CHAP, iv.] THE VASCULAR MECHANISM. 133 



rapidly again when the banks close in. Exactly in the same way 

 the velocity of the blood-stream slackens from the aorta to the 

 capillaries corresponding with the increased total bed, but hurries 

 on again as the numerous veins are gathered into the smaller bed 

 of the venae cavse. The loss of velocity in the capillaries, as com- ' 

 pared with the arteries, is not due to there being so much more ; 

 friction in the narrow channels of the former than in the wide j 

 canals of the latter. For the peripheral resistance caused by the j 

 friction in the capillaries and small arteries is an obstacle not only 

 to the flow of blood through these small vessels where the resist- 

 ance is actually generated, but also to the escape of the blood from 

 the large into the small arteries, and indeed from the heart intoi 

 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 re- [ 

 covery of velocity along the venous tract. The rapidity of the flow 

 in arteries, capillaries, and veins, is in each case determined by the 

 total sectional area of the channels. There is, however, a loss of 

 velocity on the whole course. At each stroke as much blood enters 

 the right auricle as issues from the left ventricle ; but the sectional 

 area of the venae cavae is greater than that of the aorta, so that 

 even if the auricle were tilled in exactly the same time as the 

 ventricle is emptied, the blood must pass more rapidly through the 

 narrow aorta than through the broad venae cava?, in order that the 

 same quantity of blood should pass each in the same time. The 

 diastole of the auricle, however, is distinctly longer than the systole 

 of the ventricle; the time during which the auricle is being filled is 

 greater than that during which the ventricle is being emptied, and 

 hence the velocity of the venous flow into the auricle must be still \ 

 less than that of the arterial blood in the commencing aorta. 



The temporary variations of the velocity of the stream in any 

 given' channel, and these we have already (p. 127) seen to be very 

 considerable in the case of the arteries at least, are dependent on a 

 variety of circumstances. In a tube of constant calibre, the velo- 

 city with which fluid flows from one point to another, for instance 

 from the point a to the point 6, will be in main dependent on the 

 difference between the pressures existing at a and b. The lower 

 the pressure at b as compared with a the greater the rapidity with 

 which the fluid flows from a to 6. And temporary variations of 

 pressures form undoubtedly the main cause of the temporary varia- 

 tions observable in the velocity of the arterial flow. Thus with 

 each systole of the ventricle there is an increase of velocity in the 

 whole arterial flow followed by a diminution during the diastole. 

 So also if the peripheral resistance in the minute arteries into 

 which a larger artery divides be suddenly lowered (by the action of 

 vaso-motor nerves, in a manner which we shall presently discuss), 

 without the calibre of the larger artery itself being changed, the 

 pressure on the distal (peripheral) side of the artery may be much 



