PORTAL CIRCULATION OF THE KIDNEY. 



591 



Fig. 175.* 



and the calices are composed of two coats, an external or fibrous coat, 

 the tunica propria ; and an internal mucous coat, which is continuous 

 with the mucous membrane of the bladder inferiorly, and with the 

 tubuli uriniferi above. 



Vessels and Nerves. The renal artery is derived from the aorta ; it 

 divides into several large branches before entering the hilus, and 

 within the organ ramifies in an arborescent manner, terminating in 

 nutrient twigs, and in the small infer- 

 ent vessels of the corpora Malpighiana. 

 In the Malpighian bodies the inferent 

 vessels divide into several primary 

 twigs, which subdivide into capillaries, 

 and the capillaries, after forming loops, 

 converge to the efferent vein, which is 

 generally smaller than the correspond- 

 ing artery. The efferent veins pro- 

 ceed to and form a capillary venous 

 plexus, which surrounds the tortuous 

 tubuli uriniferi, and from this venous 

 plexus the blood is conveyed by con- 

 verging branches into the renal vein. 



" Thus," remarks Mr. Bowman, 

 " there are in the kidney two perfectly 

 distinct systems of capillary vessels, 

 through both of which the blood passes in its course from the arteries 

 into the veins : the first, that inserted into the dilated extremities 

 of the uriniferous tubes and in immediate connection with the arte- 

 ries ; the second, that enveloping the convolutions of the tubes and 

 communicating directly with the veins. The efferent vessels of the 

 Malpighian bodies, that carry the blood between these two systems 

 may collectively be termed the portal system of the kidney." The 

 inferences drawn by Mr. Bowman from his investigations are pecu- 

 liarly interesting ; they are : that the capillary tufts of the Malpi- 

 ghian bodies are the part of the kidney specially acted on by diuretics ; 

 that they are the medium by which water, certain salts, and other 

 substances pass out of the system ; that they are, moreover, the 

 means of escape of certain morbid products, such as sugar, albumen, 

 and the red particles of the blood. Respecting the capillary venous 

 plexus, we have proof that the principal proximate constituents of 

 urine, such as, urea, lithic acid, &c. are, like the bile, derived from 

 venous (portal) blood. 



* Plan of the renal circulation ; copied from Mr. Bowman's paper, a. A 

 branch of the renal artery giving off several Malpighian twigs. 1. An efferent 

 twig to the capillary tuft contained in the Malpighian body, m ; from the Mal- 

 pighian body the uriniferous tube is seen taking its tortuous course to t. 2, 2 

 Efferent veins ; that which proceeds from the Malpighian body is seen to be 

 smaller than the corresponding artery, p, p. The capillary venous plexus, ra- 

 mifying upon the uriniferous tube. This plexus receives its blood from the 

 efferent veins, 2, 2, and transmits it to the branch of the renal vein, v. 



