154 THE VASCULAR MECHANISM. 



composed of an India-rubber membrane, is introduced between the two cut ends 

 of an artery. A long light lever pierces the India-rubber membrane. The short 

 expanded arm of this lever projecting within the tube is moved on its fulcrum in 

 the India-rubber ring by tne current of blood passing through the tube, the 

 greater the velocity of the current the larger being the excursion of the lever. 

 The movements of the short arm give rise to corresponding movements in the oppo- 

 site direction of the long arm outside the tube, and these, by means of a marker 

 attached to the end of the long arm, may be directly inscribed on a recording sur- 

 face. 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 micrometer, and is found to 

 be about half a millimeter 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 0.75 

 millimetres a second, but is probably quicker than even 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. 



112. 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 101, 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 determines 

 the general velocity of the flow at various parts of the system. The slowness 

 of the flow in the capillaries is not due to there being so much more friction 

 in their narrow channels than in the wider canals of the larger arteries. 

 For the peripheral resistance caused by the friction in the capillaries and 

 small arteries is an obstacle not only to the flow of blood through these 

 small vessels where the resistance is actually generated, but also to the 

 escape of the blood from the large into the small arteries, and indeed from 

 the heart into the large arteries. It exerts its influence along the whole 

 arterial tract. And it is obvious that if it were this peripheral resistance 

 which checked the flow in the capillaries, there could be no recovery of 

 velocity along the venous tract. 



The blood is flowing through a closed system of tubes, the bloodvessels, 

 under the influence of one propelling force, the systole of the ventricle, for 

 this is the force which drives the blood from ventricle to auricle, though, as 

 we have seen, its action is modified in the several parts of the system. In 

 such a system the same quantity of fluid must pass each section of the sys- 

 tem at the same time, otherwise there would be a block at one place and a 



deficiency at another. If, for instance, a fluid 

 FIG. 49. is made to flow by some one force, pressure or 



gravity, through a tube A (Fig. 49) with an 

 enlargement B, it is obvious that the same 

 quantity of fluid must pass through the sec- 

 tion b as passes through the section a in the 

 same time for instance, a second. Other- 

 a I c wise, if less passes through b than a, the 



fluid would accumulate in B, or if more, 

 B would be emptied. In the same way just as much must pass in the 



