194 OUTLINES OF EQUINE ANATOMY. 



best by laving open tbe left side of the thorax. From the 

 base of the heart, between the two ventricles the aorta 

 passes upwards for about two inches. It is a large vessel 

 with stout elastic walls, and gives off the coronary arteries 

 to the substance of the heart in a manner which will be 

 hereafter described. It terminates superiorly in the anterior 

 and posterior aortge. The terminal portion of the trachea 

 lies in contact with its right side, while on the left is the 

 pulmonaiy artery and also the left pneumogastric, which on 

 passing gives off its recurrent branch, coursing from without 

 inwards around the posterior margin of the aorta, and 

 running forwards and up the neck to supply all the mus- 

 cles of the larynx with the exception of the crico-thyroideus. 

 In consequence of this arrangement many authorities con- 

 sider that pressure upon this nerve durmg the heart's 

 action, by producing atrophy of the muscles it supplies, 

 causes that defect in respiration termed " roaring." 



The anterior aorta, after coursing for a short distance 

 through the anterior mediastinum, situated below and 

 slightly to the left of the oesophagus and trachea, terminates 

 in forming the two arterise innominatse, the right of which is 

 much the largest. The right arteria innominata runs in 

 a forward direction as far as the first rib. The first branch 

 it gives off superiorly is the common root of the anterior 

 dorsal and lyosterior cervical arteries (which have been already 

 noticed) ; around this from without inwards the recurrent 

 branch of the right pneumogastric winds prior to passing 

 uj) the neck. These vessels arise se23arately on the left side, 

 for, as just noticed, the recurrent branch of that side winds 

 round the aorta. From its anterior part the right arteria 

 innominata gives off the vertebral, inferior cervical, axillary, 

 and common carotid arteries. The left arteria innominata 

 gives off corresponding branches with the exception of the 

 common carotid, and from the inferior part of each vessel is 

 given off an internal thoracic artery, which runs along the 

 floor of the thorax at the lateral j^art of the superior sur- 

 face of the sternum. It gives off branches outwards to 

 anastomose with the external pectoral and intercostal 

 branches, and opposite the ensiform cartilage divides, one 

 of its branches running along the floor of the abdomen to 

 anastomose with the epigastric artery ; the other along the 

 inferior margins of the false ribs, anastomoses with the 

 terminal extremities of the posterior intercostal arteries, 



