KIDNEY AND SKIN AS EXCRETORY ORGANS. 841 



be considered as an indication of cellular activity, this result would 

 tend to show that some -diuretics may cause a genuine secretion, 

 while others influence the amount of urine through mechanical or 

 physical influences alone.* In the case of the inorganic salts it 

 may be said (Magnus) that there is for each salt a " secretion thresh- 

 old." An increase in concentration above this level leads to the 

 elimination of the excess of salt and an increased secretion of water. 



The Blood-flow through the Kidneys. It will be inferred 

 from the discussion above that, other conditions remaining the same, 

 the secretion of the kidney varies with the quantity of blood flowing 

 through it. It is, therefore, important to refer briefly to the nature 

 and especially the regulation of the blood-flow through this organ, 

 although the same subject is referred to in connection with the 

 general description of vasomotor regulation (see Circulation). It 

 has been shown by Landergrenf and Tigerstedt that the kidney 

 is a very vascular organ, at least when it is in strong functional activ- 

 ity such as may be produced by the action of diuretics. They esti- 

 mate that in a minute's time, under the action of diuretics, an amount 

 of blood flows through the kidney equal to the weight of the organ; 

 this is an amount from four to nineteen times as great as occurs in 

 the average supply of the other organs in the systemic circulation. 

 Taking both kidneys into account, their figures show that (in strong 

 diuresis) 5.6 per cent, of the total quantity of blood sent out of the 

 left heart in a minute may pass through the kidneys, although the 

 combined weight of these organs makes only 0.56 per cent, of that 

 of the body (see table p. 485). 



The nature of the supply of vasomotor nerves to the kidney and 

 the conditions which bring them into activity are fairly well known, 

 owing to the useful invention of the oncometer by Roy. This in- 

 strument is, in principle, a plethysmograph especially modified 

 for use upon the kidney of the living animal. It is a kidney-shaped 

 box of thin brass made in two parts, hinged at the back, and with 

 a clasp in front to hold them together. In the interior of the box 

 thin peritoneal membrane is so fastened to each half that a layer 

 of olive oil may be placed between it and the brass walls. There 

 is thus formed in each half a soft pad of oil upon which the kidney 

 rests. When the kidney, freed as far as possible from fat and sur- 

 rounding connective tissue, but with the blood-vessels and nerves 

 entering at the hilus entirely uninjured, is laid in one-half of the on- 

 cometer, and the other half is shut down upon it and tightly fas- 

 tened, the organ is surrounded by oil in a box which is liquid-tight 

 at every point except one, from which a tube is led off to some suitable 

 recorder such as a tambour. Under these conditions every increase 



* Barcroft and Straub, ibid., 1910, xli., 213. 



t "Skandinavisches Archiv f. Physiologie," 4, 241, 1892. 



