EXCRETION 407 



suggests that the water secreted during the elimination of 

 urea after ligature of the renal arteries may really come 

 through the Malpighian tufts. At the same time, this 

 objection does not touch the conclusion of Nussbaum, that 

 the glomeruli are alone concerned in the separation of the 

 other bodies mentioned. For his operation, whether it com- 

 pletely cut off the circulation in the tufts or not, interfered 

 with it so much as to stop the excretion of these substances, 

 while leaving the epithelium of the tubules as able to con- 

 tinue that function, if it possessed it, as it was before. 

 Adami himself has shown that haemoglobin when free in the 

 blood-plasma is excreted by the glomeruli, even when the 

 renal artery has been ligatured, and that menisci of this sub- 

 stance may be coagulated within the lumen of the Bowman's 

 capsules by plunging the kidney into boiling water. In the 

 dog, too, haemoglobin is excreted by the glomeruli, and may 

 be washed out of the capsule by the increased quantity of 

 water secreted when sodium nitrate is administered. This 

 shows that a diuretic may act upon the glomerular epithelium, 

 which is thus brought into line with the ' rodded ' epithelium 

 of the tubules. 



What, then, is the significance of the peculiar arrangement of the 

 glomerular bloodvessels, if the epithelium of the capsules has secretive 

 powers like that of ordinary glands ? It is difficult to believe that 

 these unique vascular tufts have not a near and important relation to 

 the renal function ; but it is by no means clear what that relation is. 

 The secretion of water, and even its rapid secretion, is not at all 

 bound up with any set arrangement of bloodvessels. Gland-cells all 

 over the body secrete water under the most varied conditions of 

 blood-pressure, although a comparatively high pressure is upon the 

 whole favourable to a copious outflow. 



But the kidney has, as we now know, other functions than mere 

 excretion (p. 483). And it may be that the simplest part of the latter 

 process, the elimination of water and salts, is largely thrown upon 

 the Malpighian corpuscles, as a physiologically cheaper machine 

 than the epithelium of the tubules, which is left free for more complex 

 labours. These may include not only the separation of nitrogenous 

 metabolites, but perhaps the building up of urea, or of less completely 

 metabolized substances which precede it, into higher combinations, 

 and the consequent regulation of the quantity of urea finally excreted, 

 and the ultimate proteid waste which this expresses. The epithelium 

 of the glomerulus, being a less highly organized and less delicately 

 selective mechanism than that of the convoluted tubules, may more 



