112 THE ANATOMY OF THE HORSE. 



fa;t\is brings the two arteries into comnmnication. The puhiionary 

 artery conveys venous blood to the lungs to be purified. 



The Common Aorta. This is the primary trunk of the syxtemic 

 arteries. It is of great calibre, but not more than three inches in length. 

 It springs from the left ventricle, and divides into two unequal vessels — 

 the anterior and the posterior aorta. Whei-e the vessel springs from 

 the ventricle, it shows, when injected, three bulgings, each corresponding 

 to a dnus of Valsalva. From two of these sinuses spring the right and 

 left coronary arteries of the heart. These, wliich are the first collateral 

 branches of the arterial tree, are described with the heart. 



The Posterior Aorta is by far the longer of the two terminal branches 

 of the common trunk, and it has also the greater calibre. It passes 

 backwai'ds and upwards, describing a curve — the arch of the aorta, and 

 reaches the spine at the 10th dorsal vertebra. From that point it is 

 continued backwards along the vertebral bodies, being at first a little to 

 the left of the middle line ; but it gradually inclines to the right, imtil, 

 at the 14th dorsal vertebra, it lies almost entirely to the right of the 

 median plane of the body. It passes into the abdominal cavity through 

 the hiatus aorticus — an opening between the pillars of the diaphragm. 

 The arch of the vessel is crossed to the right by the oesophagus, and by 

 the termination of the trachea. The remaining portion of the artery 

 is related on its right to the thoracic duct and vena azygos, the duct 

 being usually between the vein and artery, but sometimes to the left of 

 the latter. The thoracic branches of the posterior aorta are : — 



1. The Broncho-desojyhageal Artery. — This vessel will be more con- 

 veniently dissected with the right side of the chest. It is described at 

 page 118. 



2. Intercostal Arteries. — The last thirteen of these generally have this 

 origin. They spring from the upper aspect of the artery, and pass over the 

 vertebral bodies, crossing beneath the dorsal cord of the sympathetic to 

 gain the upper end of an intercostal space. Here each gives off a large 

 dorso-spinal branch, and places itself at the posterior border of a rib, 

 along which it descends. The latter part of the intercostals and their 

 dorso-spinal branches have already been followed in the dissection of the 

 chest-wall and back. 



The Anterior Aorta. This vessel, after a course of not more than 

 three inches, divides into two vessels of unequal size. The left and smaller 

 of the two is the left axillary artery ; the other is the arteria innomin- 

 ata. The direction of the anterior aorta is oblique upwards and for- 

 wards, and it is in great pai-t included within the pericardial sac. It has 

 no collateral branches of a size meriting description. Of its terminal 

 branches only the left axillary will be followed now The left axillary 

 is the vessel for the supply of the neck, the fore limb, and the subjacent 

 part of the chest-wall on the left side ; while the arteries innominata, 



