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The cloaca is the dilated terminal portion of the alimentary canal, which opens through the vent, and besides the faeces, discharges the urine and-the genital products. The whole cloaca of most birds is divided by transverse folds into a vestibulum, a urino-genital or middle, and a rectal or innermost chamber.
Diagram of Cloaca
Diagram of the Cloaca of a Bird

BF. ..... Bursa of Fabricius (Bursa Fabricii) ;
CD. ..... Coprodaeum ;
UD. ..... Urodaeum ;
PD. ..... Proctodaeum
R. ..... Rectum ;
u. ..... Ureter ;
v.d. ..... Vas de ferens

The urino-genital chamber or "urodaeum" is small, and receives in its dorso-lateral walls the ureters and the genital ducts, which are protected by papillae. Above their orifices is a circular fold, most prominent on the ventral side; below them, towards the vent, is another well-marked circular fold, which, towards the ventral aspect, passes into the coating of the copulatory organ, when such is present. The space between this fold and the outer anal opening, which is closed by a strong sphincter muscle, lodges the copulatory organ, and on its dorsal wall leads through a wide opening into the bursa of Fabricius or bursa Fabricii. This organ is peculiar to birds, is most developed in the young of both sexes, and often becomes more or less obliterated in the adult.  Its function is to produce "B cells" (which are normally produced in the bone marrow in mammals).

The innermost chamber, or "coprodaeum," is situated above the urodaeum, is mostly an oval dilatation of the rectum, and is of considerable size in those birds whose faeces are very fluid, as Accipitres, Herodii, and Steganopodes. In the Cassowary and Rhea it passes gradually into the rectum above, but in many Carinatae, as well as in Struthio, the upper end is marked by a strong circular fold, and the inner surface of the walls is smooth and different from that of the rectum proper. In Struthio this chamber is followed by another, which is smaller and less defined, resembling in this respect some Saurians.

It follows from the arrangement described above, that in Birds the urine is not retained in the small urodaeum, but that, as in Saurians, it passes into the next chamber above. Through this the faeces pass; if they are very fluid, they collect in the then very capacious space, together with the urine, and transform the chamber into a physiological cloaca.  If the faeces are more solid, as for instance in Geese, they are retained in the rectum proper, and simply pass through the cloaca.  In the Ostriches defaecation and micturition are mostly separate acts, especially when the largely-developed and persistent bursa of Fabricius acts as a physiological bladder.  A true urinary bladder, i.e. a ventral dilatation of the urodaeum, is absent in Birds.

The copulatory organ in the male, and the corresponding part in the female, are developed from the ventral wall of the vestibulum or "proctodaeum." It is present in two different forms.

In the Ratitae, except Rhea, it consists of a right and left united half, with a deep longitudinal furrow on the dorsal side, and strongly resembles the same organ in Crocodiles and Tortoises; it can be protruded and retracted by special muscles which in the Ratitae are partly attached to the pelvic bones. In Rhea, and among the Carinatae in the Anseres only, the copulatory organ consists like-wise of two halves with a longitudinal furrow, but is greatly specialized by being spirally twisted and being reversible like the finger of a glove; its muscles are derived solely from the sphincter muscle of the vent. In other Carinatae, for instance in the Tinamidae, Cracidae, in Platalea, Ciconia, and Phoenicopterus, the penis is much smaller and simpler in structure, with all the appearance of a degraded organ.  In  the majority of Birds, especially in the highest, it has disappeared, and the primitive way of everting the cloaca is resorted to during copulation (H. Gadow, Phil. Trans. 1887, page 32).

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