210 ELEMENTS O! OENITHOLOeX. 



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 wary, 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 — oviduct — which conveys the 

 eggs outwards is widely open at its upper or anterior end. 

 The more posterior part of its interior is lined 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 Eespieatoey 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 t. 



Altliough 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 tradiea, 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 a7ite, p. 176. 



