324 



EXCRETION 



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

 organs are classed as 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 the urine from the 

 blood. It is difficult to determine precisely where the secreting tubes 



Fig. 71. Longitudinal section of the pyramidal 

 substance of the kidney ofthefcetus (Sappey). 



i, trunk of a large uriniferous tube; 2, 2, pri- 

 mary branches of this tube ; 3, 3, 3, secondary 

 branches ; 4, 4, 5, 5, 6, 6, j, j, 7, 7, branches be- 

 coming smaller and smaller; 8, 8, 8, 8, loops of 

 the tubes of Henle. 



Fig. 72. Longitudinal section of the cortical sub- 

 stance of the same kidney (Sappey) . 



I, i, limit of the cortical substance and base 

 of the pyramids ; 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 branches (7, 7, 7) ; 9, 9, fibrous covering 

 of the kidney. 



merge into the excretory ducts ; but it is a common idea, which prob- 

 ably is 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 openings, ^-$ to g 1 ^ of an inch 

 (85 to 425 IJL) in diameter. The tubes leading from the pelvis immedi- 



