1000 PHYSIOLOGY 



during diastole. The heart output 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 peri- 

 pheral 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 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, has been most used. This instrument consists of two bulbs of 



