932 PHYSIOLOGY 



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 sub- 

 maxillary 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 pulsatile 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 wide- 

 spread 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 

 Fio. 398. Diagram of variations in the velocity which occur in the course of each 

 Lud wig's ' Stromuhr.' 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. 398), 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 <\ which is pierced by two 

 openings serving to connect the loucr orilices of the bulbs with the tubes /, /, cemented 

 into the lower disc ab. 



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



