822 A MANUAL OF ANATOMY 



11. The straight collecting tubule is lined with epithelial cells, which, in its 

 earlier part, are clear and cubical, but in its later part they are columnar, and 

 the lumen is very distinct. 



12. The excretory tube or duct of Bellini is lined with epithelial cells which 

 are clear and columnar. 



Medullary Rays or Pyramids of Ferrein. — ^These take the form of 

 pyramidal bundles of uriniferous tubules, which are separated from 

 each other by portions of the labyrinth. The tubules are straight, 

 and emerge from the boundary zones of the Malpighian pyramids. 

 As these medullary rays are followed from the boundary zones to 

 the periphery, each gradiially diminishes in breadth, and ultimately 

 tapers to a point, which stops a short distance from the fibrous 

 capsule, being separated from it by a portion of the labyrinth. 

 The rays are thus conical, their bases being at the boundary zones 

 of the Malpighian pyramids, and their apices near the fibrous 

 capsule — hence the name ' pyramids.' The explanation of this 

 shape is that the tubules at the circumference of a medullary ray 

 enter the labyrinth sooner than those in the centre, which latter 

 form the apex of the ray. 



Summary of a Medullary Ray. — Each contains the following tubules : 

 (i) spiral tubules; (2) ascending tubules of Henle ; and (3) straight collecting 

 tubules. 



Medulla of the Kidney. — The medullary portion has been already 

 generally described. It is composed of Malpighian pyramids, con- 

 sisting of uriniferous tubules and bloodvessels (true and false). 



The tubules in the various parts of the Malpighian pyramids are 

 as follows : 



1. Boundary Zone. — ^This contains the following tubules : 



Portions of the descending tubules of Henle. 

 Portions of the ascending tubules of Henle. 

 Straight collecting tubules. 



2. Papillary Zone. — ^This contains the following tubules : 



Portions of the descending tubules of Henle. 

 The loops of Henle's tubules. 

 Portions of the ascending tubules of Henle. 

 The excretory tubes or ducts of Bellini. 



3. Apex of the Papillary Zone. — This contains only the excretory 

 tubes or ducts of Bellini. 



Bloodvessels of the Kidneys — Arteries. — Each kidney receives a 

 large amount of blood from the renal artery, which is a branch 

 of the abdominal aorta. This vessel, as it approaches the hilum, 

 divides into four or five branches, which enter the sinus, where 

 they are embedded in fat along with the calices. They then 

 subdivide into interlobar branches, which pass between the Mal- 

 pighian pyramids, where they continue to subdivide. On arriving 

 at the junction of the cortex and boundary layer they form a series 

 of arches, which are independent of one another, and are called 

 the cortico-medullary arches. Alongside of these there are venous 

 arches, which, unlike the arterial arches, anastomose freely with 



