160 



HANDBOOK OF PHYSIOLOGY. 



been filled in a given time is known, the velocity of the current can be 

 calculated. 



Chauveau's instrument (Fig. 142) 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, 



FIG. 142. Diagram of Chauveau's Instrument, a, Brass tube for the 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. 



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 HcBmatocho meter of Vierordt, and the instrument of Lortet, re- 

 semble in principle that of Chauveau. 



(b.) In the Capillaries The observations of Hales, E. H. Weber, 

 and Valentin agree very closely as to the rate of the blood-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 -gV to T jf F inch (.5 to .73 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 capil- 

 lary vessels, through which any given portion of blood has to pass, prob- 

 ably does not exceed from -^th to -g^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. 



(c.) 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 



