6 DISEASES OF DOMESTICATED BIRDS 
of mammals. It is principally membranous with fine muscular 
bundles attached to the ribs. Two thin, delicate membranes com- 
pose the diaphragm and divide it into two planes, a pulmonary and 
a thoracic-abdominal plane. The former is spread over the in- 
ferior face of the lungs and is attached to the ribs on its border. It 
assists in respiration. The thoracic-abdominal plane has its origin 
on the dorsal spine in common with the pulmonary plane. It is 
convex anteriorly and extends to the sternum, dividing the body 
cavity into a thoracic and an abdominal cavity. It also assists in 
respiration and is particularly concerned with the abdominal air 
sacs. 
The pectineus muscle assists in the mechanical flexion of the toes 
in the act of perching on the roost. It originates on a tuberosity 
of the hip bone in front of the acetabulum and is continued by a 
long thin tendon to a short distance distad from the head of the 
tibia where it joins the flexor perforatus of the three anterior digits. 
CIRCULATORY SYSTEM 
The heart of birds is cone shaped, short and wide in the ostrich, 
elongated and possessing a more acute apex in the common fowl. It 
is placed in the anterior extremity of the thoracic cavity. Three 
veins, two anterior vense cave and one posterior vena cava, terminate 
in the sinus of the right auricle. A peculiarity of the heart of 
birds is the absence of a tricuspid valve at the right auriculo-ven- 
tricular opening. Instead, a strong muscular fold which becomes 
closely applied to the ventricular septum during systole prevents re- 
gurgitation of blood into the auricular cavity. Two pulmonary veins 
empty into the left auricle. The left auriculo-ventricular opening 
is provided with a bi-cuspid or mitral valve. The pulmonary artery 
and aorta each has a semilunar valve. 
Arteries. The aorta gives off two innominate arteries, right and 
left brachio-cephalic, on leaving the heart and then passes over the 
right bronchus to become the descending or dorsal aorta. It con- 
tinues along the spine, giving off numerous branches, to the coccygeal 
region where it separates into three terminal arteries. 
The two carotids leave the innominate arteries and curve toward 
the ventral median line of the neck where they ascend in close prox- 
imity to each other to near the head. At the angle of the jaw each 
carotid divides into the external and internal carotid arteries. The 
arteries, especially those of the head and viscera, are distinguished 
