The =pulmonary artery= (A. pulmonalis) springs from the conus arteriosus
at the left side of the base of the right ventricle. It curves upward,
backward, and inward, and divides behind the arch of the aorta into
right and left branches. It is related in front to the right auricle
(appendix), behind to the left auricle (appendix), and internally to the
aorta. It is enveloped with the latter in a common sheath of the
visceral layer of the serous pericardium. Near the bifurcation it is
connected with the arch of the aorta by a fibrous band about half an
inch (ca. 1.2 cm.) in width; this is the =ligamentum arteriosum=, a
remnant of the large =ductus arteriosus=, which conducts most of the
blood from the pulmonary artery to the aorta in the fœtus. The artery is
bulbous at its origin, and forms three =sinuses= or pouches, which
correspond to the cusps of the semilunar valve. Beyond this it gradually
diminishes in caliber.
In a horse of medium size the artery is about seven inches (ca. 17 to
18 cm.) long. At the origin it is about two and a half inches (ca. 6
to 6.5 cm.) in width; at the bifurcation its caliber is about one and
a half inches (ca. 3.5 to 4 cm.). The wall is relatively thin,
especially at the origin.
The =right branch= (Ramus dexter) of the pulmonary artery is a little
longer and wider than the left one. It passes over the fore part of the
left atrium and below the bifurcation of the trachea to the hilus of the
right lung, and enters the latter below the right bronchus. In the lung
it passes to the outer and lower side of the stem bronchus and
accompanies it to the base of the organ. The branches correspond to the
ramification of the bronchi. The =left branch= (Ramus sinister) is very
short. It passes backward and enters the lung below the left bronchus.
Its branches within the lung are arranged like that of the right one.