THE NERVOUS CONTROL OF THE BLOOD VESSELS 1035 



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

 from it in a given time. 



Of these methods th<3 two most used are those based on determination 

 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 experiment. 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. 



to oncometer 



FIG. 469. Diagram of oncometer. 



FIG. 470. Diagram of oncograph. 



For this purpose "we may use a kidney plethysmograph or oncometer. The structure 

 of Roy's oncometer is shown in Fig. 469. The oncometer is a metal capsule, the two 

 halves of which are hinged together and come in contact at the whole of their circum- 

 ference except at h, where a small depression is left in each half for the passage of the 

 kidney vessels and ureter. A piece of peritoneal membrane is attached to the rim of 

 each half of the oncometer, the space between this and the brass capsule being filled 

 with warm oil. The kidney rests in the oncometer on this bed of warm oil, from which 

 it is separated by a membrane. A tube leads from the cavity between the brass capsule 

 and membrane to a registering apparatus, or oncograph (Fig. 470), which is a piston 

 recorder containing oil. Any swelling of the kidney will drive oil out of the oncometer 

 into the cylinder of the oncograph and so raise the piston, the excursions of which are 

 recorded by a lever writing on a blackened surface. 



Schafer's plethysmograph (Fig. 471), which can be adapted to almost any organ of 

 the body, is made of vulcanite 1 previously moulded to the size of the organ whose 

 volume is the object of investigation. In one side of the box a depression is left sufficient 

 to accommodate easily the vessels, nerves, or ureter going to the organ. The oncometer 

 is covered with a glass lid which is made air-tight by means of vaseline, the space 

 between the lid and the vessels being also packed with cotton-wool and vaseline. A 

 glass tube is fixed into one corner of the plethysmograph and leads to a piston recorder 

 or tambour. Every variation in the volume of the organ causes a movement of air into 

 or out of the oncometer and thus gives rise to a corresponding movement of the recording 

 lever. 



1 A very good material for this purpose is ' Stent's composition,' used by dentists 

 for taking a mould of the jaw in fitting artificial teeth. 



