1 68 THE ENTODEKJVIAL CANAL AND THE BODY CAVITIES 



stem buds of the lungs grow out laterad, carrying with them folds of the mesoderm. 

 The branching of the bronchial buds takes place within this tissue which is cov- 

 ered by the mesothelial Hning of the body cavity. The terminal branches of 

 the bronchi are lined with entodermal cells which flatten out and form the res- 

 piratory epithelium of the adult lungs. The surrounding mesenchyme differen- 

 tiates into the muscle, connective tissue, and cartilage plates of the lung, tracheal, 

 and bronchial walls. Into it grow blood vessels and nerve fibers. When the 

 pleural cavities are separated from the pericardial and peritoneal cavities, the 

 mesothehum covering the lungs, with the connective tissue underlying it, becomes 

 the visceral pleura. The corresponding layers lining the thoracic wall form the 

 parietal pleura. These layers are derived respectively from the visceral (splanch- 

 nic) and parietal (somatic) mesoderm of the embryo. 



:--\ -Pulmonary artery 



■■'., \ 



■\ \ -^ 

 V '' ^' ,„ , Pulmonary vein 



Fig. 176. — Ventral view of the lungs of a 10.5 mm. embryo showing the pulmonary arteries and veins 

 (His in McMurrich). X 27. Ep., Apical bronchus; /, II, primary bronchi. 



In 1 1 mm. embryos the two pulmonary arteries, from the sixth pair of aortic 

 arches, course lateral then dorsal to the stem bronchi (Fig. 176). The right 

 pulmonary artery passes ventral to the apical bronchus of the right lung. The 

 single pulmonary vein receives two branches from each lung, two larger veins 

 from each lower lobe, two smaller veins from each upper lobe and the middle lobe 

 of the right side. These four pulmonary branches course ventrad and drain into 

 the pulmonary trunk. When this common stem is taken up into the wall of the 

 left atrium, the four pulmonary veins open directly into the latter. 



According to KoUiker, the air cells or alveoli of the lungs begin to form in the sixth 

 month and their development is completed during pregnancy. Elastic tissue appears during 

 the fourth month in the largest bronchi. The abundant connective tissue found between the- 

 bronchial branches in early fetal hfe becomes reduced in its relative amount as the alveoli 

 of the lungs are developed. 



Before birth the lungs are relatively small, compact, and possess sharp margins. They 

 lie in the dorsal portion of the pleural cavities. After birth they normally fill with air ex- 



