386 THE EXCRETIONS OF THE BODY 



branches, each of which by repeated division forms a lobule composed of sev- 

 eral collateral vessels. These vessels do not anastomose, but unite finally to 

 form a simple vas eff er ens, the beginning of which lies in the middle of the 

 glomerulus. They have a simple wall and hence are to be regarded as capillaries. 

 In the kidney therefore the blood passes through two sets of capillaries, one in 

 the glomeruli and the other between the secreting elements. 



The secreting and conducting elements of the kidney are numerous, much- 

 convoluted tubules, which begin at the glomeruli and end on the free surface 

 of the papillae. The glomerulus is surrounded by a thin capsule (the capsule of 

 Bowman), the whole constituting a Malpighian corpuscle (Fig. 144, I). The 

 capsule is a vesicle composed of thin epithelial cells of 0.13 to 0.22 mm. diameter, 

 and, like the serous sacs, consists of two layers, a visceral and a parietal. The 

 former layer is closely applied to the surface of the glomerulus and is reflected 

 at the place where the vessels enter the glomerulus to form the latter layer. 

 From the point opposite the entrance of the vessels the capsule is continued into 

 the renal tubule. In the transition to this there comes first a short, narrow 

 neck; then follows a much convoluted portion (II) 0.045 mm. in diameter which 

 reaches down to and enters the outer layer of the medullary substance. Here 

 the tubule suddenly diminishes in size very considerably (the diameter is only 

 0.014 mm.) and passes into the medullary substance, then turns back, forming 

 a loop (loop of Henle, III and IV) and runs toward the cortex. Sooner or 

 later it becomes enlarged (0.026 mm.) and soon thereafter becomes convoluted 

 again (V). Then it unites by means of a narrow connecting portion with a 

 collecting tubule (VI). 



Up to this point each tubule is independent of every other, forming no 

 anastomoses. The collecting tubes, however, in their course through the medul- 

 lary substance, unite repeatedly with others, so that finally the number of tubes 

 opening on the surface of a papilla is only about fourteen to twenty, whereas 

 there are from 4,000 to 6,000 collecting tubules tributary to them. 



The epithelium of the urinary tubule and of the collecting tubule is dif- 

 ferent in different divisions. In the human foetus Bowman's capsule is com- 

 posed of cubical cells; in the newborn child the cells are natter, and later they 

 become very thin. The convoluted tubules, the thicker portion of Henle's loop, 

 and the collecting tubule are lined with fairly tall epithelial cells, which pre- 

 sent minor differences in the different divisions named. In the narrower por- 

 tion of Henle's loop the epithelium consists of clear, flat, spindle-shaped cells. 



B. MECHANISM OF THE EXCRETION OF URINE 



Any attempt to explain theoretically the process of secretion in the kidneys 

 must take into account the remarkable arrangement of its blood vessels and 

 the renal tubules. 



In the glomeruli the blood flowing in is suddenly divided into a consid- 

 erable number of tiny streams, which of course must favor the passage of 

 constituents into the capsule. Moreover, the vas efferens has a smaller diam- 

 eter than the vas afferens, and it is divided up within a short space into 

 another true capillary network. The resistance distal to the glomeruli must 

 therefore be much greater than the resistance proximal to them, which means 

 that the blood must flow through the glomeruli under a relatively high pres- 

 sure. If now the further fact that the renal tubules begin with the Bowman's 

 capsule surrounding the glomeruli be considered, it cannot readily be denied 

 that, seen merely from the anatomical point of view, the glomerulus and the 



