252 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



of the coelom on each side forms the pleura. The part of the 

 coelomic membrane which is invaginated on the lung bud 

 becomes the visceral pleura. The invaginating or ensheathing 

 lining of the isthmus becomes the parietal layer. As the lung 

 buds grow, they distend the originally small pleural parts of the 

 coelom until at the time of birth the right and left pleurae almost 

 meet in front of the heart. They meet after birth under the 

 sternum, enclosing between them the anterior mediastinum. 



(2) As will be seen from Fig. 201, the lung bud sprouts out 

 from the dorsal mesentery just behind the duct of Cuvier. This 

 relationship is retained in the adult, the vena azygos and 

 superior vena cava lying above and in front of the root of 

 the right lung. If the left duct of Cuvier persisted it would 

 lie above and in front of the root of the left lung. The 

 lung bud springs from the pulmonary diverticulum just be- 

 hind the 5th visceral arch of the pharynx. This arch is 

 involved in the formation of the pulmonary groove and pul- 

 monary diverticulum. It is from this arch (5 th aortic arch) that 

 vessels arise, perforate the lung, and become the pulmonary 

 arteries. The fifth arch — part of the branchial pharynx — takes 

 a chief part in the formation of the pulmonary apparatus. The 

 ductus arteriosus — part of the 5 th arch — lies over the root 

 of the left lung. At this stage (4th week) the pleural cavity is 

 still in communication with the peritoneal above the septum 

 trans versum (Fig. 205). 



Formation of the Bronchi and Lungs. — The bronchi are 

 the stalks of the right and left lung buds. The right bud is 

 the bigger; the left is probably repressed by the heart turning 

 to the left side. The right shows three secondary buds — the 

 forerunners of the upper, middle and lower lobes of the lung; 

 the left, two, which form the upper and lower lobes. 



The condition of the lung buds at the end of the 5th week is 

 shown in Fig. 206. The right and left bronchi are formed, so 

 are the chief bronchial ramifications. Each ramification ends in 

 a bud, which divides again and again and keeps on dividing 

 until the fourth month. The terminal buds form the infundibula. 

 Each bud is solid, and carries its sheath of mesoblast. At the 

 sixth month sacular evaginations occur from the infundibula; 

 they form the air cells, or alveoli. 



