278 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



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

 277 representing the effect of partial, and fig. 278 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 -^ 

 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 

 blood has to pass probably does not exceed from -^ to ^V inch 

 (0'5 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. It is during this time that the 

 blood does its -duties in reference to nutrition. 



In the larger vessels direct observations of this kind are not 

 possible, and it is necessary to have recourse to some instrumental 

 method. 



Volkmann was the first to make more or less accurate measure- 

 ments by introducing a long (J -shaped glass tube into the course of 

 an artery. A diagram of this Jicemodromometer, as it was termed, is 

 shown in the accompanying diagram (fig. 279); this is filled with 

 salt solution, and provided with a stop-cock a ; this tap is so arranged 



