The primary physiologic role of the esophagus is the physical transport of ingesta from the oral cavity to the stomach. The esophageal mucosa is composed of a superficial layer of stratified squamous epithelium. Beneath the mucosa lie variable numbers of mucous glands, which produce mucus to lubricate and facilitate passage of digesta.
The constituents of the tunica muscularis varies based on species and location along the length of the esophagus. For example, in species such as the cat and horse, the tunic muscularis is primarily composed of skeletal muscle with distal segments incorporating smooth muscle. Dogs and ruminants have uniform skeletal muscle along the entire length. Pigs, similar to humans, primarily contain skeletal muscle in the orad 1/3 of the esophagus which transitions to a combination of skeletal and smooth muscle, and terminates in uniform smooth muscle, distally.
Chickens have a specialized outpouching, or diverticulum, located in the distal esophagus (at the ventral base of the neck): the crop. The anatomy of the crop is identical to the esophagus. The crop serves as storage for ingesta, allowing for the chicken to consume a large amount of food and digest later.
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