m 



BOUNDARY ZONE. BLOOD VESSELS. 81 



Kidney. Dog, for boundary zone. (p. 12, s. 22 & 24, c. P., 

 . B.) (L) Between the cortex and medulla find the 

 boundary zone, rendered distinct from either of these by the 

 closely packed parallel tubes, chiefly ascending and descending 

 limbs of Henle's loop. Observe the large blood-vessels on a level 

 with this zone. (H) Find loops at different levels in this stratum, 

 and note the character of the epithelium with which they are 

 lined. Find a glomerulus, cut through its attachment to 

 Bowman's capsule', observe that the former is compounded of 

 clusters of capillaries, and also that its surface is covered by a 

 layer of epithelium, the reflected part of the capsule, the nuclei 

 of which are superficial to those of the capillaries. Search for 

 the connection of the capsule with a convoluted tube, and inspect 

 its epithelium. Next note the shape of the cells in the collecting 

 tubes of the medullary rays, and compare them with those of 

 Henle's loops in the boundary zone. 



Injected Kidney of a mammal. Cat. (Blue or red gelatin 

 mass, s. 17 or 22, c. G. or P., m. B. or F.) (L) Find the arching 

 arteries and veins in the intermediate zone and trace their two- 

 fold distribution. (1) Outwards to the cortex, the interlobular 

 arteries give off efferent branches to the glomeruli, the rich 

 cluster of capillaries which form the latter and the efferent 

 vessel which leaves them to divide into a second set of capil- 

 laries, the intertubular plexus of capillaries. The venous return 

 from these occurs through the interlobular veins, which com- 

 mence on the surface of the cortex as the stellate veins. From 

 the lower glomeruli the efferent vessel forms a leash of 

 capillaries, pseudo-arterial rectcv, which pass into the medulla. 

 (2) Inwards branches break up at once into parallel clusters 

 of vessels, the arterice, rectw, which return their blood through. 

 a similar set of small vessels into the arching veins. 



