360 



EXCRETION BY THE SKIN AND KIDNEYS. 



straight course from the bases to the apices of the pyramids. As these strise 

 indicate the direction of the little tubes that constitute the greatest part of 

 the medullary substance, this is sometimes called the tubular portion of the 

 kidney. 



From the arrangement of the secreting portion of the kidneys, these 

 organs are classed among the tubular glands, presenting a system of tubes, 

 or canals, some of which are supposed simply to carry off the urine, while 

 others separate the excrementitious constituents of this fluid from the blood. 

 It is difficult to determine precisely where the secreting tubes merge into the 

 excretory ducts, but it is the common idea, which is probably correct, that 

 the cortical substance is the active portion, while the tubes of the pyramidal 

 portion simply carry off the excretion. 



Pyramidal Substance. Each papilla, as it projects into the pelvis of the 

 kidney, presents ten to twenty-five little openings, -^ to -^ of an inch (85 

 to 425 p) in diameter. The tubes leading from the pelvis immediately 



FIG. 111. Longitudinal section of the py- 

 ramidal substance of the kidney of the 

 foetus (Sappey). 



1, trunk of a large uriniferous tube ; 2, 2, 

 primary branches of this tube ; 3, 3, 3, 

 secondary branches ; 4, 4, 5, 5, 6, 6, 7, 7, 

 7, 7, branches becoming smaller and 

 smaller ; 8, 8, 8, 8, loops of the tubes of 

 Henle. 



FIG. 115. Longitudinal section of the cortical sub 

 stance of the same kidney (Sappey). 



1,1, limit of the cortical substance and base of the pyr- 

 amids ; 2, 2, 2, tubes passing toward the surface of 

 the kidney ; 3, 3. 3, 8, 8, 8, convoluted tubes ; 4, 4, 4, 

 4, 5, Malpighian bodies ; 6, 6, artery, with its branch- 

 es (7, 7, 7) ; 9, 9, fibrous covering of the kidney. 



