THE ARTERIES—THE PULMONARY ARTERY—THE SYSTEMIC ARTERIES 629 
through stomata with the cavity of the pericardium. There is a less distinct 
subendocardial network. The vessels converge usually to two trunks, which ac- 
company the blood-vessels in the grooves and enter the glands at the bifurcation 
of the trachea. The nerves are derived from the vagus and sympathetic through 
the cardiac plexus. 
THE ARTERIES 
THE PULMONARY ARTERY 
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 medi- 
ally, and divides behind the arch of the aorta into right and left branches. It is 
related in front to the right auricle, behind to the left auricle, and medially 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 em.) in width; this is the liga- 
mentum arteriosum (Fig. 542), a remnant of the large ductus arteriosus, which 
conducts most of the blood from the pulmonary artery to the aorta in the foetus.! 
The artery is bulbous at its origin, and forms three pouches, the sinuses of the 
pulmonary artery, which correspond to the cusps of the pulmonary valve. Beyond 
this it gradually diminishes in caliber. 
In a horse of medium size the artery is about seven inches (ca. 17-18 em.) long. At the 
origin it is about two and a half inches (ca. 6-6.5 cm.) in width; at the bifurcation its caliber 
is about one and a half inches (ca. 3.5-4 cm.). The wall is relatively thin, especially at the 
origin. 
The right branch (Ramus dexter) of the pulmonary artery is longer and a little 
wider than the left one. It passes over the forepart of the left atrium and 
under 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 ventro-lateral 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 
THE SYSTEMIC ARTERIES 
The aorta is the main systemic arterial trunk. It begins at the base of the left 
ventricle and is almost median at its origin. Its first part, the ascending aorta 
(Aorta ascendens), passes upward and forward between the pulmonary artery on 
the left and right atrium on the right. It then curves sharply backward and dor- 
sally, inclmes somewhat to the left, forming the arch of the aorta (Arcus aortz), 
and reaches the ventral surface of the spine at the eighth or ninth thoracic vertebra. 
After passing backward along the ventral aspect of the bodies of the vertebre and 
between the lungs, it traverses the hiatus aorticus and enters the abdominal cavity, 
where it lies ventral to the vertebral bodies and the psoas minor, just to the left of 
the median plane. It divides under the fifth lumbar vertebra into the two internal 
iliac arteries. 
From the bifurcation a small vessel, the middle sacral artery (A. sacralis media), sometimes 
passes backward on the pelvic surface of the sacrum. It becomes lost in the periosteum or joins 
the coceygeal artery, or in exceptional cases is traceable to the sphincter ani externus. 
The caliber of the aorta is greatest at its origin, which is termed the bulbus 
_' There is a depression in the interior of the artery which corresponds to the attachment of 
the lig. arteriosum, and in some cases the latter has a very small lumen. 
