642 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the blood which has passed through this series of capillaries, again un- 

 dergoes subdivision into a second net-work of capillaries distributed 

 around the urinary tubules. In the glomerulus is represented the filtra- 

 tion apparatus, in which, through the influence of blood pressure, the 

 substances held in solution in the blood-serum may be removed from the 

 blood and forced into the interior of the commencement of the urinary 

 tubules. The conditions are, therefore, evidently different here, where 

 transudations directly enter into the excretory ducts, from what holds in 

 the case of other glands, where the transudations simply pass into lymph 

 and require some other force for their transference to the secretion. 



Cortex. 



Boundary or 

 marginal zone. 



Papillary zone. 



FIG. 266. LONGITUDINAL SECTION ov A MAI/PIGHIAN PYRAMID. (Landois.) 



PF, pyramids of Ferrein ; RA, branch of renal artery ; RV, lumen of a renal vein receiving an inter- 

 lobular vein ; VR, vasa recta; PA, apex of a renal papilla; b b embrace the bases of the renal lobules. 



That this separation of the constituents of the blood through the glom- 

 eruli of the kidney is actually dependent upon the blood pressure is 

 shown by the fact that if the blood pressure be reduced below fifty milli- 

 meters of mercury secretion ceases, while if it be increased the secretion 

 is correspondingly augmented. The contrast between this fact and the 

 secretion of saliva is, therefore, very striking. 



If, however, pressure is increased by venous obstruction, then, in- 

 stead of an increased secretion, the reverse takes place. That this does 

 not contradict the filtration hypothesis is explainable by the fact that ob- 

 struction of the veins increases the pressure in the capillaries, and these 



