210 ELEMENTS OF ORNITHOLOGY. 
the ureters. In Water Birds, such as the Drake, there is a 
special intromittent organ, spirally arranged, which can be pro- 
truded from the cloaca or retracted within it, as the finger of a 
glove may be everted and the reverse. 
In a few Birds, such as the Ostrich, a more solid organ of 
the kind, grooved beneath, is attached to the front wall of the 
cloaca. 
The ovary, or essential female organ, is generally single ; its 
companion aborting. It is usually the right ovary which atro- 
phies. This female organ has somewhat the appearance of a 
small bunch of grapes, the grape-like structures being the more 
or less developed eggs. The duct—ovzduct—which conveys the 
eggs outwards is widely open at its upper or anterior end. 
The more posterior part of its interior is lied with long deli- 
cate processes, or close-set villi, which secrete the material of 
the egg-shell. Posteriorly the oviduct opens into the cloaca. 
The eggs or ova will be further noticed when we come to 
speak of the development of Birds. 
THE RESPIRATORY SYSTEM. 
This system is wonderfully developed in Birds which, as they 
are specially modified to move in the air external to them, are 
also specially modified to receive air extensively within them, 
as has been already stated *. 
Birds breathe by the alternate approximation and separation 
of the sternum and the back. Their separation tends to produce 
a vacuum, and causes air to rush into the body, while their 
approximation expels it by contracting the space into which it 
has been received. This movement is greatly facilitated by the 
joints which exist at the junction of the vertebral and sternal 
ribs f. 
Although this action is called “breathing,” respiration or - 
breathing really consists in the purification of the blood by the 
elimination of carbonic-acid gas and the absorption of oxygen. 
Air is introduced into the body through the “ glottis,” 
which is the external aperture of the windpipe or trachea. The 
uppermost part of the trachea is slightly dilated, and contains 
parts which correspond with those which exist in a dilatation 
* See ante, p. 167. t See ante, p. 176. 
