STRUCTURE AND FUNCTION OF THE KIDNEYS. 37 T 



B. Secretion of urine may be diminished 



a. By diminishing the general Hood-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 Wood-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. 



FIG. 262. Curve taken by renal oncometer compared with that of ordinary blood-pressure, 

 a, Kidney curve; 6, blood-pressure curve. (Roy.) 



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 ligature 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 certain 

 constituent parts of the blood to be filtered off, and not others. 



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

 cess 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 an 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 endothe- 

 lium. The two functions are, then, suggested by the differences of epi- 

 thelium, 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 two parts of the tubules, is that the cells of the 

 convoluted tubes, by a process of true secretion, separate from the blood 



