CH. XXII.] VELOCITY OF BLOOD-FLOW 281 



anaesthesia, when the forces which counteract the bad effects of 

 gravity may not be working efficiently; if the legs are hanging 

 down, the result may be serious. 



The pressure in the Pulmonary Circulation varies from J to -J 

 (mean J) of that in the systemic vessels. 



The influence of the Cardiac Nerves on blood-pressure. The 

 importance of the heart's action in the maintenance of blood-pressure 

 is well shown by the effect that stimulation of the vagus nerve has 

 on the blood-pressure curve. If the vagus of an animal is exposed 

 and cut through, and the peripheral end stimulated, the result is that 

 the heart is slowed or stopped; the arterial blood-pressure conse- 

 quently falls ; the fall is especially sudden and great if the heart is 

 completely stopped. There is a rise in venous pressure. The effect 

 on arterial pressure is shown in the two accompanying tracings ; fig. 

 246 represents the effect of partial, and fig. 247 of complete stoppage 

 of the heart ; in both cases the animal used was a rabbit, and the 

 artery the carotid. 



On stimulating the cardiac sympathetic (accelerator and augmentor 

 fibres) the increased action of the heart causes a rise of arterial pres- 

 sure. 



The effects of stimulating the central end of the vagus and other 

 nerves cannot be understood until we have studied the vaso-motor 

 nervous system. 



The Velocity of the Blood-flow. 



We have already seen that the velocity of the current of blood is 

 inversely proportional to the sectional area of the bed through which 

 it flows. The flow is therefore swiftest in the aorta and arteries, and 

 slowest in the capillaries. In very round numbers, the rate is about 

 a foot per second in the aorta, and about an inch per minute in the 

 capillaries. The capacity of the veins is about twice or thrice that of 

 the arteries ; so the velocity in the veins is from a half to a third of 

 that in the corresponding arteries. The rate in the veins increases as 

 we approach the heart, as the total sectional area of the venous trunks 

 becomes less and less. 



The question of velocity is one of great importance, for it is on 

 velocity that the actual amount of blood supplying the tissues mainly 

 depends. In the capillaries the rate can be measured by direct micro- 

 scopic investigation of the transparent portions of animals. E. H. 

 Weber and Valentin were among the earliest to make these measure- 

 ments in the frog, and the mean of their estimates gives the velocity 

 as 25 mms. per minute in the systemic capillaries. In warm-blooded 

 animals the velocity is somewhat greater ; in the dog it is -^ to y^ 

 inch (0'5 to 075 mm.) per second. It must be remembered that the 

 total length of capillary vessels through which any given portion of 



