2 28 CHORDATE ANATOMY 



The submucosa is a layer of loose connective tissue containing glands 

 and many blood and lymph vessels. The tunica muscularis consists of an 

 inner layer of circular muscles and an outer longitudinal layer. The 

 connective tissue between them contains a plexus of sympathetic nerve 

 fibers. By the wave-like peristalsis of the circular muscles food is con- 

 veyed from the pharynx to the stomach. 



Development of the Esophagus. Beginning with the fourth week, 

 the esophagus develops as an elongation of the fore-gut between pharynx 

 and stomach. Its single-layered columnar epithelium becomes gradually 

 converted into a stratified squamous epithelium like that which lines 

 the pharynx. 



History of the Esophagus. There is little to distinguish the esophagus 

 of a fish from its stomach, except the relative scarcity of glands, and the 

 fact that its muscle fibers, like those of the pharynx, are striated, while 

 those of the stomach are smooth. In amphibians, the esophagus becomes 

 slightly elongated. Its considerable elongation in reptiles and mammals 

 is correlated with the elongation of the neck. In these groups, it becomes 

 constricted in diameter and most of its muscle fibers become smooth. 



The Stomach 



The stomach, lying between the esophagus and small intestine, is the 

 most expanded part of the alimentary canal. Its shape in man varies 

 greatly, depending upon the quantity of food contained. The human 

 stomach lies almost transversely across the abdominal cavity with a 

 greater curvature on the left side of the body and a lesser curvature to 

 the right. The opening of the esophagus into the stomach is the cardiac 

 orifice, that into the small intestine is the pylorus. 



The anterior more enlarged portion of the stomach is the cardiac 

 portion, the posterior more constricted region is the pyloric portion. 

 The pyloric portion of the stomach diminishes in size towards the pylorus, 

 which is reduced to a small aperture by a local ring-like thickening of the 

 mucous lining and of the layer of circular muscle. The wall of the 

 stomach contains the same four layers of tissue as are seen in the esopha- 

 gus, plus an external serous layer. 



The tunica muscularis contains three layers of muscle, longitudinal, 

 circular, and obUque. By their combined action under the stimulus of 

 the sympathetic nerves, the stomach maintains a peristaltic churning 

 action as long as food is present. 



The simple mucous epithelium which lines the stomach joins abruptly 

 the stratified epithelium of the esophagus. Viewed with a hand-lens, 

 the inner surface of the stomach appears to be filled with minute pores, 

 which are the apertures of the ducts of the gastric glands. Three kinds of 

 stomach glands are distinguished, cardiac, gastric, and pyloric. The 



