374 SAUROPSiDA (birds). 



{(kie), which are generally jDreseiit in birds. The cloaca is the 

 cavity wliioli receives the rectum, ureters, and sexual ducts, 

 there being no distinct sexual openings in the class Aves. 

 Salivary glands open into the mouth, the liver {L), and pancreas 

 (P) into the small intestine near the gizzard. 



Rvnjiiratory onjcaiy in birds not only consist of l«ngs, but also 

 of " air-sacs " and spaces in the bones. The lungs differ from 

 those of mammals by not being freely suspended in the pleural 

 cavity : they are two in number, spongy, and bright red in 

 colour. The air-sacs are prolongations of the lining membrane 

 of the bronchi, which spring from the bronchial tubes just be- 

 fore they enter the lung. These air-receptacles penetrate the 

 thorax and abdominal cavity. They become filled with air prior 

 to the bird taking flight, and thus reduce its specific gravity, 

 at the same time bringing air in direct contact with the blood 

 in other parts of the body besides the lungs. The air-sacs are 

 continued in adult birds into the bones. In young birds the 

 pneumatic bones do not exist to the same extent. 



The heart in birds consists of four chambers, two auriclet^ 

 and two ventricles. The heart essentially agrees with that of 

 mammals, as does the general circulation of the blood, so no 

 further reference need be made to this subject. 



There is no urinary bladder in birds : the kidneys are two in 

 number and elongated ; their ducts open into the cloaca. In 

 regard to the repi-odiictire organs, the chief characteristic in 

 birds is that there is only one functional ovary (fig. 214) and 

 Fallopian tube in the female ; this single ovary is that of the 

 left side. I have sometimes in the fowl found the right ovary 

 more or less developed. The oviduct is a. long tortuous tube in 

 which the ovum receives first the " white ' or albuminous cover- 

 ing of the yolk. Towards the end of the egg-tube dilatations 

 appear : in these the eggs recei^•e the shell, which consists of 

 carbonate of lime. A deficiency of this in the bird's system 

 results in so-called "soft eggs." The egg is then passed into 

 the cloaca, and so out of the vent. The eggs of birds are hatched 



