14 DISEASES OF DOMESTICATED BIRDS 
gizzard is a tough horn-like covering formed by the hardening of 
the products secreted by the underlying simple glands. 
The mucous membrane of the intestine presents innumerable 
cylindrical elevations known as villi. It is through these that the 
digested food is assimilated and carried by means of the chyle- 
vessels or lacteals to the portal vein and thence to the liver. Nu- 
merous goblet cells for the secretion of mucin are present in the 
villi. 
Liver. This organ is dark brown in color and composed of a 
right and left lobe, the former being the larger. The right lobe 
has a gall bladder on its ventral surface, while the bile from the 
left lobe is poured directly into the duodenum through a separate 
bile duct, the hepatic or choledic duct, which empties a short dis- 
tance behind the two pancreatic ducts. The cystic duct opens caudad 
of the hepatic duct and drains the gall bladder. 
Pancreas. This organ is long, narrow and lobulated. In color 
it is pale red. It is highly developed in birds and occupies the 
space between the two branches of the duodenum. Two or some- 
times three ducts carry its secretion into the ascending arm of the 
duodenum near its extremity and just anterior to the hepatic duct. 
RESPIRATORY SYSTEM 
The organs of respiration in birds differ markedly from those of 
mammals. They resemble more nearly the reptilian type of 
formation. 
Air passages. The nostrils are represented by two narrow 
openings at the base of the upper mandible of the beak. The nasal 
chambers are short and narrow and are separated by a septum which 
is partly bony and partly cartilaginous. The turbinated bones of 
mammals are represented by three cartilaginous structures. The 
nasal cavities open into the mouth by a common, elongated slit in 
the median line of the hard palate. Cavities known as the sub- 
orbital or infraorbital sinuses occupy a position on either side of 
the head. They communicate with the nasal cavity by narrow 
passages which extend from a comparatively low level in the sinus, 
upward into the nasal cavity. The arrangement prevents natural 
drainage of the sinuses. 
Birds possess an upper and a lower larynx. The former cor- 
responds to the larynx of mammals but serves only as an opening 
for the passage of air. An epiglottis is absent but its function is 
