ANATOMY OF BIRDS 15 
assumed by two lateral lips which close the laryngeal opening dur- 
ing the passage of food through the pharynx. The inferior larynx, 
or syrinx, is the voice organ. It is located in the anterior thorax 
at the division of the trachea into two bronchi. The end of the 
trachea, in fowls, is compressed from side to side and forms the 
wall of the syrinx. In certain birds the syrinx is complicated in 
structure and may be formed of a number of tracheal rings ex- 
panded to form large bony cavities. Sound is produced by the 
vibration of membranes within the walls of the lower larynx. 
The trachea is long in birds and its cartilaginous rings are com- 
plete. In the bronchi the cartilaginous rings are incomplete, a 
portion of the wall being formed by a membrane. The bronchi 
enter the lungs at the anterior extremities, become membranous and 
give off branches to these organs and to the air cells of the thorax 
and abdomen. 
The lungs are pale or bright red in color and of a spongy tex- 
ture. They are closely applied to the dorsal surface of the thorax 
and present several deep indentations into which the ribs fit. The 
ventral surface is smooth and is covered by a fold of the diaphragm. 
Only a small portion of the thoracic cavity is occupied by the lungs. 
Air sacs. These are membranous receptacles into which the air is 
carried by terminal branches of the bronchi. Through these branches 
the cells are in direct communication with the lungs. They also, 
with certain exceptions, communicate with canals or spaces in the 
various bones, to which they supply air. The membranous wall 
is composed of two delicate layers, an external serous and an in- 
ternal mucous layer. The latter is a continuation of the mucosa of 
the bronchi, the former is regarded as a reflection of the peritoneum 
and pleura. Nine air sacs are found in the thoracic and abdominal 
cavities. The single anterior thoracic air sac surrounds the inferior 
larynx, bronchi and large blood vessels of the region. It receives 
air from the anterior part of the lungs by two openings and com- 
municates with deep seated air cells of the neck and with the cla- 
vicle, coracoid, scapula and sternum. Two lateral thoracic sacs 
receive air from the internal edge of each lung opposite the base 
of the heart. They transmit air currents to the air cells of the 
axillary region, vertebre, ribs and humerus. The diaphragmatic 
air cells are four in number, two anterior and two posterior. They 
are inclosed between the two diaphragmatic folds. Air is received 
from the lungs but is not communicated to bony structures from 
these. Two abdominal air sacs each receive a long voluminous bron- 
