The cloaca is divided by transverse circular folds, which project from
its inner walls, into three successive chambers. The innermost, the
coprodaeum, is an oval dilatation of the end of the rectum, and
attains its greatest size in those birds whose faeces are very fluid;
it serves entirely as the temporary receptacle of the faeces and the
urine. The next chamber, the urodaeum, is small, and receives in its
dorso-lateral wall the ureters and the genital ducts; above and below
this chamber is closed by circular folds, the lower of which, towards
the ventral side, passes into the coating of the copulatory organ when
such is present. The urodaeum serves only as a passage, the urine
being mixed with the faeces in the chamber above. The third or
outermost chamber, the proctodaeum, is closed externally by the
sphincter ani; the orifice is quite circular. It lodges the copulatory
organ, and on its dorsal wall lies the _bursa Fabricii_, an organ
peculiar to birds. It is most developed in the young of both sexes, is
of unknown function, and becomes more or less obliterated in the
adult. Only in the ostrich it remains throughout life, being
specialized into a large receptacle for the urine, an absolutely
unique arrangement. A true urinary bladder, i.e. a ventral dilatation
of the urodaeum, is absent in all birds. It is significant that the
whole type of their cloaca much resembles that of the Crocodilia and
Chelonia, in opposition to that of the Lacertilia.
The penis, and its much reduced vestige of the female, is developed
from the ventral wall of the proctodaeum. It occurs in two different
forms. In the Ratitae, except _Rhea_, it consists mainly of a right
and left united half (_corpora fibrosa_), with a deep longitudinal
furrow on the dorsal side, and much resembles the same organ in
crocodiles and tortoises. It is protruded and retracted by special
muscles which are partly attached to the ventral, distal end of the
ilium. Another type exists in _Rhea_ and in the _Anseriformes_,
greatly specialized by being spirally twisted and partly reversible
like the finger of a glove. This is mainly due to the greater
development of an unpaired, median portion, analogous to the mammalian
_corpus spongiosum_, which is much less prominent in the Ratitae; the
muscles of this type are derived solely from the anal sphincter. In
other Carinatae, e.g. tinamous and storks, the penis is very much
smaller and simpler, with every appearance of a degenerated organ. In
the great majority of birds it has disappeared completely and the
primitive way of everting the cloaca is resorted to.
Both right and left testes are functional. They become greatly
enlarged in the breeding season; in the sparrow, for instance, from
the size of a mustard seed to that of a small cherry. The vas deferens
descends with many undulations down the lateral side of the ureter of
the same side, and opens upon a small papilla into the urodaeum.
Extraordinary increase in length during the breeding season causes the
vasa deferentia in some of the African weaver-birds to protrude, or to
bulge out the cloacal walls beyond the vent. The spermatozoa exhibit
many differences in shape, size and proportions, in the various groups
of birds. They have been studied minutely by E. Ballowitz.
Only the left ovary becomes functional, with rare individual
exceptions. Both present the appearance of diminutive clusters of
grapes, at the anterior end of the kidneys, close to the suprarenal
bodies, separated from each other by the descending aorta and by the
vena cava where this is formed by the right and left _vena iliaca
communis_. During the breeding season many more eggs are developed
than reach maturity, amounting in most birds to several dozens. Those
germs which do not ripen during the season undergo a process of
resorption, and in the winter the whole ovary dwindles to often a
diminutive size. In young birds both oviducts are almost equal in
size, but the right soon degenerates into an insignificant strand.
During every laying season the left duct increases enormously by new
formation of its component fibres. For instance, in the fowl its
volume increases about fifty-fold, growing from some 6 in. in length
and scarcely one line in width to more than 2 ft. in length and 1/2 in.
in thickness. The upper, wide opening of the duct is attached by
elastic, peritoneal lamellae to the hinder margin of the left lung;
the middle portion of the duct is glandular and thick-walled, for the
deposition of the albumen; it is connected by a short, constricted
"isthmus" (where the shell-membrane is formed) with a dilated "uterus"
in which the egg receives its calcareous shell and eventual
pigmentation.
BIBLIOGRAPHY.--A. v. Brunn, _Rueckbildung nicht ausgestossener
Eierstockseier, Henle Festschrift_ (Bonn, 1882); E. Ballowitz, "Die
Spermatozoen der Voegel," _Arch. Mikr. Anat._ xxxii., 1888, pls. 14-18;
M. Sacchi, "Contribuzione all' istiologia del ovidotto dei
saurop-sidi," _Att. Soc. Ital._, Milano, vol. xxx.; W.A. Forbes, "On
the Bursa Fabricii in Birds," _P.Z.S._, 1877, pp. 304-318; H. Gadow,
"Remarks on the Cloaca and on the Copulatory Organs of the Amniota."
_Phil. Trans._, 1887, pp. 5-37, pls. 2-5; Martin Saint Ange, "Etude de
l'appareil reproducteur dans les cinq classes d'animaux vertebres,"
_Mem. Ac. Soc._, Paris, xiv., 1856; E. Retterer, "Contribution a
l'etude du cloaque et de la bourse de Fabricius," _Robin's Journ. de
l'anat. et physiol._, 1885, pp. 369-454, pls. 17-19.