MANUAL OF ZOOLOGY. 



The heart in all Birds consists of four chambers, two auricles 

 and two ventricles. The right auricle and ventricle, constitut- 

 ing the right side of the heart, are wholly concerned with the 

 pulmonary circulation ; the left auricle and ventricle, forming 

 the left side of the heart, are altogether occupied with the 

 systemic circulation; and no communication normally exists 

 i in adult life between the two sides of the heart. In all essen- 

 ; tial details, both as regards the structure of the heart itself 

 I and the course taken by the circulating fluid, Birds agree with 

 Mammals. The venous blood namely, that which has circu- 

 lated through the body is returned by the venae cavae to the 

 right auricle, whence it is poured into the right ventricle. The 

 right ventricle propels it through the pulmonary artery to the 

 lungs, where it is aerated, and becomes arterial. It is then 

 sent back by the pulmonary veins to the left auricle, whence it 

 is driven into the left ventricle. Finally, the left ventricle pro- 

 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 circulation is, that in the Birds the two sides 

 of the heart are completely separated frorJI 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 consid- 

 erable 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 along- 

 side of the ureters. A male organ (penis] may or may not 

 be present, but there is no perfect urethra. The female bird 



