NERVOUS CONTROL OF THE BLOOD-VESSELS 1115 



which will 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 in- 

 creased 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 



FIG. 464. 



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 arterioles A in the organ B dilate (Fig. 464), 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 c remain 

 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 con- 

 stricted. 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 



