EXCRETION 397 



from the materials brought to it by the blood. And here the 

 same questions arise as have already been discussed in the 

 case of the salivary and other digestive glands : (i) Are the 

 urinary constituents, or any of them, present as such in the 

 blood? (2) If they do exist in the blood, can they be shown 

 to be separated from it by processes mainly physical or 

 mainly vital in other words, by ordinary filtration, diffusion 

 and osmosis, or by the selective action of living cells ? In 

 the case of the digestive juices it has been seen that some 

 constituents are already present in the blood, but that 

 physical laws alone, so far as we at present understand 

 them, cannot explain the proportions in which they occur 

 in the secretions, nor the conditions under which they are 

 separated; while other constituents and these the more 

 specific and important are manufactured in the gland- 

 cells. 



In the kidneys the conditions seem at first sight favourable 

 to physical separation, as opposed to physiological secretion. 

 Urine has been described as essentially a solution of urea 

 and salts, and both are ready formed in the blood. The 

 arrangement of the bloodvessels, too, suggests an apparatus 

 for filtering under pressure. 



Bloodvessels and Secreting Tubules of Kidney. The renal artery 

 splits up at the hilus into several branches, which pass in between 

 the Malpighian pyramids, and form at the boundary of the cortex 

 and medulla vascular arches, from which spring, on the one side, inter- 

 lobular arteries running up into the cortex between the pyramids of 

 Ferrein, and, on the other, vasa recta running down into the boundary 

 layer of the medulla (Fig. 124). The interlobular arteries give off at 

 intervals afferent vessels ; each of these soon breaks up into a glomer- 

 ulus or tuft of vascular loops, which gather themselves up again into 

 a single efferent vessel of somewhat smaller calibre than the afferent. 

 The glomerulus is fitted into a cup or capsule (of Bowman), which is 

 closely reflected over it, except where the afferent and efferent vessels 

 pass through, and forms the beginning of a urinary tubule. If we 

 suppose the tuft pushed into the blind end of the tubule so as to 

 indent it, it will be easily understood that the single layer of flattened 

 epithelium reflected on the glomerulus is continuous with that lining 

 the capsule, which in its turn is continuous with the epithelial layer 

 of the rest of the urinary tubule. This has been divided by histo- 

 logists into a number of parts which it is unnecessary to enumerate 

 here, further than to say that the urinary tubule proper begins in the 

 cortex in Bowman's capsule and the proximal convoluted tubule 



