80 SECRETION. 



3. Division of the splanchnic nerves ; but the polyuria pro- 



duced is less than in 1 or 2, as these 'nerves are distrib- 

 uted to a wider area, and the dilatation of the renal 

 artery is accompanied by dilatation of other vessels, and 

 therefore with a somewhat diminished general blood- 

 supply. 



4. Puncture of the floor of fourth ventricle or mechanical 



irritation of the superior cervical ganglion of the sympa- 

 thetic, possibly from the production of dilatation of the 

 renal arteries. 

 B. Secretion of urine may be diminished 



a. By diminishing the general blood-pressure by 



1. Diminution of the force or frequency of the heart-beats. 



2. Dilatation of capillary areas other than that of the kidney. 



3. Division of spinal cord below the medulla, which causes 



dilatation of general abdominal area, and urine generally 

 ceases being secreted. 



b. By increasing the blood-pressure by stimulation of the spinal 



cord below the medulla, the constriction of the renal 

 artery which follows not being compensated for by the 

 increase of general blood-pressure. 



c. By constriction of the renal artery by stimulating the renal or 



splanchnic nerves or the spinal cord. 



What two methods of elimination, then, do we find in the 

 kidneys ? 



(1) The process of filtration, depending upon blood-pressure and 

 acting almost solely upon the aqueous elements. 



(2) Secretion proper, by which cell-activity performs the func- 

 tion of excreting the solid matters of the urine. 



What is the source of the urea ? 



It is not fully known whether the urea is taken from the blood 

 as such by the kidneys, or if it is made up by the cells of the kid- 

 neys from elements taken from the blood. It is probable that the 

 former is for the most part the source of urea, and a less amount 

 is really composed in the cells of the kidney-tubules. At any rate, 

 the source of the urea is the nitrogenous matters of the body : 1, 

 those taken in as food, urea being greatly increased by a nitrog- 

 enous meal ; 2, by the metabolism of the tissues. The measure 

 of the amount of work done is, however, not found in the quantity 



