Chap. XVII] URINARY SYSTEM 327 



The substance of the kithiey is readily seen by the naked eye 

 to consist of two distinct parts: (1) An outer, Hghter, and more 

 soHd portion, called the cortex (bark). (2) An inner, darker, 

 striated portion, called the medulla (marrow), which is not a solid 

 mass, but more or less distinctly divided into pyramidal-shaped sec- 

 tions. The pointed projections, or papillae, of the pyramids are re- 

 ceived by the cup-like cavities or calyces of the pelvis. The bulk 

 of the kidney substance, both in the cortex and medulla, is com- 

 posed of little tubes or tubules, closely packed together, having only 

 just so much connective tissue as is sufficient to carry a large supply 

 of lilood-vessels and a certain number of lymphatics and nerves. 



Urinif erous tubules. — Examined under the microscope, it is 

 seen that the uriniferous tubules begin as little hollow globes, called 

 capsules, in the cortex of the kidney. These capsules are joined 

 to the tubules by a constricted neck, and the tubules, after running 

 a very irregular course, open into straight collecting tubes, which 

 pour their contents through their openings in the pointed ends or 

 papillte of the pyramids, into the calyces of the kidney. 



The tubules are composed of basement membrane, lined through- 

 out by epithelial cells. The cells vary in the different parts of a 

 tubule, those of the capsule and convoluted or irregular parts being 

 more especially adapted to secretory purposes than the straight 

 parts of the tubule. 



Pyramid. — These collecting tubules en masse, together with 

 interstitial tissue, blood-vessels, and lymphatics, make a pyramid. 

 The number of pyramids varies from eight to twelve. 



Renal or Malpighian corpuscles. — In the cortical portion of the 

 kidney are found renal corpuscles which consist of two parts : (1) a 

 minute tuft of capillaries called a glomerulus, surrounded by (2) a 

 closed capsule which is the beginning of a uriniferous tubule. 

 The investment of the glomerulus by the capsule is double and 

 quite complete except at one point where an afferent vessel enters, 

 and an efferent vessel leaves. 



The blood supply of the kidney. — For its size, the kidney is 

 abundantly supplied with blood. The renal artery, coming di- 

 rectly from the aorta, divides, before it enters the hilus of the kid- 

 ney, into several branches, which pass into the tissue of the organ. 

 Branches from these arteries have two destinations : (1) into 

 the cortex, and (2) into the pyramids. 



