THE PULMONARY ARTERY THE SYSTEMIC ARTERIES 535 



smooth and glistening and is formed by a layer of endothelial cells. The latter 

 rest on a thin layer of connective tissue, which is connected with the myocardium 

 by a subendocardial elastic tissue containing vessels and nerves. 



Vessels and Nerves. — The heart receives its blood-supply through the two 

 coronary arteries which arise from the aorta opposite to the anterior and left cusps 

 of the aortic \alve. Most of the blood is returned by the coronary veins, which 

 open into the right atrium by the coronary sinus. ^ A few small veins open directly 

 into the right atrium, and others are said to open into the left atrium and the 

 ventricles. The lymph-vessels form a subepicardial network which conmiunicates 

 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 atrio-ventricular 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 

 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 })y a fibrous l)and al^out 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 

 foetus. 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 an<l a lialf inches (ca. 6 to 6.5 cm.) in width; at the bifurcation its cahber 

 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 l)ranches 

 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 ai'terial trunk. It begins at the base of the left 

 ventricle and is practically median at its origin. It passes upward and slightly 

 forward between the pulmonary artery on the left and right atrium on the right. 

 It then curves sharply backward and upward and inclines somewhat to the left, 

 forming the arch of the aorta ( Arcus aortse) , 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 vertebrae between the lungs it traverses the hiatus 



^ These vessels will be described later in their systematic order. 



