352 HAND-BOOK OF PHYSIOLOGY. 



Klein, however, believes that the pigment passes through the intercellular 

 substances, and not through the cells. 



In some places, it is stated that a distinct membrane of flattened cells 

 can be made out lining the lumen of the tubes (centrotubular membrane). 



Blood-vessels of Kidneys. In connection with the general distri- 

 bution of blood-vessels to the kidney, the Malpighian Corpuscles may be 

 further considered. They (Fig. 243) are found only in the cortical part 

 of the kidney, and are confined to the central part, which, however, 

 makes up about seven-eighths of the whole cortex. On a section of the 

 organ, some of them are just visible to the naked eye as minute red points; 

 others are too small to be thus seen. Their average diameter is about 

 T |~o of an inch. Each of them is composed, as we have seen above, of 

 the dilated extremity of a urinary tube, or Malpighian capsule, enclosing 

 a tuft of blood-vessels. 



The renal artery divides into several branches, which, passing in at 

 the hilus of the kidney, and covered by a fine sheath of areolar tissue 

 derived from the capsule, enter the substance of the organ in the inter- 

 vals between the papillae, chiefly at ths junction between the cortex and 

 the boundary layer. The chief branches then pass almost horizontally, 

 giving off smaller branches upward to the cortex and downward to the 

 medulla. The former are for the most part straight, they pass almost 

 vertically to the surface of the kidney, giving off laterally in all directions 

 longer or shorter branches, which supply the afferent arteries to the Mal- 

 pighiaii bodies. 



The small afferent artery (Figs. 243 and 245) which enters the Mal- 

 pighian corpuscle, breaks up as before mentioned in the interior into a 

 dense and convoluted and looped capillary plexus, which is ultimately 

 gathered up again into a single small efferent vessel, comparable to a min- 

 ute vein, which leaves the Malpighian capsule just by the point at which 

 the afferent artery enters it. On leaving, it does not immediately join 

 other small veins as might have been expected, but again breaking up into 

 a network of capillary vessels, is distributed on the exterior of the tubule, 

 from whose dilated end it had just emerged. After this second breaking 

 up it is finally collected into a small vein, which, by union with others 

 like it, helps to form the radicles of the renal vein. Thus, in the kidney, 

 the blood entering by the renal artery traverses two sets of capillaries be- 

 fore emerging by the renal vein, an arrangement which may be compared 

 to the portal system in miniature. 



The tuft of vessels in the course of development is, as . were, thrust 

 into the dilated extremity of the urinary tubule, which finally completely 

 invests it just as the pleura invests the lungs or the tunica vaginalis the 

 testicle. Thus the Malpighian capsule is lined by a parietal layer of 

 squamous cells and a visceral or reflected layer immediately covering the 

 vascular tuft (Fig. 241), and sometimes dipping down into its interstices. 



