1034 



PHYSIOLOGY 



nearly all the vessels of the body receive vaso-constrictor fibres, and that 

 many receive also vaso-dilator fibres. In order to determine the course 

 and distribution of the vascular nerves, it is necessary to have means at our 

 disposal for investigating the condition of the blood flow through different 



parts and organs of the body. 

 Let us see what effects will 

 ensue on the local circulation 

 by constriction or dilatation 

 of the arterioles with which 

 it is supplied. If the arte- 

 rioles A in the organ B 

 dilate (Fig. 468), the first 

 effect is a diminution of the 

 resistance to the flow of 

 blood into the capillaries 

 beyond. Supposing that the 

 arterial pressure in the trunk 



FIQ 46g c remains constant, a local 



diminution of resistance in 



A will at once determine an increased flow of blood through the arterioles, 

 and the fall of pressure from A to the capillaries will be less than when 

 the arteriole was constricted. If the organ is distensible and elastic, the 

 increased pressure in the arterioles and capillaries will cause dilatation of 

 these vessels, and a consequent dilatation of the whole organ. The same 

 effect on intracapillary pressure, and therefore on the volume of the part, 

 may be caused by obstruction to the flow of blood from the veins. 

 Provided that there is no obstruction -to the flow of blood through the vein, and 

 that the general blood pressure in c remains constant, dilatation of an 

 organ may be taken as an expression of vaso-dilatation in the arteries 

 with which it is supplied. The diminution of the resistance in A may also 

 increase the velocity of the flow through the part, since the amount of blood 

 flowing in a given period of time through any vessel varies directly as the 

 difference of pressure, and inversely as the resistance in the vessel. 



We can therefore use the following criteria for the occurrence of a vaso- 

 dilatation in the arterial supply to any part or organ : 



(1) If the surface of the part is translucent, the increased filling of the 

 blood vessels will cause redness or blushing. 



(2) The increased size of the vessels will cause an increase in the volume 

 of the organ concerned. 



(3) An increased velocity of blood flow will, if the part be normally 

 below the temperature obtaining in the central organs of the body, raise its 

 temperature, and vaso-dilatation can thus be detected by the application of 

 the hand or of a thermometer. 



(4) Any of the methods mentioned in a previous chapter may be used 

 to determine the velocity in the arteries going to the part, and an increased 

 velocity may be interpreted as due to vaso-dilatation. 



(5) The increased flow through the part may be detected by cutting 



