STRUCTURE OF THE KIDNEYS. 263 



border of each kidney proceeds also the ureter, U, a slender 

 tube opening below into the bladder, Vu, near its lower 

 end. From the bladder proceeds the urethra, at Ua. The 

 channel of each ureter passes very obliquely through the 

 wall of the bladder; accordingly if the pressure inside the 

 latter organ rises above that of the liquid in the ureter the 

 walls of the oblique passage are pressed together and it is 

 closed. Usually the bladder (which contains muscular 

 tissue in its walls) is relaxed, and the urine flows readily 

 into it from the ureters. The commencement of the 

 urethra being kept closed by elastic tissue around it (which 

 can voluntarily be reinforced by muscles which compress 

 the tube), the urine accumulates in the bladder. When 

 this latter contracts and presses on its contents the ureters 

 are closed in the way above indicated, the elasticity of the 

 fibres closing the urethral exit from the bladder is over- 

 come, and the liquid is forced out. 



Naked Eye Structure of the Kidneys. "When a section 

 is made through a kidney from its outer to its inner border 

 (Fig. 74) it is seen that a deep fissure, the hilus, leads 

 into the latter. In the hilus the ureter widens out to form 

 the pelvis of the kidney, which breaks up into a number of 

 smaller divisions, the cups or calices. The cut surface of 

 the kidney proper is seen to consist of two distinct parts; 

 an outer or cortical portion, and an inner or medullary. 

 The medullary portion is less red and more glistening to 

 the eye, is finely striated in a radial direction, and does not 



What is the ureter? Under what circumstances is its opening 

 into the bladder closed? What is the usual state of things? 

 How is the commencement of the urethra closed? What results? 

 What happens when the bladder contracts? 



What is seen on a section made through a kidney ? How is the 

 pelvis of the kidney formed? What are the calices? What is seen 

 on the cut surface of the kidney proper? How does the medullary 

 part differ in appearance from the cortical? 



