NERVOUS CONTROL OF THE BLOOD-VESSELS 



991 



increase the state of constriction of the vessels ; another set inhibitor or 

 dilator which will diminish the tone of the arteriole muscle and cause 

 vascular dilatation. Our knowledge of the existence of this second class 

 of nerve fibres to the vessels we owe also to Claude Bernard, who observed 

 that stimulation of the chorda tympani nerve not only evoked secretion 

 from the submaxillary gland but also increased the blood-flow through its 

 vessels five- or sixfold. Subsequent researches have revealed the fact that 

 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 con- 

 striction or dilatation of the arte- 

 rioles with which it is supplied. 

 If the arterioles A in the organ B 

 dilate (Fig. 459), the first effect is 

 a diminution of the resistance to 

 the flow of blood into the capil- 

 laries beyond. Supposing that 

 the arterial pressure in the trunk 

 c remain constant, a local diminu- 

 tion of resistance in A will at once 

 determine an increased flow of 

 blood through the arterioles, and FIG. 459. 



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 there- 

 fore on the volume of the part, may be caused by obstruction to the flow 

 of blood from the veins. Provided that there t's 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. 



