324 HISTOLOGY 



many cellular elements and some lymphocytes, and passes without a 

 definite boundary into the loose connective tissue of the submucosa. 



The tunica muscularis has considerable connective tissue among its 

 smooth muscle bundles. The latter form an inner longitudinal and an 

 outer circular layer. In the lower half of the ureter there is a third, outer 

 longitudinal layer, specially thickened along the last 5 cm. Around the 

 papillae of the kidney the circular fibers form a "sphincter." The part 

 of the ureter which passes obliquely through the wall of the bladder has 

 only longitudinal fibers, ending in the tunica propria of the bladder. By 

 contracting they open the outlet of the ureter. The adventitia consists 

 of loose fibro-elastic connective tissue. 



Lymphatics and blood vessels are numerous. There are sympathetic 

 nerves to the muscles, and free sensory endings in the tunica propria 

 and epithelium. 



BLADDER. 



The development of the bladder from the ventral part of the cloaca 

 has been described on page 245. Its epithelium is entodermal whereas 

 that of the ureters opening into it is mesodermal. There is however no 

 demarcation between the layers in the adult, since both produce the same 

 sort of "transitional epithelium." (This term, introduced by Henle 

 (Allg. Anat., 1841) as a designation for epithelia which are intermediate 

 between stratified squamous and simple columnar, such as occur at the 

 cardia and elsewhere, is now generally restricted to the peculiar epithelium 

 of the bladder, ureter and renal pelvis.) 



The bladder consists of a mucosa, submucosa, muscularis and serosa. 

 The epithelium has been described as two-layered in the distended bladder, 

 the outer cells having terminal bars; in the contracted condition it becomes 

 several-layered and the bars form a net extending into the epithelium. 

 Thus it is not believed that during distention the layers shown in Fig. 322 

 merely flatten; they are thought to "slip by each other." The columnar 

 cells may, however, become extremely flat. The appearances of the 

 epithelium in the bladder and ureter of the dog under various conditions 

 of distention and contraction have been figured by Harvey (Anat. Record, 

 1909, vol. 3, pp. 296-307). The superficial cells have a cuticular border; 

 they often contain two nuclei, and their darkly granular protoplasm has been 

 considered suggestive of secretory activity. Round or oval pockets extend 

 into the tunica propria (Fig. 324). Some of them have no lumen, or are 

 detached from the epithelium, but others are pits containing a colloid 

 substance. The pits are rudimentary glands. In the adult, branched 

 tubules lined with cylindrical epithelium may sprout from the bottom of 

 the pits, thus forming true glands. Their occurrence is limited to the 



