TEE CRANIAL OR ENCEPHALIC NERVES. 839 



of the principal cardiac nerves, with some of which it contracts intimate 

 relations. 



On the left side, it is only when the pneumogastric nerve arrives near the 

 root of the Imig that it gives off its recurrent. To be reflected forward, the 

 latter turns from left to right behind the arch of the aorta, and arrives beneath 

 the inferior face of the trachea, among the cardiac nerves, with which it com- 

 municates, like the right. 



The inferior laryngeal nerves are in this way mixed, at a greater or less dis- 

 tance from their commencement, with those branches of the sympathetic (see 

 the description of the si/mpathetic) which collectively constitute the tracheal 

 plexus. They are soon disengaged, however, and leave the chest, but always in 

 proceeding along the inferior face of the trachea, then ascending on its sides, 

 below the carotid arteries, which they gradually approach, and finally attain the 

 larynx in penetrating beneath the crico-pharyngeal muscle. 



According to Goubaux, the left recurrent must be situated more superficially 

 than the right in the lower part of the neck, and for this reason should be more 

 exposed to compression. He thus explains why, in chronic "roaring," the 

 alterations observed are nearly always in the left muscles of the larynx. 



The terminal divisions of the recurrents are distributed to the posterior and 

 lateral crico-aryttenoid, and the arytainoid and thyro-arytasnoid muscles, as well 

 as to the subglottic mucous membrane. 



In their long course, they emit collateral ramuscules that also ascend, and 

 are distributed to the mucous membrane and muscular layer of the trachea, as 

 well as to that of the oesophagus.' 



The recurrent msophageal ramuscules are all sensitive, and form five groups : 

 the first and second arise, on the left side, close to the arch of the aorta, ascend 

 on the sides of the trachea, and pass to the portion of the oesophagus lying 

 between that tube and the thoracic section of the longus colli muscle. The 

 third, more considerable, springs from a branch given off at the brachial trunks ; 

 it is markedly recurrent, and forms on the side of the trachea, with the second, 

 a small plexus, and then lies closely alongside the oesophagus, which it accom- 

 panies for a distance of eight inches from the first rib. The branch constituting 

 the fourth group is the longest of all ; it is detached about two or two and a half 

 inches in front of the first rib, and after emitting several long tracheal filaments, 

 it ascends on the side of the trachea, close to the border of the oesophagus, where 

 it generally disappears at about six or eight inches from the pharynx. The fifth 

 group is composed of a branch detached from the recurrent, about the point 

 where the preceding terminates ; it goes entirely to the origin of the tube. 



6. Cardiac Filaments. — These will be alluded to when describing the 

 sympathetic nerves. 



Terminal Branches of the Pneumogastric Nerves. 1. Bronchial 

 Plexus (Fig. 480, 29). — The bronchial plexus is formed by several branches 

 from the pneumogastric, on the arrival of that nerve above the roots of the 

 lungs ; these interlace into a network and ramify around the bronchi, following 

 these into the texture of the lungs. It is these branches which give to the 

 mucous membrane its great sensibility, and which provoke those evident con- 

 tractions of which the bronchioles are the seat. 



2. (Esophageal Branches (Fig. 480, 30, 31).— After emitting the ramus- 



(' For a detailed description of the superior and inferior laryngeal nerves, see my treatise 

 on "■Roaring in Horses." London : 1889.) 



