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. 
Inaddition tocystictumours f~ 
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- Fic. 117.—The Cloaca of a Hen: 
sages of animal bodies possess the stump of the right 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 velvetycovering 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. 
