. Evolution and disease . cystic conditions arising in functionless ductsare described in Chapter III. In addition to cystic tumoursarising from retention of fluid,or in functionless ducts, thereis an interesting class knownas diverticula, or false cysts :these demand some considera-tion. Many of the tubes and pas- ^lo. ny.-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 witha soft velvetycovering known as mucous membrane. Thisdiffers from the out

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. Evolution and disease . cystic conditions arising in functionless ductsare described in Chapter III. In addition to cystic tumoursarising from retention of fluid, or in functionless ducts, thereis an interesting class knownas diverticula, or false cysts :these demand some considera-tion. Many of the tubes and pas- ^lo. ny.-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 witha soft velvetycovering known as mucous membrane. Thisdiffers from the outer tube in being usually distensible andeasily separable from the outer or protecting wall. Notinfrequently from strain, accumulations of fluid or air, the inner, moveable, elastic mucous membrane liningsuch a tube will be forced through a weak or defectivespot in the outer tube and form a soft rounded bulgingor diverticulum ; the cavity of such a diverticulum stillretains its connection with the tube to which it belongs.. 232 evolutiojsj and disease. One of the most remarkable diverticula known to meexists in the neck of the Emu {Dromcsiis novce-Jiollandice), which demands consideration, as it serves as an admir-able physiological type of diverticula in general.