THE LUNGS 335 



(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 within it. On the right side a small 

 eparterial bronchus is given off from the right bronchus to 

 the well-developed superior lobe, entering the lung anterior 

 to 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 

 portion, or hilus, of each organ. The artery penetrates 

 deeply almost at once, anterior and then dorsal to the main 

 bronchus, but some large venous tributaries have a con- 

 siderable superficial course. 



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

 giving off a number of branches, which accompany the 

 bronchus to the lung. 



These branches contain both afferent nerve fibres for the mucous 



membrane and efferent fibres to the smooth muscles of the bronchioles, 



the latter producing bronchoconstriction when stimulated and assisting 



expiration. 



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 may be 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 

 position, 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. In the posterior part of the thorax, 

 both cords lie dorsolatera,l to the oesophagus and after 

 passing through the diaphragm in this position the left 

 nerve crosses 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. 230). 



