398 RELATIONS TO BLOOD-PRESSURE. [BOOK n. 



which is a kind of filtration, will be directly affected by the amount 

 of arterial pressure in the renal arteries, while the effect of varia- 

 tions of arterial pressure on that part of the process which is a real 

 active secretion will be an indirect one only. Hence, the discharge 

 of urine by the kidneys must be to a much greater extent than is 

 the case with the secretion of saliva or of gastric juice a mere 

 matter of pressure ; and it will consequently be of advantage 

 to study the relations of urinary secretion to blood-pressure before 

 we enter upon the discussion of the active secretion itself. 



The Relation of the Secretion of Urine to Arterial Pressure. 



Eecent observations have shewn experimentally that the kidney 

 is supplied with a vaso-motor mechanism as well developed perhaps 

 as that of any other part of the body. 



By means of a modification of the plethysmograph, we can readily 

 observe the variations which take place in the volume of the 

 kidney and the same method can be applied also to other internal 

 organs. 



FIG. 63. BENAL ONCOMETER. Seen in section (semi-diagrammatic). K. kidney, 

 V. vessels and nerves imbedded in fat, &c. entering hilus of organ, O.C. and I.C. 

 outer and inner metal capsules screwed together by the screw S, and holding between 



