VELOCITY OF BLOOD IN VASCULAR SYSTEM 887 



is therefore also lessened, so that a final result of a dilatation of the arterioles 

 may be a diminished instead of an increased velocity throughout the 

 system. 



The foregoing discussion of the factors which determine the average 

 velocity across a given cross- section of the whole vascular system must not be 

 applied directly to the changes in the velocity following on local alterations 

 in the resistance presented by some particular vascular area. In this case 

 the local changes are insufficient to affect the general arterial blood-pressure, 

 and the effect of diminution of peripheral resistance is to furnish a short cut 

 for a small portion of the total output of the heart from the arterial to the 

 venous side. Thus dilatation of the vessels of the submaxillary gland, while 

 not altering the general blood-pressure as registered in the carotid artery, 

 causes the blood-flow through the gland to be increased six to eight times, 

 and the peripheral resistance in the gland may be so far diminished that the 

 blood passes through the capillaries into the veins without losing the pulsa- 

 tile force imparted to it by each heart-beat. The pressures therefore in 

 arterioles, capillaries, and veins are all increased by this local vaso-dilatation. 

 On the other hand, constriction of the arterioles of any given part will 

 diminish the velocity of the blood through this part and also the pressure in 

 its capillaries. 



The larger the area affected by the change in the peripheral resistance, 

 the more difficult it is to predict a priori what will be the result on the 

 velocity of the blood and on the circulation as a whole, or in the parts 

 specially affected. Thus section of one splanchnic nerve in the dog causes 

 an increased flow of urine from the kidney on the same side, the paralysis of 

 the vessels in this organ causing an increased flow of blood through it and an 

 increased pressure in its capillaries. Section of the corresponding nerve 

 of the rabbit may cause a diminution rather than an increase in the amount 

 of urine secreted, owing to the fact that the total area supplied by the 

 splanchnic nerve is much greater relatively in the rabbit than in the dog. 

 Thus section of this nerve may cause such a widespread dilatation that the 

 blood- pressure falls ; and although the vessels in the kidney are relaxed, 

 the arterial pressure is not sufficient to drive through these relaxed vessels as 

 much blood as was previously driven through the normally contracted 

 arterioles. 



METHODS OF MEASURING THE VELOCITY OF THE BLOOD 



The velocity in an artery is measured by placing some apparatus in the path of 

 the blood without intercepting its flow ; such an apparatus may be used to give the 

 quick variations in the velocity which occur in the course of each heart-beat, or the 

 average flow of blood through the cross-section of the artery in a given space of time. For 

 the latter purpose Ludwig's Stromuhr, or current clock (Fig. 387), has been most used. This 

 instrument consists of two bulbs of equal size, a and b, communicating with one another 

 above ; their lower ends are clamped in the disc c, which is pierced by two openings 

 serving to connect the lower orifices of the bulbs with the tubes t, t, cemented into the 

 lower disc db, 



An artery such as the carotid, being clamped at its central end and divided, a is 

 inserted into its central end, and b into its peripheral cut end. The tube a is filled with 



