THE CIRCULATION OF THE BLOOD. 227 



Chauveau's instrument, fig. 190, consists of a thin brass tube, a in one side 

 of which is a small perforation closed by thin vulcanized india-rubber. Pass- 

 ing through the rubber is a fine lever, one end of which, slightly flattened, 

 extends into the lumen of the tube, while the other moves over the face of a 

 dial. The tube is inserted into the interior of an artery, and ligatures applied to 

 fix it, so that the movement of the blood may, in flowing through the tube, be 

 indicated by the movement of the outer extremity of the lever on the face of 

 the dial. 



The Hcematochometer of Vierordt, and the instrument of Lortet, resemble in 

 principle that of Chauveau. 



In the Capillaries. The observation of Hales, E. H. Weber, and Val- 

 entin agree very closely as to the rate of the blood-current in the capil- 

 laries of the frog; and the mean of their estimates gives the velocity of 

 the systemic capillary circulation at about one inch (25 mm.) per min- 

 ute. The velocity in the capillaries of warm-blooded animals is greater. 

 In the dog ^ ff to T f^ inch (.5 to .75 mm.) a second. This may seem 

 inconsistent with the facts, which show that the whole circulation is 

 accomplished in about half a minute. But the whole length of capillary 

 vessels, through which any given portion of blood has to pass, probably 

 does not exceed from -g^th to -^th of an inch (.5 mm.); and therefore 

 the time required for each quantity of blood to traverse its own appointed 

 portion of the general capillary system will scarcely amount to a second. 



In the Veins. The velocity of the blood is greater in the veins than 

 in the capillaries, but less than in the arteries : this fact depending upon 

 the relative capacities of the arterial and venous systems. If an accurate 

 estimate of the proportionate areas of arteries and the veins correspond- 

 ing to them could be made, we might, from the velocity of the arterial 

 current, calculate that of the venous. A usual estimate is, that the ca- 

 pacity of the veins is about twice or three times as great as that of the 

 arteries, and that the velocity of the blood's motion is, therefore, about 

 twice or three times as great in the arteries as in the veins, 8 inches 

 (200 mm.) a second. The rate at which the blood moves in the veins 

 gradually increases the nearer it approaches the heart, for the sectional 

 area of the venous trunks, compared with that of the branches opening 

 into them, becomes gradually less as the trunks advance toward the heart. 



Of the Circulation as a Wliole. It would appear that a portion of 

 blood can traverse the entire course of the circulation, in the horse, in 

 half a minute. Of course it would require longer to traverse the vessels 

 of the most distant part of the extremities than to go through those of 

 the neck; but taking an average length of vessels to be traversed, 

 it may be concluded that half a minute represents the average rate. 

 \l Stewart states that the circulation time in man is probably not less than 

 twelve nor more than fifteen seconds. 



