74 THE SURGICAL ANATOMY OF THE HORSE 



descends below the longus colli muscle to the entrance to the chest, 

 where it presents a slight bend, and passes into the thorax between the 

 two first ribs. In the chest it is placed between the layers of the 

 anterior mediastinum, and continues its course backwards until it 

 arrives at the heart, above the left auricle of which it is placed to 

 the right of the posterior aorta. Here it divides into the right 

 and left bronchi. 



In the neck it is related superiorly to the oesophagus and longus 

 colli muscle, and inferiorly it is clothed by the sterno-thyro-hyoideii 

 muscles. Its lateral aspect is crossed obliquely upwards and forwards 

 by the sterno-maxillaris muscle and downwards and forwards by the 

 subscapulo-hyoideus. Its left side is also related in the inferior 

 portion of the neck to the oesophagus, which here leaves the middle 

 line and deviates towards the side of the trachea. 



The scalenii muscles clothe its lateral aspects near the root ot 

 the neck. 



In the chest the inferior cervical ganglia of the sympathetic lie on 

 its lateral surfaces, which are also crossed by the dorsal and superior 

 cervical arteries on the left side, and on the right side by the common 

 trunk of these vessels. These surfaces are also crossed by the veterbral 

 arteries. 



Above the thoracic portion is the oesophagus and also the longus 

 colli muscle. Inferiorly the trachea crosses the angle formed by the 

 anterior and posterior divisions of the aorta, where it lies over the 

 anterior vena cava. Immediately in front of its bifurcation it is 

 crossed obliquely downwards on its right side by the great azygos 

 vein and frequently also by the thoracic duct, and the oesophagus 

 here takes a downward course between the trachea and the posterior 

 aorta. 



The somewhat elliptical outline of the trachea will be apparent 

 if a transverse section be made, and such a section will also reveal the 

 presence of the trachealis muscle which consists of fibres running 



