486 CIRCULATION OF BLOOD AND LYMPH. 



the velocity of the blood and the pressure in the arteries to vary in 

 the same sense. (2) Whatever increases or diminishes the resis- 

 tance offered to the blood in passing from the arteries (to the veins) 

 will cause the velocity and the arterial pressure to vary in an 

 inverse sense as regards each other. That is, an increased re- 

 sistance diminishes the velocity in the arteries while increasing 

 the pressure, and vice versa. 



The Time Necessary for a Complete Circulation of the 

 Blood. — It is a matter of interest in connection with many physio- 

 logical questions to have an approximate idea of the time necessary 

 for the blood to make a complete circuit of the vascular system, — ■ 

 that is, starting from any one point to determine how long it will 

 take for a particle of blood to arrive again at the same spot. In 

 considering such a question it must be borne in mind that many 

 different paths are open to the blood, and that the time for a 

 complete circulation will vary somewhat with the circuit actually 

 followed. For example, blood leaving the left ventricle may pass 

 through the coronary system to the right heart and thence through 

 the puhnonary system to the left heart again, or it may pass to the 

 extreniities of the toes before getting to the right heart, or it may pass 

 through the intestines, in which case it will have to traverse three 

 capillary areas before completing the circuit. It is obvious, there- 

 fore, that any figures obtained can only be regarded as approxima- 

 tions more or less exact. The experiments that have been made, 

 however, are valuable in indicating bow very rapidly any substance 

 that enters the blood may be distributed over the body. The 

 method first employed by Hering was to inject into the jugu- 

 lar vein of one side a solution of potassium ferrocyanid, and then 

 from time to time specmiens of blood were taken from the jugular 

 vein of the opposite side. The first specimen in which the ferrocy- 

 anid could be detected by its reaction with iron salts gave the least 

 time necessary for a complete circuit. The method was subse- 

 quently improved in its technical details bj^ Vierordt, and such 

 results as the following were obtained: Dog, 16.32 seconds; horse, 

 28.8 seconds; rabbit, 7.46 seconds; man (calculated), 23 seconds. 

 The time required is less in the small than in the large animals, and 

 Hering and Vierordt concluded that in general it requires from 26 to 

 28 beats of the heart to effect a complete circulation. Stewart has 

 devised a simpler and better method,* based upon the electrical 

 conductivity of the blood. 



The Pressure Relations in the Vascular System. — That the 

 blood is under different pressures in the several parts of the vascu- 

 lar system has long been known and is easily demonstrated. When 

 * "Journal of Physiology," 15, 1, 1894. 



