64 



ZOOLOGY. 



plainly distinguishable by tlie naked eye : these are called 

 the medulla and the cortex. The pyramid is simply a 

 projecting portion of the medulla. 



Considering now the minute structure, the whole kidney 

 is essentially a mass of secretory tubes the uriniferous 

 tubules united together by connective tissue containing 

 blood-vessels. Each tubule ends in one direction by opening 

 into the pelvis, the openings being on the surface of the 

 pyramid and in the other direction it terminates blindly 

 in a structure called a malpighian body. The difference 

 of appearance between medulla and cortex is due to the 

 fact that while in the former the tubules have a straight 

 course in a radial direction, as soon as they enter the cortex 

 they curve and bend. It is also in the cortex only that 

 malpighian bodies occur. 



Blood is brought to the kidney by the renal artery : the 

 ultimate branches of this end for the most 

 part in a little bunch of looped capillaries 

 called a glomerulus (fig. 26). From this 

 the blood passes to a network of capillaries 

 around the tubules and thence to the 

 factors of the renal vein. A malpighian 

 body consists of a double-walled cap- 

 sule formed by the end of the tubule 

 (Bowman's capsule) and containing a 

 glomerulus. 



Water is drained off the blood as it 

 passes through the glomeruli ; urea is re- 

 moved as it circulates around the tubules. 

 The action of an excretory epithelium like 

 this of the kidney differs from that of an 

 ordinary secretory gland in that its function is to select 

 from the blood substances already present in it, and not 

 to manufacture new materials. The selection, however, 

 involves work it is not a mere passive process of filtering, 

 either in the*case"of the capsules or of the tubules. For 

 example, if the epithelial cells of the capsules are unable 

 to perform their work properly, the water drained off 

 from the blood will retain the proteids dissolved in it 

 and the disease called " albuminuria " is produced. 



Fig. 26. BOWMAN'S 

 CAPSULE AND 

 GLOMEBULUS. 



