48f HANDBOOK OF PHYSIOLOGY. 



increased by clamping the renal vein, in which circumstance the secre- 

 tion of urine is suspended. 



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. 



Secretion. That there is a second part in the process of the excre- 

 tion 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 secre- 

 tory epithelium of other glands, whereas the Malpighian capsules and 

 portions of the loops of Henle are lined simply by flattened epithelium. 

 The two functions of the different parts of an uriniferous tube are, then, 

 suggested by the differences of epithelium, and also by the fact that the 

 blood supply to the different parts is different, since, as we have seen, 



Fig. 309. Curve taken by renal oncometer compressed with that of ordinary blood-pressure, 

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



the convoluted tubes are surrounded by capillary vessels derived from 

 the breaking up of the efferent vessels of the Malpighian tufts. As to 

 the functions of the different parts of the uriniferous tubes in the 

 secretion of urine, two chief theories have been brought forward. The 

 first, suggested by Bowman (1842), and still generally accepted, is that 

 the cells of the convoluted tubes, by a process of true secretion, separate 

 from the blood substances such as urea, whereas from the glomeruli 

 are separated the water and the inorganic salts. The second, suggested 

 by Ludwig (1844), is that in the glomeruli are filtered off from the 

 blood all the constituents of the urine in a very diluted condition. 

 When this passes along the tortuous uriniferous tube, part of the water 

 is re-absorbed into the vessels surrounding them, leaving the urine in 

 a more concentrated condition retaining all its proper constituents. 

 This osmosis is promoted by the high specific gravity of the blood in 



