DISSECTION OF THE BACK AND THORAX. Ill 



passing between the axillary artery and the origin of its inferior 

 cervical branch. Continuing backwards between the layers of the medi- 

 astinum, it crosses the common trunk of the dorsal, superior cervical, 

 and vertebral veins, and the pericardium at the level of the common 

 aorta. Behind the heart it passes under the root of the lung, through 

 the posterior mediastinum, and is distributed to the left half of 

 the diaphragm (muscular rin) and pillar), of which it is the motor nerve. 



The left Pnbumogastbic, Vagus, or 10th Cranial Nerve. At the 

 entrance to the chest this nerve lies on the trachea, at the upper edge 

 of the cephalic trunk, and a little below the sympathetic. It crosses 

 in beneath the arch of the left axillary, in company with a cardiac nerve. 

 It is continued backwards across the angle of separation between the 

 anterior and posterior aortse ; and, crossing the root of the latter vessel, 

 it reaches the root of the lung, where it divides. The upper division is 

 continued backwards to fuse above the oesophagus with the corresponding 

 branch from the right vagus, this fusion taking place about the middle 

 of the posterior mediastinum. The lower division unites in the same 

 way with a branch from the nerve of the opposite side, the fusion 

 taking place on the left bronchus. The resulting nerves are termed 

 the superior and inferior oesophageal nerves, and they are continued 

 backwards, the one above, and the other below, the gullet, giving 

 branches to it and accompanying it through the foramen sinistrum of 

 the diaphragm.^ 



In this part of its course the vagus detaches the following branches: — 



1. A Branch of Communication with the middle cervical ganglion of 

 the sympathetic (or with the inferior ganglion when the middle is not 

 developed). It is given oft' within the 1st rib. 



2. The left Inferior {Recurrent) Laryngeal Nerve. — This is detached at 

 the root of the posterior aorta ; and, turning round behind the vessel at 

 that point, it gains its inner side, to be included between the artery and 

 the left bronchus, where it receives twigs from the cardiac nerves. It 

 then passes forwards along the lower face of the trachea, in company 

 with a cardiac nerve ; and, issuing from the chest, it is continued up 

 the neck to the larynx. As it is included between the aorta and left 

 bronchus, it is related to the bronchial lymphatic glands. Within the 

 thorax the nerve gives branches that pass upwards and forwards to the 

 trachea and oesophagus. The left recurrent is the nerve implicated in 



"roaring." 



3. Pulmonary Branches.— These form at the root of the lung a plexus 

 from which filaments are continued into the lung along the ramifications 

 of the air tube. 



1 The superior oesophageal nerve is generally, if not always, larger than the inferior; 

 and in most cases the upper nerve is formed in greater proportion by the left vagus than 

 by the'right, while the lower is formed about equally from each. 



