rises and the velocity becomes less. 



If the output of the heart increases and the resistance remains 

 constant, then both the tension and the velocity become greater. 



Ludwig and Dogiel state that the velocity of the blood does not 

 depend on the blood-pressure. They state that the velocity in 

 a vessel depends on (1) the vis a tergo that is, the action of the 

 heart; and (2) on the peripheral resistance. 



In an animal in repose, the average quickness of the blood con- 

 tinually varies at short intervals. These variations are not dependent 

 upon the temperature of the blood, for it can be made to vary from 104 

 degrees F. to a very cool limit without perceptibly changing the 

 rapidity. The variations of swiftness are entirely independent of the 

 rapidity of the heart, for the rapidity is different while the beat of the 

 heart has not altered. The changes of rapidity in the different 

 vessels are independent of the average aortic pressure or the pressure 

 which exists in the arteries. There is no proportional relation between 

 the variations of swiftness in the different arteries, for comparative 

 experiments made at the same time in the carotid and crural artery 

 show that the rapidity is greater now in one, now in the other of 

 these vessels. The variations of swiftness in these vessels are at the 

 same time independent of each other. The changes of rapidity of the 

 flow in the arteries depends solely upon the obstacles which the blood, 

 when injected, meets in its course, especially when the obstacles are 

 encountered by the flow of blood in other arteries at the same time; 

 for example, the resistance in the arteries of the intestine increases 

 the rapidity of the flow in the vessels of the brain notwithstanding 

 the obstacles in these vessels. The intestinal circulation regulates the 

 flow in other arteries, and particularly in the carotid. The obstacles 

 met with in the rapidity of the circulation in vessels depend upon the 

 innervation of these vessels by the different vasomotor nerves. This 

 innervation oscillates; hence the swiftness is not the same in two 

 arteries of the same caliber. Section of a vasomotor nerve does not 

 prevent variations in the rapidity in the territory innervated by that 

 nerve, since neighboring territories under the control of other vaso- 

 motor nerves can react upon the flow of blood in the part with the 

 divided vasomotor nerve. When one carotid is ligated the velocity in 

 the other increases. 



