Esophagus

The esophagus transports food from the oral cavity to the stomach. It is lined by a stratified squamous epithelium. Be sure you can identify the major layers of the esophageal wall in this image: the mucosa, consisting of the epithelium, lamina propria and muscularis mucosa, the submucosa, and the inner circular and outer longitudinal smooth muscle of the muscularis externa. How does the composition of the muscularis externa change between the first 1/3 and the final 2/3 of the esophagus?