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EXCRETION. 



stance of the kidney is composed of two distinctly-marked portions called the cortical 



substance, and the medullary, or pyramidal substance. 



The cortical substance is reddish and gran- 

 ular, rather softer than the pyramidal sub- 

 stance, and is about one-sixth of an inch in 

 thickness. This occupies the exterior of the 

 kidney and sends little prolongations (col- 

 umns of Bertin) between the pyramids. The 

 surface of the kidney is marked by little po- 

 lygonal divisions, giving it a lobulated appear- 

 ance. This, however, is simply due to the 

 arrangement of the superficial blood-vessels. 

 The medullary substance is arranged in the 

 form of pyramids, sometimes called the pyra- 

 mids of Malpighi, from twelve to fifteen or 

 eighteen in number, their bases presenting 

 toward the cortical substance, and their apices 

 being received into the calices at the pelvis. 

 Ferrein subdivided the pyramids of Malpighi 

 into smaller pyramids (the pyramids of Fer- 

 rein), each formed by about one hundred 

 tubes radiating from the openings at the 

 summit of the pyramids toward their bases. 

 The tubes composing these pyramids were 

 supposed to pass into the cortical substance, 

 forming corresponding pyramids of convo- 

 Fm. us. Vertical section of the kidney. (Sappey.) luted tubes, thus dividing this portion of the 



1,1,2,2,3,3,3,4,4,4,4, pyramids of MaipigW; 5 5, kidney into lobules, more or less distinct. 



5, 5, 5, 5, apices ot the pyramids surrounded by 



the calices ; 6, 6, columns of Bertin ; 7, pelvis of The medullary substance is firm, of a dark- 



thekidney; 8, upper extremity of the ureter. ^ ^ ^^ ^ ^ ^.^ ^^^ and 



is marked by tolerably distinct striae, which take a nearly straight course from the bases 

 to the apices ol the pyramids. As these striaa indicate the direction of the little tubes 

 that, constitute tne 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, they 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 con- 

 stituents 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, pre- 

 sents from ten to twenty-five little openings, measuring from -g-i to -gV of an inch in 

 diameter. The tubes leading from the pelvis immediately divide at very acute angles, 

 generally dichotomatously, until a bundle of tubes arises, as it were, from each opening. 

 These bundles constitute the pyramids of Ferrein. In their course, the tubes are slightly 

 wavy and are nearly parallel with each other. These are called the straight tubes of the 

 kidney, or the tubes of Bellini. They extend from the apices of the pyramids to their 

 bases and pass then into the cortical substance. The pyramids contain, in addition to 

 the straight tubes, a delicate fibrous matrix and numerous blood-vessels ; which latter, 

 for the most part, pass beyond the pyramids, to be finally distributed in the cortical 

 substance. Kecent researches have shown that some of the convoluted tubes dip down 



