TUMOURS AND CANCERS. 231 



of this kind of cyst. I have already described the ves- 

 tige of the right oviduct which may be detected on the 

 right side of the cloaca (see p. 64). This slender and 

 apparently innocent tube may now and then dilate to 

 form^a cyst as large as a walnut (fig. 117). As a rule 

 such cysts are harmless, but at times they inflame and 

 become filled with pus ; should the cyst rupture the pus 

 escapes among the intestines and the bird dies from 

 peritonitis. 



Other cystic conditions arising in functionless ducts 

 are described in Chapter III. 



In addition to cystic tumours 

 arising from retention of fluid, 

 or in functionless ducts, there 

 is an interesting class known 

 as diverticula, or false cysts : 

 these demand some considera- 

 tion. 



Many of the tubes and pas- FlG . II7 ._ Th e Cloaca of a H en : 

 sages of animal bodies possess the stum P of the ri s ht oviduct 



is dilated into a cyst. 



two walls, an outer one, more or 



less rigid, which gives it shape, and an inner one lined with 

 a soft velvety covering known as mucous membrane. This 

 differs from the outer tube in being usually distensible and 

 easily separable from the outer or protecting wall. Not 

 infrequently from strain, accumulations of fluid or air, 

 the inner, moveable, elastic mucous membrane lining 

 such a tube will be forced through a weak or defective 

 spot in the outer tube and form a soft rounded bulging 

 or diverticulum ; the cavity of such a diverticulum still 

 retains its connection with the tube to which it belongs. 



