The Structure of the Mammalian Csophagus. 9 
unstriated muscle. At the middle of the esophagus it forms a con- 
tinuous layer of considerable thickness (205 micra in an animal of 
3 kg.) and increasing to 420 micra at the cardiac orifice of the stom- 
ach. In the lower portion, by reason of the fact that the glands are 
practically confined to the mucosa, the lamina muscularis mucose 
is separated by only a narrow band of collagenic connective tissue 
representing the tela submucosa, from the tunica muscularis. 
Mucous glands are present throughout the whole length of the 
esophagus and in the pharynx. In the upper part of the esophagus 
Demilune cells 
Lumen 
Mucous cells 
Fic. 3. A. Section of a tubule of an csophageal gland of Didelphys show- 
ing mucous cells and demilunes. B. Group of demilune cells with central 
lumen and intercellular canaliculi. > 500. 
they are located in the submucosa, but at the lower end where the 
transverse folds occur they are found in the lamina propria of these 
folds, superficial to the 1. muscularis mucose, although here a few 
tubules may extend into the lamina muscularis mucosze and even 
into the tunica muscularis. 
The glands in all parts of the cesophagus of the opossum are mixed 
glands, that is to say they consist of mucous cells and demilunes or 
erescents. The latter are few in number at the upper end of the 
cesophagus, but at the lower end where the glands are located in the 
lamina propria mucose they are very abundant, as shown in Fig. 3. 
