438 MANUAL OF ZOOLOGY. 



pels the aerated blood to all parts of the body through the 

 great systemic aorta. 



The chief difference between Birds and Reptiles as regards 

 the course of the circulation is, that in the Birds the two sides 

 of the heart are completely separated from one another, the 

 blood sent to the lungs being exclusively venous, whereas that 

 which is sent to the body is exclusively arterial. In Reptiles, 

 on the other hand, the pulmonary and systemic circulations 

 are connected together either in, or in the immediate neigh- 

 bourhood of, the heart; so that mixed venous and arterial 

 blood is propelled both through the lungs and through every 

 part of the body. 



In accordance with their extended respiration and high mus- 

 cular activity, the complete separation of the greater and lesser 

 circulations, and the perfect structure of the heart, Birds main- 

 tain a higher average temperature than is the case with any 

 other class of the Vertebrata. This result is also to a consider- 

 able extent conditioned by the non-conducting nature of the 

 combined down and feathers which form the integumentary 

 covering of Birds. 



The urinary organs of Birds consist of two elongated kid- 

 neys and two ureters, but there is no urinary bladder. The 

 ureters open into the cloaca, or into a small urogenital sac 

 which communicates with the cloaca. 



As regards the reproductive organs, the males have two testes 

 placed above the upper extremities of the kidneys, and their 

 efferent ducts (vasa deferentid] open into the cloaca alongside 

 of the ureters. A male organ (penis] may or may not be pre- 

 sent, but there is no perfect urethra. The female bird, as a 

 general rule, is provided with only one ovary and oviduct 

 that of the left side the corresponding organs of the right side 

 being rudimentary or absent. The oviduct is very long and 

 tortuous, and the egg, during its passage through it, receives 

 the albuminous covering which serves for the nutrition of the 

 embryo, and which is known as the " white " of the egg. The 

 lower portion of the oviduct is dilated, and the egg receives 

 here the calcareous covering which constitutes the " shell." 

 Finally, the oviduct debouches into the cloaca, into which the 

 egg, when ready, is expelled. The further development of the 

 chick is secured by the process of " incubation " or brooding, 

 for which birds are peculiarly adapted, in consequence of the 

 high temperature of their bodies. 



The development of the ovum belongs to physiology, and 

 does not concern us here. It is sufficient to notice the means 

 by which in many cases the chick is ultimately enabled to escape 



