622 THE CRANIAL NERVES. 



it divides into branches, which, conjointly with others derived from the 

 glosso-pharyngeal, the superior laryngeal, and the sympathetic nerves, form 

 a plexus (pharyngeal) behind the middle constrictor of the pharynx. From 

 the plexus branches are given to the muscular structure, and to the mucous 

 membrane of the pharynx. As the pharyngeal nerve crosses the carotid 

 artery, it joins filaments which the glosso-pharyngeal distributes on the same 

 vessel. There is sometimes a second pharyngeal branch. 



SUPERIOR PHARYNGEAL BRANCH. 



This nerve springs from the middle of the ganglion of the trunk of the 

 pneumo-gastric nerve. It is directed inwards to the larynx beneath the 

 internal carotid artery, and divides beneath that vessel into two branches, 

 distinguished as external and internal laryngeal, both of which ramify in 

 the structures of the larynx. 



The external laryngeal branch, the smaller of the two divisions, gives 

 backwards, at the side of the pharynx, filaments to the pharyugeal plexus 

 and the lower constrictor muscle ; and it is finally prolonged beneath the 

 muscles on the side of the larynx to the crico thyroid muscle in which it 

 ends. In the neck this branch joins the upper cardiac nerve of the 

 sympathetic. 



The internal laryngeal branch is continued to the interval between the 

 hyoid bone and the thyroid cartilage, where it perforates the thyro-hyoid 

 membrane with the laryngeal branch of the superior thyroid artery, and dis- 

 tributes filaments to the mucous membrane : some of these are directed up- 

 wards in the aryteno-epiglottidean fold of mucous membrane to the base of the 

 tongue, the epiglottis, and the epiglottidean glands; while others are reflected 

 downwards in the lining membrane of the larnyx, extending to the corda 

 vocalis, on the inner side of the laryngeal pouch. A slender communicating 

 branch to the recurrent laryngeal nerve descends beneath the lateral part of 

 the thyroid cartilage. A branch enters the arytenoid muscle, some fila- 

 ments of which seem to end in the muscle, while others proceed through 

 it to the mucous membrane. 



RECURRENT LARYNGEAL BRANCH. 



The recurrent or inferior laryngeal branch of the vagus nerve, as the 

 name expresses, has a reflex course to the larnyx. 



The nerve on the right side arises at the top of the thorax, winds round 

 the subclavian artery, and passes beneath the common carotid and inferior 

 thyroid arteries in its course towards the trachea. On the left side the re- 

 current nerve is bent round, below and behind the arch of the aorta, imme- 

 diately beyond the point where the obliterated ductus arteriosus is con- 

 nected with the arch, and is thence continued upwards to the trachea. 



Each nerve in its course to the larynx is placed between the trachea and 

 oesophagus, supplying branches to both tubes ; and each, while making its 

 turn round the artery, gives nerves to the deep cardiac plexus. At the 

 lower part of the cricoid cartilage the recurrent nerve distributes branches 

 to supply all the special muscles of the larynx, except the crico-thyroid 

 muscle, which is supplied from the upper laryngeal nerve. It likewise 

 gives a few offsets to the mucous membrane, and a single communicating 

 filament which joins the long branch of the upper laryngeal nerve beneath 

 the side of the thyroid cartilage. 



