THE CIRCULATION. 317 



Section of the vagi increases the velocity by increasing the amount 

 of blood ejected by the ventricle. 



Systole of the heart increases the velocity ; diastole decreases it. 



Duration of the Circulation as a Unit. The general rapidity of 

 the circulation that is, how long a time an entire circulation occupies 

 may be easily determined experimentally in a living animal. This 

 was first accomplished by Hering, whose principle of action was to 

 compute the time required for the circuit of an injected, harmless sub- 

 stance. The substance taken is one that may be easily recognized by 

 chemical test; sodium ferrocyanide is the one least injurious to the 

 heart. He injected a 2-per-cent. solution into the central end of a 

 divided jugular vein,, and the time of injection was carefully noted. 

 From the opposite jugular, samples were taken as quickly as possible, 

 the time of each being noted. When the Prussian blue reaction was 

 obtained in any sample, the time of its withdrawal gave the duration 

 of the entire circuit. In this experiment the blood containing the 

 solution passed to the right side of the heart, through the lungs to the 

 left side of the heart, from thence into the aorta to be distributed 

 through the smaller vessels and capillaries of the head and face, to 

 return by the jugular veins. 



This jugular-to-jugular result does not represent the circulation 

 of the entire blood-supply of the body, but the shortest time that a 

 drop of blood may traverse the shortest pathway along both the 

 systemic and pulmonic circulations. It is impossible thus to determine 

 the circulation time of the entire blood. 



From the result of experiments it has been ascertained that 

 the circulation time in the horse is 31.5 seconds; in the dog, 16.7 

 seconds; in the rabbit, 7.79 seconds. 



Another method is that of Professor Gr. N". Stewart. He injects 

 into a rabbit methylene blue per jugular, and then, watches the 

 appearance of the coloring matter in the opposite carotid. Under 

 the carotid he places a thin sheet of India rubber and between this 

 and the artery a little piece of white, glazed paper. Then, noting 

 the time when blood is injected per jugular and the time of its 

 arrival in the opposite carotid, he determines the duration of the cir- 

 culation. In the rabbit he made the jugular-to-jugular time from 

 5 to 7 seconds. 



The above method merely gives the shortest possible time in 

 which a particle of blood can travel through the shortest pathway. 



In man the time it takes the blood to make a complete circuit of 

 the body is about 32 seconds. 



