126 AVES. 



in Birds and Amphibia, and is the common cavity receiving the ori- 

 fices of the intestinal, sexual, and urinary organs. 



In Birds the cloaca usually forms a very wide vesicular dilatation, 

 a continuation properly of the intestinal tube, nearly as long as it is 

 broad, and invested externally to a considerable extent by peritoneum 

 and lined with mucous membrane, a muscular layer being interposed 

 between these two. The rectum enters the upper and anterior part 

 of the cavity, protected by a circular fold, and to the left behind it 

 the oviducts in the female, or the two vasa deferentia in the male, 

 terminate from either side upon papilliform eminences, provided with 

 similar folds ; behind these papillae is found a well-developed plexus 

 of vessels, or rete mirabile. Between and posterior to the orifices of 

 the excretory ducts of the sexual organs, those of the ureters are 

 found in juxtaposition with each other, while behind them is situated 

 a very prominent circular valve, beneath which the bursa Fabricii 

 opens directly by a small aperture ; last of all, is the large circular 

 aperture of the cloaca which at the same time constitutes the external 

 anal outlet. The 'space between the two circular folds guarding the 

 urethro-sexual cavity and the rectum, has been regarded as a rudi- 

 mentary receptacle for the urine. The penis, when present, arises 

 from the lower wall of the cloaca, where it is either surrounded by 

 folds, or situated, as in the Ostrich, in a special pouch. 



The cloaca is closed by a circular sphincter muscle. From the 

 longitudinal muscular layer particular fasciculi are developed, or, as 

 in the large Brevipennes or short-winged birds, muscles of consider- 

 able size, which are inserted into the ischium and expand or open 

 the cloaca. Peculiar muscles are connected with those of the cloaca 

 for moving the penis when present, such as an elevator and retractor 

 of that organ. 



