362 EXCRETION BY THE SKIN AND KIDNEYS. 



from these bodies to their connections with the straight tubes of the pyram- 

 idal substance. 



Malpiyhian Bodies. These are ovoid or rounded, terminal dilatations of 

 the convoluted tubes, and are ^ T to T ^ F of an inch (100 to 250 //,), in diam- 

 eter. They are composed of a membrane, which is continuous with the ex- 

 ternal membrane of the convoluted tubes, and is of the same homogeneous 

 character, but somewhat thicker. This sac, called the capsule of M tiller or 

 of Bowman, encloses a mass of convoluted blood-vessels and is lined with a 

 layer of nucleated epithelial cells. In addition to the cells lining the cap- 

 sule, there are other cells which are applied to the blood-vessels. 



The cells attached to the capsule of Muller are smaller and more trans- 

 parent than those lining the convoluted tubes. They are ovoid, nucleated 

 and finely granular. The cells covering the vessels, however, are larger and 

 more opaque, and they resemble the epithelium lining the tubes. They 

 measure y^^ to 10 * 00 of an inch (16 to 25 /A), in diameter, by about ^-gVrr f 

 an inch (10 /A) in thickness. 



Tubes of the Cortical Substance. Passing from the Malpighian bodies, 

 the tubes present first a short, constricted portion, called the neck of the 

 capsule, which soon dilates to the diameter of about -^-g- of an inch (50 p,), 

 when their course becomes quite intricate and convoluted. These are what 

 are known as the convoluted tubes of the kidney. The membrane of these 

 tubes is transparent and homogeneous, but quite firm and resisting. It is 

 lined throughout with a single layer of epithelial cells, 14 1 00 to 10 ^ of an 

 inch (16 to 25 /A) in diameter, somewhat larger, consequently, than the cells 

 lining the straight tubes. The cells lining the convoluted tubes present two 

 tolerably distinct portions. The inner portion or zone, which is next the 

 lumen of the tube, is finely granular, with sometimes a few small oil-glob- 

 ules. The outer zone presents little fibrils or rods, which are perpendicular 

 to the tubular membrane. These are called " rodded " cells, and a similar 

 appearance is presented by some of the cells of the pancreas and of the sali- 

 vary glands. The nucleus is usually situated between the granular and thf 

 rodded zones. 



The researches of Heidenhain and others have shown that the greatest 

 part of the solid excrementitious constituents of the urine, such as urea 

 and the urates, is separated from the blood by the cells of the convoluted 

 tubes of the cortical substance and perhaps by the dilated portions of the 

 tubes of Henle, while the water and a certain portion of the inorganic salts 

 of the urine transude through the blood-vessels in the Malpighian bodies. 

 This view was first advanced by Bowman, in 1842. 



Narrow Tubes of Henle. The convoluted tubes above described, after a 

 tortuous course in the cortical substance, become continuous, near the pyra- 

 mids, with the tubes of much smaller diameter, which form loops extending 

 to a greater or less depth into the pyramids. The loops formed by these 

 canals (the narrow tubes of Henle), are nearly parallel with the tubes of Bel- 

 lini and are much greater in number near the bases of the pyramids than 

 toward the apices. The diameter of these tubes is very variable, and they 



