ESOPHAGUS, STOMACH AND INTESTINE 177 



separated from the pericardial and peritoneal cavities, the mesothelium 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 (splanchnic) and parietal (soma- 

 tic) mesoderm of the embryo. 



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. 170). 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 Kolliker, the air cells of the lungs begin to form at the sixth month and their 

 development is completed during pregnancy. Elastic tissue may be recognized at the third 

 month in the walls of the vessels and during the fourth month it appears in the largest bronchi. 

 The abundant connective tissue found between the bronchial branches in early fetal life 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, expanding 

 and completely filling the pleural cavities. Their margins become rounded and the compact 

 fetal lung tissue which resembles that of a gland in structure becomes light and spongy, owing 

 to the enormous increase in the size of the alveoli and blood-vessels. Because of the greater 

 amount of blood admitted to the lungs after birth their weight is suddenly increased. 



In the most common anomaly involving the esophagus and trachea the former is divided 

 transversely, the trachea opening into the lower portion of the esophagus, while the upper 

 portion of the esophagus ends blindly. According to Lewis (in Keibel and Mall, vol. 2, p. 367), 

 the anomaly may be produced by the abnormal development of lateral esophageal grooves 

 which occlude the lumen of the esophagus. These grooves, though small, were found present 

 in 4 mm. human embryos. 



ESOPHAGUS, STOMACH AND INTESTINE 

 Esophagus. — The esophagus in 4 to 5 mm. embryos is a short tube, flattened 

 laterally, and extending from the pharynx to the stomach. Its epithelium is 

 composed of two layers of columnar cells. The esophagus grows rapidly in length 

 and in 7.5 mm. embryos its diameter decreases both relatively and absolutely 

 (Forssner) . 



In embryos of from 8 to 16 mm. its laryngeal end is crescent-shaped and concave toward 

 the trachea. Its middle, portion is round or oval and opposite the bifurcation of the trachea 

 it begins to enlarge and is flattened laterally. Its lumen is open throughout and shows from 



