DISSECTION OF THE DOG 59 



The branches derived from each vagus nerve while within the chest are 

 cardiac nerves, a recurrent nerve for the larynx, bronchial branches for the 

 lungs, and small branches for the oesophagus. 



N. recurrens. — The right recurrent nerve turns round either the 

 commencement of the right subclavian or the termination of the brachio- 

 cephalic artery, gains the side or the right ventral border of the trachea, and 

 passes to the thoracic inlet dorsal to the right carotid artery. The left nerve 

 winds round the aortic arch, and lies at first along the ventral border of the 

 trachea. At the entrance to the chest it is placed in a groove formed by the 

 trachea and oesophagus. Pulmonary and cardiac branches (rami pulmonales 

 et rami cardiaci) arise from the recurrent nerves. Bronchial branches (rami 

 bronchiales) leave the vagus just aboral to the aortic arch, and enter the lung 

 along with the other constituents of the root. 



(Esophagus. — The oesophagus enters the chest along with and to the left 

 of the trachea. This relationship is maintained for some distance, but later 

 the oesophagus, inclining to the right, crosses the left bronchus and'so reaches 

 the median plane. After this there is a very slight deviation again to the left. 

 The aortic arch traverses the left face of the oesophagus, and the thoracic aorta 

 runs for a short distance in the same relation, but gradually becomes dorsal. 

 The gullet and the vagus nerves are intimately related the one to the other. 



Dissection.— Cut across the oesophagus a short distance before it pierces the 

 diaphragm, and turn the cut ends as far apart as possible. Some idea of 

 the structure of the tube may be obtained from an examination of the cut 

 surface. Now clean the thoracic aorta and the vena azygos, taking 

 particular care not to destroy the narrow, thin-walled thoracic duct 

 which lies between them. 



Structure of the (esophagus. — A naked-eye examination of a transverse 

 section of the oesophagus shows that, like other tubular parts of the alimentary 

 system, its wall is constructed in layers. On the outside of the tube is a loose 

 areolar covering by which, in the thorax, the mediastinal pleura is attached to 

 the oesophagus. A considerable proportion of the thickness of the wall is formed 

 of muscular tissue (tunica muscularis), red in colour in the fresh state and with 

 fibres coursing in various directions. The interior of the oesophagus is lined by 

 a thick, pale mucous membrane (tunica mucosa), thrown into longitudinal 

 folds when the tube is not distended. The mucous membrane is attached to the 

 muscular coat by a loose submucous tissue (tela submucosa). 



Aorta. — The thoracic part of the aorta as it continues the aortic arch 

 onwards, is at first in contact with the longus colli muscle and decidedly to the 

 left of the median plane ; but, gradually inclining to the right as the diaphragm 

 is approached, it finally travels along the middle plane of the body. 



The oesophagus is at first to the right of the aorta, but later it becomes ven- 

 tral in position. To the right also are the vena azygos and the thoracic duct. 



