204 THE EXCRETORY SYSTEM 



In the case of mammals, the ureter has three distinct coats. The 

 Hning, Hke that of the kidney pelvis with which it is continuous, is 

 composed of transitional epithelium. This with an underlying 

 tunica propria of loose fibroelastic connective tissue constitutes the 

 mucosa, (Fig. 129.) It has longitudinal folds, so that in cross- 

 section the lumen apjDcars to be star-shaped. Outside the mucosa 

 there is a muscularis coat which usually exliibits an inner layer of 

 scattered longitudinally arranged strands of smooth muscle cells 

 and an outer, thicker zone of circularly disposed smooth muscle 

 cells. These two sets of muscle cells produce the peristaltic motion 

 which continuously carries drops of urine away from the kidney. 

 Covering the muscularis coat is the fibrosa which is composed of 

 fibroelastic tissue and connects the ureter with adjacent structures. 



THE BLADDER. 



In most vertebrates there is some provision made for the tem- 

 porary storage of the urine which is contimiously being excreted 

 from the kidney. The urine is collected in sacs, called liladders, of 

 which there are three types, namely, the tubal, the cloacal, and the 

 specialized bladder of the Amniotes. 



The tubal bladder is found in many fishes where the posterior 

 ends of the ureters (Wolffian ducts in this case) are enlarged for 

 temporary urine storage. In some fishes there is a partial fusion 

 of the two exjDanded ends, and in others there is a fusion of the two 

 into a single large sac with which the ureters are connected. In the 

 latter case there is a single exit located in a little eminence, called 

 the urogenital papilla, which is in the wall of the cloaca and into 

 which the genital ducts also open. 



A simple cloacal bladder is found in the lung fishes and amphib- 

 ians. It is a thin-walled sac formed as an evagination from the 

 ventral wall of the cloaca. In the frog, it is lined with a pseudo- 

 stratified epithelium. A thin layer of connective tissue containing 

 irregularly- arranged smooth muscle cells is subjacent to the epithelial 

 covering. An outermost region of connective tissue similar to the 

 peritoneum completes the structure of its wall. Such a bladder is 

 anchored to the bod\' wall by a dorsal mesentery of peritoneum and 

 similar attachments from the lateral faces to the l)0(ly wall. This 

 type of bladder stores urine which collects in it from the closed 

 cloaca. Stored urine is ejected by contractions of the body wall. 

 The allantois of the amniotes also arises as an evagination of the 

 cloacal wall and is homologous with the amphibian cloacal bladder. 



