2IO THE BIOLOGY OF THE FROG chap. 



resting upon a membrane of connective tissue. Numerous 

 goblet cells occur among the other epithelial cells. The 

 middle layer of the bladder consists of a network of smooth 

 muscle fibers. The fibers are sometimes single, and some- 

 times united into bundles, and they extend in all directions. 

 Outside of the muscle layer is a thin sheet of connective 

 tissue which is covered externally by the peritoneum. 



The bladder is very distensible, as may readily be shown 

 in a recently killed frog, by inflating it by means of a blow- 

 pipe introduced into the cloaca. When entirely empty, the 

 bladder shrinks to an inconspicuous size. It was formerly 

 doubted whether the bladder of the frog serves as a recep- 

 tacle for urine, as it has no direct connection with the ducts 

 from the kidneys. Townson, whose conclusions were fol- 

 lowed by Dumeril in his great work on reptiles and amphibia, 

 regarded the bladder as a sort of reservoir for water absorbed 

 through the skin. The contents of the bladder were stated 

 to be nearly pure water, and the urine proper was supposed 

 to pass out and through the cloaca without entering the 

 bladder at all. According to Dumeril, ^ " la pr^tendue 

 vessie urinaire des Grenouilles, des Rainettes et des Cra- 

 pauds, ainsi que celle des Salamandres, est une sorte de 

 citerne ou une humeur aqueuse, presque pure, destinee a 

 I'exhalation cutan^e, semble etre apportie, soit par les veins 

 sanguines, soit par les lymphatiques." Subsequent investiga- 

 tions by Davy, Nussbaum, and Adami have shown that there 

 is no doubt that the fluid contained in the bladder is derived 

 from the kidneys, since it contains urea and other sub- 

 stances characteristic of renal excretion. 



The end of the cloaca is commonly held closed by the 

 contraction of its circular muscles, and the urine which is 

 thus prevented from passing out collects in the bladder. 



1 Dumeril, " Erpetologie generale." 



