THE PLETHYSMOGRAM OF THE KIDNEY 265 



the microscope, then adjust the high power to a field which shows one or 

 more small arteries. Make a drawing to record the diameter of these 

 arteries, using pigment cells for land- marks. Now quickly stimulate the 

 exposed sciatic nerve while keeping the selected artery under constant 

 observation. After a short stimulation the diameter of the vessels will be 

 seen to decrease considerably, sometimes to the point of complete occlusion. 

 When the stimulation ceases, the vessel will remain contracted for a few 

 seconds, then will slowly regain its usual caliber, figure 201. This is an 

 exceptionally good method for direct observation of the vaso-motor changes 

 in the smaller vessels. 



23. The Pie thy smo gram of the Kidney. Anesthetize a dog or cat, see Ex- 

 periments 1 2 and 1 9 above, and take blood-pressure tracings on the continuous- 

 paper kymograph. Now open the abdominal wall by an incision along the 

 median line, expose the left kidney and carefully dissect off its capsule, taking 

 care not to injure its artery and vein. Enclose the kidney in the renal onkom- 

 eter, fill the onkometer with oil, and connect it with a recording apparatus. 

 Brodie's bellows recorder is probably the best recording apparatus for this 

 purpose. Adjust the recording apparatus in the vertical line with the manom- 

 eter and signal pens. 



Stimulation of the nerves which affect the general blood pressure through 

 the medium of the heart will be found to produce changes in the volume of 

 the kidney in the same direction as the blood-pressure change. On the 

 other hand, stimuli which give variations of the blood pressure without 

 direct change in the heart itself affect the volume of the kidney independent 

 of the blood pressure: 



1. Dissect out and stimulate the splanchnic nerves just where they pass 

 through the pillars of the diaphragm. Stimulation of these nerves will 

 cause vaso-constriction in the kidney, which takes place without sharply 

 affecting the blood pressure. 



2. Stimulate the depressor nerve or the central end of the divided vagus. 

 In this case the volume of the kidney will increase though the general blood 

 pressure decreases, showing that the fall of blood pressure is due to peripheral 

 vascular dilatation. 



3. Stimulate the peripheral end of the divided vagus so as to slow or 

 even completely to stop the heart. The sharp fall in blood pressure is now 

 accompanied by decrease in the volume of the kidney, showing that the 

 kidney change is merely passively following that of the blood pressure. 



