CHAP, iv.] THE VASCULAR MECHANISM. 



223 



the excursion of the lever. The movements of the short arm give rise 

 to corresponding movements in the opposite direction of the long arm 

 outside the tube, and these, by means of a marker attached to the end 



FlG. 33. H^EMATACHOMETER OF ClIAUVEAU AND LORTET. 



of the long arm, may be directly inscribed on a recording surface. This 

 instrument is very well adapted for observing changes in the velocity of 

 the flow. In determining actual velocities, for which purpose it has to 

 be experimentally graduated, it is not so useful. 



In the capillaries, the rate is slowest of all. In the web of the 

 frog the flow as judged by the movement of the red corpuscles may 

 be directly measured under the microscope by means of a micro- 

 meter, and is found to be about half a millimetre in a second; 

 but this is probably a low estimate, since it is only when the 

 circulation is somewhat slow, slower perhaps than what ought to 

 be considered the normal rate, that the red corpuscles can be 

 distinctly seen. In the mammal the rate has been estimated 

 at about 75 millimetres a second but is probably quicker even 

 than this. 



As regards the veins, the flow is very slow in the small veins 

 emerging from the capillaries but increases as these join into larger 

 trunks, until in a large vein, such as the jugular of the dog, the 

 rate is about 200 mm. a second. 



123. It will be seen then that the velocity of the flow is in 

 inverse proportion to the width of the bed, to the united sectional 

 areas of the vessels. It is greatest at the aorta, it diminishes 

 along the arterial system to the capillaries, to the united bases 

 of the cones spoken of in 112, where it is least, and from thence 

 increases again along the venous system. 



And indeed it is this width of the bed and this alone which 



