SECRETION OF THE URINE. 167 



break up into another capillary network enveloping the uri- 

 niferous tubules in the pyramids and interpyramidal regions 

 of the cortex. These capillaries then unite again and empty 

 into the interlobular veins, which occupy a situation corre- 

 sponding to that of the interlobular arteries, and empty into 

 the venous arches. The capillaries beneath the capsule unite 

 in venous branches, the stellate veins, which meet in a star- 

 shaped manner and empty into the interlobular veins. 



The arteriae rectas are small straight arterioles which arise 

 in large numbers from the concave sides of the arterial arches 

 and pass into the Malpighian pyramids, converging to the 

 apices of the latter. They divide into elongated capillary 

 meshes embracing the uriniferous tubules of the medulla. 

 A few arteriolae recta? are also derived from afferent vessels 

 of some of the glomeruli, which pass into the medulla, instead 

 of breaking up into capillaries in the cortex. The blood from 

 the medullary capillaries is collected by the vence rectce, which 

 are arranged similarly to the arteriae recta? and empty into 

 the venous arches. 



The structures composing the different parts of the kidney 

 may be recapitulated as follows : the interpyramidal portions 

 of the cortex contain the Malpighian bodies, the proximal 

 and distal convoluted tubules, the irregular tubules, the curved 

 tubules, and the interlobular vessels. 



The pyramids of Ferrein contain the spiral tubules, part of 

 the ascending limbs of Henle's loops, and portions of the 

 straight collecting tubes. 



The medulla contains the descending and part of the 

 ascending limbs of Henle's loops, part of the straight collect- 

 ing tubes, the tubes of Bellini, and the vasa recta. 



The secretion of the urine is accomplished somewhat as fol- 

 lows : In the Malpighian bodies fluid transudes or filters from 

 the blood through the thin capillary- walls and squamous cells 

 into the cavity of the capsule of Bowman. The fluid thus 

 separated is supposed to be mainly water, with very little 

 solids. The glandular epithelium of other portions of the 

 tubules, the convoluted tubules especially, then remove from 

 the blood in the surrounding capillaries the solid constituents 

 of the urine by a true secretory process ; these solids are dis- 

 solved by the fluid passing down the tubules. 



