The Thorax. 263 



(d) The trachea divides at its posterior end into two portions, the 

 right and left bronchi, one for each lung. Each bronchus 

 is again divided into smaller portions, the bronchial rami, 

 which penetrate the substance of the organ and redivide into 

 smaller tubes. On the right side a small eparterial 

 bronchus is given off from the right bronchus. It passes 

 to the superior lobe of this side, and lies in front of the right 

 pulmonary artery. 



(e) The branches of the pulmonary artery and the pulmonary 

 veins may be traced for a short' distance on the medial por- 

 tion, or hilus, of each organ. 



(f) The vagus nerve passes to the dorsal side of the bronchus, 

 giving off a number of branches, which accompany the 

 bronchus to the lung. 



The lungs may be removed, together with a portion of the 

 trachea, care being taken to leave the vagus nerves in place. The 

 lungs may then be examined to better advantage, and the surface 

 also prepared for the next dissection. 



5. The following structures may now be traced on the dorsal 

 wall of the thorax: 



(a) The oesophagus. It traverses the thorax in a median posi- 

 tion, entering the diaphragm at the hiatus oesophageus. 



(b) The vagus nerves. The right and. left nerves pass backward 

 along the lateral walls of the oesophagus, and are connected 

 with one another through nerve plexuses lying on its dorsal 

 and ventral surfaces. The left cord is that appearing at the 

 posterior end of the oesophagus in a ventral position and 

 passing to the ventral surface of the stomach. The right 

 cord occupies a corresponding dorsal position and passes to 

 the dorsal surface of the stomach (p. 185). 



(c) The thoracic aorta (aorta thoracalis) passes backward on 

 the ventral surface of the vertebral column, leaving the 

 thorax through the hiatus aorticus, the latter being the 

 aperture enclosed by the crura of the diaphragm. Its 

 branches in the thorax are the intercostal arteries (aa. 

 intercostales), beginning with the fourth, which are given off 

 metamerically in the intercostal spaces, and pass laterad to 

 the thoracic wall. 



