488 CIRCULATION OF BLOOD AND LYMPH. 



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

 extremities 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 averages 

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

 are valuable in indicating how very rapidly any substance that 

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

 first employed by Hering (1829) was to inject into the jugular vein 

 of one side a solution of potassium ferrocyanid, and then from time 

 to time specimens of blood were taken from the jugular vein of the 

 opposite side. The first specimen in which the ferrocyanid could be 

 detected by its reaction with iron salts gave the least time necessary 

 for a complete circuit. The method was subsequently improved in 

 its technical details by 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 con- 

 cluded 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. 

 If a solution of a neutral salt, such as sodium chlorid, more concen- 

 trated than the blood, is injected into the circulation, the con- 

 ductivity of the blood is increased. If the injection is made at a 

 given moment and a portion of the vessel to be examined is properly 

 connected with a galvanometer so as to measure the electrical 

 conductivity through it, then the instant that the solution of salt 

 reaches this latter vessel the fact will be indicated by a deflection 

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



