170 THE ENTODERMAL CANAL AND THE BODY CAVITIES 



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 (somatic) 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. 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, a larger vein from each lower lobe, a smaller vein from each upper 

 lobe, including 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, 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 life becomes reduced in its relative amount as 

 the alveoli of the lungs are developed. 



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

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

 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, become light and 

 spongy, owing to the enormous increase in the size of the alveoli and blood vessels. Be- 

 cause of the greater amount of blood admitted to the lungs after birth, their weight is 

 suddenly increased. 



Anomalies. Variations occur in the size and number of lobes of the lungs; rarely 

 there is a third lobe on the left side. In the most common anomaly involving both esopha- 

 gus and trachea, the esophagus is divided transversely, the trachea opening into the lower 

 segment, while the upper portion ends as a blind sac. 



THE ESOPHAGUS, STOMACH AND INTESTINE 



The 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, but at birth they 

 number nine or ten. 



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

 lumen, which, however, is at no time occluded. Glands begin to develop as epithelial 

 ingrowths at 78 mm. (CR). 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). These layers 

 appear in similar time-sequence throughout the entire digestive tract. 



Anomalies. There may be atresia. This usually involves fistulous relations with 

 the trachea, as already described (p. -170). 



