236. THE CIRCULATION OF THE BLOOD 



time required for the passage of solutions of potassium ferrocyanide, mixed 

 with the blood, from one jugular vein, through the right side of the heart, the 

 pulmonary vessels, the left cavities of the heart, and the general circulation, 

 to the jugular vein of the opposite side, varies from twenty to thirty seconds 

 in the dog. The same substance is transmitted from the jugular vein to the 

 great saphenous vein in twenty seconds; from the jugular vein to the mes- 

 enteric artery in between fifteen and thirty seconds; to the facial artery, 

 in one experiment, in between ten and fifteen seconds; in another experi- 

 ment, in between twenty and twenty- five seconds; in its transit from the 

 jugular vein to the metatarsal artery, it occupies between twenty and 

 thirty seconds. The result is said to be nearly the same whatever the 

 rate of the heart's action. In more recent methods some innocuous dye 

 like methylene 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 man in about thirty seconds or even less. 



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. 



