424 VETERINARY PHYSIOLOGY 



and becoming lined by a cubical granular epithelium, it forms 

 the ascending limb, and reaching the cortex expands into the 

 distal convoluted tubule, which exactly resembles the proximal 

 (D.C.T.}. It opens into a collecting tubule (C.T.) lined by a low 

 transparent epithelium, which conducts the urine to the pelvis 

 of the kidney. 



The renal artery breaks up and gives off a series of straight 

 branches the interlobular arteries (IL.A.} which, as they 

 run towards the surface, give off short side branches which 

 terminate in the glomeruli. The efferent vein passing from 

 these breaks up again into a series of capillaries between the 

 convoluted tubules, and these pour their blood into the inter - 

 lobular veins (IL.V.). This arrangement helps to maintain a 

 high pressure in the capillary loops of the glomerular tuft. 



Physiology of Secretion 



The marked difference in the structure and vascular supply 

 of the Malpighian bodies and of the different parts of the 

 renal tubules make it probable that they perform different 

 functions. 



Malpighian Bodies. (a) It has been shown by injecting 

 into the blood vessels acid fuchsin, which is colourless in 

 alkaline solution and red in acid solution, that the urine 

 formed in these bodies is alkaline in reaction and that it 

 becomes acid as it passes down the convoluted tubules. 



(b) It is also known that these bodies are thrown out of action 

 by decreasing the flow of blood through the kidneys. The rate 

 of flow of blood depends upon the blood pressure in the renal 

 arteries and the dilatation of the renal arterioles. It may be 

 measured by measuring the amount of blood passing out by the 

 renal vein or by enclosing the kidney in an oncometer a closed 

 vessel connected with a recording tambour so that changes 

 in the volume of the kidney are recorded (see p. 249), or by a 

 combination of these methods. 



Section of the renal nerves derived from the llth, 12th, and 

 13th dorsal nerves causes a dilatation of the renal arterioles, an 

 expansion of the kidney, and an increased flow of urine. 

 Stimulation of these nerves has the opposite effect. A fall 

 in the general arterial pressure to about 50 mm. Hg in the 



