CIRCULATION OF THE BLOOD. 



165 



Chauveau's instrument (Fig. 143) consists of a thin brass tube, #, in 

 one side of which is a small perforation closed by thin vulcanized india- 

 rubber. Passing 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 



FIQ. 143. Diagram of Chauveau's Instrument, a. Brass tube for introduction into the lumen of 

 the artery, and containing an index-needle, which passes through the elastic membrane in its side, 

 and moves by the impulse of the blood-current, c. Graduated scale, for measuring the extent of the 

 oscillations of the needle. 



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. 



Velocity in the Capillaries. the observations of Hales, E. H. 

 Weber, and Valentin agree very closely as to the rate of the ftood-current 

 in the capillaries of the frog; and the mean of their estimates gives the 

 velocity of the systemic capillary circulation at about one inch (25 mm.) 

 per minute. The velocity in the capillaries of warm-blooded animals is 

 greater. In the dog -^ to T --JJ- 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 -^ th to -^th O f 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. 



Velocity 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 

 corresponding 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 capacity 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, 



