366 DISSECTION OF THE EABBIT. 



backwards along the outer side of the carotid for 

 about half an inch. It then crosses the carotid, 

 about the level of the hinder border of the thyroid 

 cartilage, and runs backwards on the sterno-thyroid 

 muscle, to which as well as to the sterno-hyoid it 

 sends branches. It receives fibres from the first 

 and second spinal nerves. 



The ramus descendens is easily found crossing the carotid 

 about the point named above, and should' be traced backwards 

 along the sterno-thyroid muscle, and forwards to the point at 

 which it leaves the main stem of the hypoglossal. 



3. The pneumogastric or vagus nerve, the tenth cranial, 

 is a stout nerve which leaves the skull by the fora- 

 men lacerum posterius, presenting a very distinct 

 ganglionic swelling near its origin. It runs down- 

 wards and backwards until it reaches the carotid 

 artery, and then straight backwards along the neck, 

 lying immediately to the outer side of, and slightly 

 dorsal to, the carotid artery. Entering the thorax, 

 it runs alongside the oesophagus to the stomach, 

 where it ends. 



Its principal branches are the following : 



Find the pneumogastric nerve in the neck, along the outer 

 side of the carotid artery. Follow it backwards along the 

 neck to the thorax, and forwards to the skull. 



a. The anterior laryngeal nerve is a small nerve, 

 arising from the pneumogastric opposite the 

 upper border of the thyroid cartilage. It runs 

 almost directly inwards, passing dorsal to the 

 carotid artery, and ends in branches distributed 

 to the mucous membrane of the larynx, and to 

 the crico-thyroid muscle. 



The sole difficulty in dissecting the anterior laryngeal nerve 

 lies in the possibility of confusing it with the ramus descendens 

 of the hypoglossal. This is avoided if it be remembered that, 

 while both nerves cross the carotid artery, the ramus descendens 



'^. ^1 



