320 A MANUAL OF VETERINARY PHYSIOLOGY 



favour in a remarkable manner the passage of fluid by nitration, 

 yet none is filtered off. Many experiments have been made to 

 prove the secretory activity of the cells of the convoluted tubes ; 

 the one by Heidenhain is generally regarded as conclusive. 



If sulphindigotate of soda be injected into the blood of the 

 dog, within a short time the urine acquires an intensely blue 

 colour, though the blood may be only slightly affected. If 

 the kidney be removed and examined, all parts excepting the 

 Malpighian bodies are found stained blue. In order to determine 

 what portion of the tubule excretes the dye, it is necessary to 

 stop the secretion in the glomeruli, otherwise the colouring 

 matter gets carried through the whole length of the tubule. 

 In order to stop glomerular secretion, the blood-pressure is 

 lowered by dividing the spinal cord in the neck, and the blue 

 colouring matter is then injected, and the kidney subsequently 

 examined. The blue is now found in the cortex only, and within 

 the striated or rodded epithelial cells of the first and second 

 convoluted tubes, in which the pigment may be seen lying in 

 granules. From this experiment it is clear that the cortical 

 tubules elected to turn out the pigment from the blood, while the 

 medullary tubules were unable to effect this. It is therefore 

 fair to assume that a specific secretory activity of the cells of 

 the convoluted tubes is shown for indigo, and it is assumed that 

 a similar function may be exercised towards other bodies — for 

 instance, urea and the other constituents of the urine. 



Stating these points briefly in connection with secretion, they 

 amount to this, that in the glomeruli the water of the urine, 

 and perhaps the salts, are passed out chiefly as the result of 

 varying glomerular blood-pressure, while in the tubules the 

 organic matter is excreted as the result of a distinctly secretory 

 activity of their cells. These substances are carried along by 

 the fluid which trickles down the tubules into the pelvis of 

 the kidney, and so becomes urine. The secretion of protein 

 in the tuft, and its reabsorption in the tubule, was at one time 

 believed to be true, but inasmuch as no protein is found in the 

 normal urine of any animal, it is safe to assume that in an un- 

 damaged state the epithelial cells of the glomerulus allow none 

 to pass. Under pathological conditions the glomeruli admit 

 of the exit of both albumin and sugar. There are no secretory 

 nerves to the kidney ; the influence of the nervous system is 

 confined to its action on the bloodvessels, and so regulating the 

 flow of blood through the kidney. 



The latest view of urinary secretion is that put forward by 

 Brodie.* He regards the glomerular activity as secretory, and 



* 'Glomerular Activity,' Proceedings of the Royal Society, June 15, 

 191 1. Dr. T. G. Brodie, F.R.S. 



