ESOPHAGUS, STOMACH AND INTESTINE 1 69 



panding 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. 



ESOPHAGUS, STOMACH AND INTESTINE 

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

 laterally, extending from the pharynx to the stomach. It grows rapidly in length 

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

 (Forssner). At this stage the esophageal epithelium is composed of two layers 

 of columnar cells. 



In 20 mm. embryos, vacuoles appear in the epithelium and increase the size of the 

 lumen which remains open throughout. In later stages the wall of the esophagus is folded, 

 and ciliated epithelial cells appear at 44 mm. (C R). The number of cell layers in the epi- 

 thelium increases, until, at birth, they number nine or ten. Glands are developed as epi- 

 theHal ingrowths. The circular muscle layer is indicated at 10 mm. but the longitudinal 

 muscle fibers do not form a definite layer until 55 mm. (C R). (F. T. Lewis in Keibel 

 and Mail, vol. 2.) These layers appear in similar time-sequence throughout the entire 

 digestive tract. 



Stomach. — The stomach appears in embryos of 4 to 5 mm. as a laterally 

 flattened, fusiform enlargement of the fore-gut caudal to the lung anlages (Figs. 

 177 and 178). Its epithelium is early thicker than that of the esophagus and is sur- 

 rounded by a thick layer of splanchnic mesoderm. It is attached dorsally to the 

 body wall by its mesentery, the greater omentum, and ventrally to the liver by 

 the lesser omentum (Fig. 190 5). The dorsal border of the stomach both enlarges 

 locally to form the fundus, and also grows more rapidly than the ventral wall 

 throughout its extent, thus producing the convex greater curvature. The whole 

 stomach becomes curved and its cranial end is displaced to the left by the en- 

 larging Uver (Fig. 168). This forms a ventral concavity, the lesser curvature, and 

 produces the first flexure of the duodenum. 



The rapid growth of the gastric wall along its greater curvature also causes 

 the stomach to rotate upon its long axis until its greater curvature, or primitive 

 dorsal wall, lies to the left, its ventral wall, the lesser curvature, to the right 

 (Fig. 201). The original right side is now dorsal, the left side ventral in position, 

 and the caudal or pyloric end of the stomach is ventral and to the right of its 

 cardiac or cephahc end. The whole organ extends obliquely across the peri-. 



