114 THE ANATOMY OF THE HORSE. 



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 aofta. Where the 

 vessel springs from the ventricle it shows, when injected, three 

 bulgiugs, each corresponding to a sinus of Valsalva. From two of 

 these sinuses spring the right and left coronary arteries of the heart. 

 These, which 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 

 backwards 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, until, 

 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-oesophageal Artery. — This vessel will be more con- 

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

 page 119. 



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 borcler 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 (page 100). 



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

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

 smaller of the two is the left axillary artery ; the other is the arteria in- 

 nominata. The direction of the anterior aorta is upwards and forwards, 

 and it is in great part included with 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 arteria innominata, 

 besides supplying the corresponding parts on the opposite side, carries 

 blood for the head. 



