SEC. 4. THE DISCHARGE OF URINE. 



424. Structure of the ureter. The ureter, like the large 

 ducts of other glands, consists of an epithelium resting on a 

 connective-tissue basis strengthened with plain muscular fibres. 

 The epithelium is in its characters intermediate between that 

 lining the ossophagus, which as we have seen ( 221) resembles 

 the epidermis of the skin and that lining the ducts of the glands 

 of the alimentary canal. It consists not of a single layer but of 

 three or four layers of cells. The lowermost cells, next to the 

 basement membrane which limits the connective-tissue basis, are 

 oval cells placed vertically, in one or two layers. The cells of the 

 next layer are irregular in form and often pear-shaped, with a 

 narrowing process dipping down between the cells below. Above 

 these, forming the surface of the epithelium, is a layer of flat or of 

 flattened cubical cells. All the cells are nucleated, and there are 

 no special features in their cell-substance. 



The connective tissue is, as in a mucous membrane, delicate 

 immediately below the epithelium, but becomes coarser and more 

 fibrous in its outer parts. The muscular fibres are arranged in 

 three layers, an inner longitudinal, a thicker middle circular, and 

 a thinner less regular outer longitudinal layer better developed in 

 the lower part of the tube than elsewhere. 



Nerves pass into the ureter at the upper end from the renal 

 plexus and at the lower end from the spermatic and hypogastric 

 plexuses, and at the two ends nerve-cells are scattered among the 

 nerve-fibres. 



The pelvis of the kidney is an expansion of the upper end of 

 the ureter, and is lined by an epithelium like that of the ureter, 

 which is continued into the calyces and over the projecting papillae 

 of the pyramids. The circular muscular fibres of the ureter are 

 continued over the pelvis but form here a relatively thinner layer, 

 while both longitudinal layers are very scanty and gradually be- 

 come lost. 



At its lower end each ureter opens by an oblique opening, 

 serving as a valve, into the cavity of the bladder. 



