1116 



PHYSIOLOGY 



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 



Fie. 465. Diagram of oncorneter. 



FIG. 466. Diagram of oncograph. 



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 

 the main efferent vein and measuring the total volume of blood which 

 flows from it in a given time. 



Of these methods the two most used are those based on determina- 

 tion either of the volume of the part, or of the venous outflow from the 

 part. A fallacy may, however, arise, unless means be taken to ensure 

 that the general arterial pressure remain constant during the experi- 

 ment. A rise of general blood pressure will cause an expansion of the 

 vessels and of the part supplied, and also increased velocity of blood- 

 flow through the part. In all cases therefore where it is desired to 

 investigate the conditions of the local circulation, it is necessary to 

 combine a determination of the general blood pressure with some means 

 of estimating changes in the local conditions. We may take as an 

 instance an experiment on the blood-supply to the kidney. 



