448 THE URINARY SYSTEM 



occurrence. The mucosa becomes gradually thinner as it is traced 

 through the infundibula and calyces and is reflected upon the surface 

 of the renal pyramids. 



The muscular coat of the ureter consists of a well-defined layer 

 of circular fibers within which are many discrete bundles of longitudinal, 

 smooth muscle. In the lower half of the ureter a third layer, whose 

 fibers also have a longitudinal direction, is found outside the circular 

 fibers. In this portion of the ureter, therefore, the muscular coat 

 consists of three layers, an inner longitudinal, middle circular, and 

 outer longitudinal. 



In the renal pelvis and calyces the muscular coat becomes pro- 

 gressively thinner toward the renal substance, and at the apex of the 

 pyramid consists chiefly of circular fibers which are slightly thickened 

 to form a 'sphincter' about the papilla. 



The outer fibrous coat of the ureter consists of areolar tissue which 

 blends with that of the surrounding parts. In the renal pelvis this 

 coat becomes continuous with the connective tissue capsule of the kidney. 

 (For development of excretory passages, see next Chapter.) 



Blood Supply. The larger blood-vessels lie in the outer fibrous 

 coat and distribute branches to the muscular coat and to the mucous 

 membrane. In the latter they form a superficial capillary plexus which 

 is in unusually intimate relation with the deeper cells of the lining 

 epithelium. 



The Lymphatics. The lymphatics begin in an intramuscular plexus 

 and, by scanty vessels, in the deeper part of the mucosa. They pass 

 to larger vessels in the outer coat, which possess valves, and convey 

 the lymph to the neighboring lymph nodes. 



The Nerves. The nerves form a coarse plexus in the outer fibrous 

 coat which contains many small ganglia. From this plexus motor 

 fibers are distributed to the muscular layers, and sensory fibers to 

 the mucosa. The latter form a plexus beneath the epithelium from 

 which terminal fibers pass to end by minute end brushes in the con- 

 nective tissue and by varicose fibrils between the deep -cells of the epi- 

 thelium. 



THE URINARY BLADDER 



The wall of the urinary bladder closely resembles that of the ureter. 

 It consists of mucous, muscular, and fibro-serous coats. Its mucous 

 membrane is lined by transitional epithelium like that of the ureter 



