COUilSE OF THE PNEUMO-GASTRIC NERVE. 550 



part of the spinal accessory nerve, and a second ganglion is formed 

 upon it, the ganglion of the trunlc of the nerve. Several communi- 

 cations are at the same time established with the surrounding nerves. 



The upper ganglion, or ganglion of the root of the pneumo- 

 gastric nerve, situated in the foramen jugulare, is of a greyish colour, 

 nearly spherical, and about two lines in diameter ; it has filaments 

 connecting it with other nerves, viz., with the facial, the petrous 

 ganglion of the glosso-pharyngeal, the spinal accessory, and the 

 sympathetic. 



The lower ganglion, or ganglion of the trunk of the pneumo-gastric 

 nerve, is placed about half an inch beyond the preceding. Occupying 

 the trunk of the nerve outside the skull, it is of a flattened cylindrical 

 form and reddish colour, and measures about ten lines in length and 

 two in breadth. Tiie ganglion does not include all the fibres of the 

 nerve ; the fasciculus, which is sent from the spinal accessory to join 

 the vagus, is the part not involved in the ganglionic substance. It 

 communicates with the hypoglossal, the spinal, and the sympathetic 

 nerves. 



The pneumo-gastric nerve descends in the neck, between and concealed 

 by the internal jugular vein and the internal carotid artery, and after- 

 wards similarly between that vein and the common carotid artery, being 

 enclosed along with them in the sheath of the vessels. As they enter 

 the thorax, the nerves of the right and left side present some points of 

 difference, of which the following are the most important. 



On the right side the nerve crosses over the first part of the right sub- 

 clavian artery, at the root of the neck, and its recurrent laryngeal 

 branch turns backwards and upwards round that vessel. The nerve 

 then enters the thorax behind the right innominate vein, and descends 

 on the side of the trachea to the back of the root of the lung, where it 

 spreads out in the posterior pulmonary plexus. It emerges from this 

 plexus in the form of two cords, which are directed to the oesophagus, 

 and by their union and subdivision on it form, with similar branches of 

 the left side, the oesophageal plexus. Near the lower part of the thorax, 

 the oranches of the nerve, which have thus interchanged fibres with the 

 nerve of the left side, are gathered again into a single trunk, which, 

 descending on the back of the oesophagus, is spread out on the posterior 

 or inferior surface of the stomach. 



On the left side the pneumo-gastric nerve, entering the thorax between 

 the left carotid and subclavian arteries and behind the left innominate 

 vein, lies further forwards than the right nerve, and crosses over the arch 

 of the aorta, while its recurrent laryngeal branch turns up behind the 

 arch. It then passes behind tlie root of the left lung, and, emerging 

 from the posterior pulmonary plexus, is distributed like its fellow on the 

 oesophagus. Inferiorly, it forms a single trunk in front of the oeso- 

 phagus, and is spread out on tiie anterior or superior surface of the 

 stomach. 



There are various circumstances in the distribution of the pneumo-gastric 

 nerves which at first sight ajjpear anomalous, but which are explained by re- 

 ference to the process of development. The recurrent direction of the inferior 

 laryngeal branches in all probability arises from the extreme shortness or rather 

 absence of the neck in the embryo at first, and from the branchial arterial arches 

 having originally occupied a position at, a higher level than the parts in which 

 those branches are ultimately distributed, and havmg di-agged them down as it 



