INTERNAL ANATOMY OF BIRDS. 343 



essential to the health of the bird, hence poultry are given 

 " grit " to eat. This grit should be natural, not sharp needle- 

 hke pieces of flint, such as some poultry merchants advertise, 

 which only damage the walls of the gizzard and do not grind 

 the food. Many inflammatory diseases of the gizzard I have 

 traced to this reprehensible practice. The intestine consists of 

 two parts, the small and large intestine, and ends in the cavity 

 called the cloaca (CL). The large intestine commences where 

 the two long blind tubes originate, the "caecal tubes" (Gae), 

 which are generally present in birds. The cloaca is the cavity 

 which 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. 



flespiratory organs in birds not only consist of lungs, 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 before 

 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 auricles 

 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 reproductive organs, the chief characteristic in 

 birds is that there is only one functional ovary (fig. 180) and 



