204 THE CIRCULATION OF THE BLOOD 



blue is used, since it permits the determination without the loss of blood, 

 the change in color being visible through the walls of the blood-vessels. 



Stewart has made most accurate measurements of the circulation time 

 by the electrical-resistance method. Strong salt solutions injected into the 

 jugular vein on one side when they reach the other jugular (or any other 

 vessel) are instantly detected by a decrease in the electrical resistance through 

 the vessel when it is laid between the poles of the proper conductivity 

 apparatus. 



In all these experiments it is assumed that the substance injected moves 

 with the blood and at the same rate, and does not move from one part of 

 the organs of circulation to another by diffusing itself through the blood or 

 tissues more quickly than the blood moves. The assumption may be ac- 

 cepted that the times above mentioned as occupied in the passage of the in- 

 jected substances are the times in which the portion of blood itself is carried 

 from one part to another of the vascular system. 



Another mode of estimating the general velocity of the circulating blood 

 is by calculating it from the quantity of blood supposed to be contained in 

 the body, and from the quantity which can pass through the heart in each 

 of its contractions. But the conclusions arrived at by this method are less 

 satisfactory. For the total quantity of blood, and the capacity of the cavities 

 of the heart, have as yet been only approximately ascertained. Still the most 

 careful of the estimates thus made accord very nearly with those already 

 mentioned; and it may be assumed that the blood may all pass through 

 the heart in about twenty-five seconds. 



THE PULSE. 



The most characteristic feature of the arterial pressure and blood flow 

 is its intermittency, and this intermittent flow is seen or felt as waves of change 

 in diameter of the arteries, known as the Pulse. 



The pulse is generally described as a wave-like expansion of the artery 

 produced by the injection of blood at each ventricular systole into the already 

 full aorta. The force of the left ventricle is expended in pressing the blood 

 forward and in dilating the aorta. With the injection of each new quantity 

 of blood into the aorta there is a wave of dilatation which passes on, expanding 

 the arteries as it goes, running as it were over the more slowly traveling blood 

 contained in them, and producing the pulse as it proceeds. -A sharp dis- 

 tinction must be made between the passage of the pulse wave along an artery 

 and the rate of flow of the blood in the vessel. The pulse produced by any 

 given beat of the heart is not felt at the same moment in all parts of the body. 

 Thus, it can be felt in the carotid a short time before it is perceptible in the 

 radial artery, and in this vessel before it occurs in the dorsal artery of the 

 foot. Careful measurements of the intervals between the time of the pulse 



