CHAPTER XIV. 

 THE URINAEY SYSTEM. 



SECTION I. 

 THE STRUCTURE OF THE KIDNEY. 



THE kidney is a compound tubular gland having a duct, the ureter, 

 which connects it with the bladder and is expanded at its upper end 

 to form the pelvis of the kidney. On dividing the kidney lengthwise 

 from its outer to its inner border, it is seen to consist of two layers, 

 an outer reddish-brown cortex, and an inner pale layer, the medulla. 

 The latter is composed of a number of pyramids, the apices of which 

 project as papillae into the pelvis of the kidney. The larger sub- 

 divisions of the renal artery and vein lie between the cortex and 

 medulla, this region being known as the boundary zone. Prolonga- 

 tions of the medullary tissue extend radially into the cortex, forming 

 the medullary rays. 



The kidney consists of a mass of tubules, held together by con- 

 nective tissue. Each tubule begins in the cortex by a blind expanded 

 end (Bowman's capsule), which may be compared with a small ball 

 indented so that its opposing walls almost touch ; these walls consist 

 of a single layer of flattened epithelium. A bunch of capillaries, known 

 as a glomerulua, projects into the indentation, and together with 

 Bowman's capsule forms a Malpighian body. At the pole opposite 

 the entrance of the blood-vessels Bowman's capsule opens into the 

 tubule proper, which at first takes a tortuous course and is known 

 as the first convoluted 'tubule ; it then becomes spiral or nearly 

 straight (spiral tubule), and passes into the medulla, where it forms a 

 loop (loop of Henle) and returns into the cortex. Here it becomes 

 irregularly zigzag (zigzag tubule), and then convoluted (second con- 

 voluted tubule), and ultimately joins a straight collecting tubule 

 (fig. 131). The collecting tubules run into the medulla, and open at 

 the apices of the pyramids into the pelvis of the kidney. 

 - ' 380 



