THE SECRETION OF URINE. 469 



THE SECRETION OF URINE. 



STRUCTURE OF THE KIDNEY. 



The kidneys are compound tubular glands (Fig. 142). 



All of the urinary tubules arise within the cortex of the kidney from Bow- 

 man's capsules, which are globular in shape, measure from 200 to 300 fi in diameter, 

 are constituted of endothelioid cells (k), and whose inner surface is lined 

 with a single layer of epithelial cells (Fig. 142, II). In the interior of the 

 capsule lies the convolution of vessels known as the glomerulus or Malpighian body. 

 Each capsule passes by means of a narrowed portion into the convoluted urinary 

 tubule, which has a diameter of 45 p (I, x). This possesses a membrana propri'a 

 constituted of extremely fine fibers and passes through the cortical structure 

 in a devious course. It is lined by characteristic epithelium, the cells contain- 

 ing a turbid protoplasm that swells readily and is occasionally filled with 

 fat-globules. That portion of the protoplasm which. is directed toward the rela- 

 tively narrow lumen of the tubule contains a distinct globular nucleus, while 

 the portion adjacent to the membrana propria, and different also chemically, 

 presents a fibrillated appearance, as if constituted of rods. Where the rods 

 are in direct contact with the membrane they diverge like the bristles of a brush 

 pressed against a surface. The free extremities of the rods of adjacent cells 

 touch one another, so that the attached surface of the cells acquires an irregular 

 radiating appearance. When secretion takes place the free surface has a brush- 

 like margin. 



Landauer describes the cells of the convoluted tubules and of the wider portion 

 of Henle's loop as provided with lateral folds (and not with the rod-like structure) , 

 by means of which adjacent cells are brought in direct contact with one another. 



At the junction of the medullary and the cortical tissue the convoluted 

 tubule becomes suddenly constricted and passes over as Henle's loop in 

 the form of an elongated arch into the medullary structure (t, t). A distinction 

 is made in the loop between the small descending limb, with a relatively large 

 lumen (14 //) and clear, flat epithelium arranged in alternating order and bulged 

 out at the middle by its nucleus (IV, S), and the wider, ascending limb. The 

 transition from the one to the other takes place in man, as a rule, in the lowermost 

 portion of the descending limb. The ascending limb becomes dilated to a diameter 

 of from 20 //. to 26 u, its lumen is relatively large and its epithelium is essentially 

 the same as that of the convoluted tubules, except that the rods are shorter. 

 Where the ascending limb penetrates into the cortical structure the canal at first 

 becomes smaller again. It then passes into the intercalated portion (n, n), 

 which has a diameter of 40 u. and in structure most nearly resembles the con- 

 voluted tubules, than which, however, it is shorter, though lined with similar 

 cells. After a second constriction the intercalated portions pass over into the 

 collecting tubules (o) , which within the medullary rays projecting into the cortex 

 have a diameter of about 45 //. In their further course downward in the papilla 

 adjacent collecting tubules unite and form tubes having a diameter of from 200 

 to 300 //, the papillary ducts or excretory tubes (O), of which from 24 to 80 

 open at the apex of each of the 1 2 or 15 papillae (foramina papillaria or cribrum 

 benedictum). 



In the lowermost and widest portion the membrana propria of the duct is 

 surrounded and fortified by a layer of delicate connective-tissue fibers. The cells 

 are large cylindrical epithelia, with well-defined, spherical nuclei (VI) and dip- 

 losomata. Further upward the constricted portion of the collecting tubules is 

 lined by low, cylindrical, rather cubical cells, with large nuclei (V) supported upon 

 a structureless membrana propria. Within the cortical structure the cells assume 

 an inclined position, so that they overlie one another like the shingles on a roof. 

 In the cells of all of the urinary tubules, excepting those of the collecting tubules, 

 a ciliated process projects from the centrosoma into the lumen of the tubule. 

 The same peculiarity is present in the epithelium of the seminal vesicles. 



The Blood-vessels of the Kidney. The renal artery with its branches 

 reaches the junction of the medullary and the cortical structure after repeated 

 division. From this point the interlobular arteries (a) arise at equal distances 

 apart and traverse the cortex vertically. Throughout their entire course they 

 give off laterally the afferent vessels (i) , each of which enters into a capsule formed 

 by the urinary tubule at a point exactly opposite to that from which the tubule 



