THORAX AND ABDOMEN OF THE HORSE 47 



intimate contact with the right and left sympathetic cords, and enter 

 the thorax between the subclavian artery and vein of their own side. 

 In the precardial and cardial mediastina the two nerves have different 

 relations. 



The right nerve passes underneath the arch formed by the sub- 

 clavian artery, and then traverses obliquely the right face of the brachio- 

 cephalic artery and the trachea. This brings it into contact with the 

 oesophagus where a division into dorsal and ventral branches (ramus 

 dorsalis et ramus ventralis) takes place. Close to the point of division 

 the nerve is crossed to the right by the vena azygos. 



The left nerve enters the thorax related to the oesophagus, passes 

 underneath the arch of the left subclavian artery, crosses the left face of 

 the common brachio-cephalic trunk and the aortic arch and the dorsal 

 face of the left bronchus, and divides into two branches — dorsal and 

 ventral. 



The dorsal branches of the two vagus nerves run along the dorsal 

 border of the oesophagus to the level of the twelfth or thirteenth thoracic 

 vertebra, where they unite to form the dorsal oesophageal trunk 

 (truncus oesophageus dorsalis), which accompanies the oesophagus 

 through the diaphragm into the abdomen. The termination of the 

 trunk in association with the stomach, &c., is examined during the 

 dissection of the abdomen. It should be noted that the right vagus 

 contributes more fibres to the dorsal oesophageal trunk than does the 

 left nerve. 



A ventral oesophageal trunk (truncus oesophageus ventralis) is in 

 like manner formed by the union of the ventral branches of the two 

 vagus nerves, but this is effected earlier, namely, close to the bifurcation 

 of the trachea. 



Both dorsal and ventral branches of the vagus send filaments (rami 

 bronchiales) to the pulm.onary plexus (plexus pulmonalis), a network 

 of small nerves, dorsal and ventral to the tracheal bifurcation, formed by 

 twigs from the caudal cervical sympathetic ganglia and some of the 

 thoracic ganglia. 



The branches derived from the thoracic part of each vagus are a 

 recurrent nerve, and cardiac, tracheal and oesophageal rami. 



The tracheal and oesophageal branches (rami tracheales, rami 

 cesophagei) concur in the formation of the tracheal and oesophageal 

 plexuses, to which the recurrent nerves and sympathetic ganglia 

 contribute branches. 



The cardiac rami (rami cardiaci) of the vagus, generally two or three 

 from each nerve, pass to the cardiac plexus (plexus cardiacus), to which 

 the sympathetic and recurrent nerves contribute. From the plexus 



