484 EXCRETION 



SECTION II. THE SECRETION OF THE URINE. 



We have now to consider the mechanism by which the urine is 

 formed in the kidney 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 selec- 

 tive 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 under- 

 stand them, cannot explain the proportions in which they occur in 

 the secretions, or 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 blood- 

 vessels, 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 Mal- 

 pighian pyramids, and form at the boundary of the cortex and medulla 

 vascular arches, from which spring, on the one side, interlobular 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. 188). The interlobular arteries give off at intervals 

 afferent vessels. Each of these soon breaks up into a glomerulus 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 histologists 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 (with its continuation, the spiral tubule), 

 and ends in the cortex with the distal convoluted tubule, the connection 

 between the two being made by a long loop (Henle's) with a descending 

 and an ascending limb (Fig. 189). Between the ascending limb and 

 the distal convoluted tube is interposed the zigzag tubule. The tubule 

 throughout its length is bounded by a basement membrane lined by a 

 single layer of epithelium, which differs in its character in different 

 parts of the tubule 



