368 HAND-BOOK OF PHYSIOLOGY. 



b. By relaxation of the renal artery without compensating relaxa- 

 tion elsewhere, by 



1. Division of the renal nerves (causing polyuria). 



2. " and afterward stimulating cord 

 below medulla (causing greater polyuria). 



3. Division of the splanchnic nerves; but polyuria is less than 



in 1 or 2, as these nerves are distributed to a wider area, 

 the dilatation of the renal artery is accompanied by dila- 

 tation of other vessels, and therefore with a somewhat 

 diminished general blood supply. 



4. Puncture of the floor of fourth ventricle or mechanical irri- 



tation of the superior cervical ganglion of the sympathetic, 

 possibly from dilatation of the renal arteries. 



B. Secretion of urine may be diminished 



a. By diminishing the general blood-pressure, by 



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



2. Dilatation of capillary areas other than the kidney. 



3. Division of spinal cord below medulla, which causes dilata- 



tion of general abdominal area, and urine generally ceases 

 being secreted. 



b. By increasing the blood-presstire, by stimulation of spinal cord 



below medulla, the constriction of the renal artery 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 by stimulating the spinal cord. 



Although it is convenient to call the processes which go on in the renal 

 glomeruli, filtration, there is reason to believe that they are not absolutely 

 mechanical, as the term might seem to imply, since, when the epithelium 

 of the Malpighian capsule has been, as it were, put out of order by liga- 

 ture of the renal artery, on removal of the ligature, the urine has been 

 found temporarily to contain albumen, indicating that a selective power 

 resides in the healthy epithelium, which allows a certain constituent part 

 of the blood to be filtered off and not others. 



(2.) Of True Secretion. That there is a second part in the process 

 of the excretion of urine, which is true secretion, is suggested by the 

 structure of the tubuli uriniferi, and the idea is supported by various 

 experiments. It will be remembered that the convoluted portions of the 

 tubules are lined with epithelium, which bears a close resemblance to the 

 secretory epithelium of other glands, whereas the Malpighian capsules 

 and portions of the loops of Henle are lined simply by endothelium. The 

 two functions are, then, suggested by the differences of epithelium, and 

 also by the fact that the blood supply is different, since the convoluted 

 tubes are surrounded by capillary vessels derived from the breaking up of 

 the efferent vessels of the Malpighian tufts. The theory first suggested 

 by Bowman (1842), and still generally accepted, of the function of the 



