Digestive Organs
Esophagus
The esophagus is a hollow muscular tube that connects the back of the throat to the top of the stomach. This tube in the adult ranges from 10 to 14 inches in length, and one inch in diameter. At rest, the esophagus is collapsed but opens readily to accept food and liquids. The upper portion of this muscular tube is composed of muscle similar to that of the arms and legs (skeletal muscle) and is therefore under voluntary control. The other two thirds of the esophagus are made of smooth muscle like the rest of the gut and is not under voluntary control. These muscles are arranged with an inner circular layer and an outer longitudinal one.
Function and Control
To keep our food from coming back from the stomach to our mouth, the esophagus has two sphincter muscles; the one at the top is called the upper esophageal sphincter (UES); the one at the bottom is the lower esophageal sphincter (LES). Functionally, the esophagus is then divided into the UES, the body of the 
esophagus, and the LES.
The act of swallowing takes a great deal of coordination, most of which is unconscious. Within the brain is an area known as the "swallowing center." This center acts to receive information from the esophagus and throat and to send out the message to activate swallowing. This message center may be affected by strokes and other injuries. The smooth muscle has sensors within the wall that detect stretch from food or liquids; also present are hormones and nerves which then make the smooth muscle contract to push the food or liquid toward the stomach. At rest, then, the UES and LES are closed and the body of the esophagus is quiet.
When we put food into our mouth, the nervous system detects this and directs the UES to open. The food within the esophagus is moved downstream by movement of the smooth muscle (primary peristalsis), and the LES relaxes for food to enter the stomach. This entire series of events takes about eight seconds. After swallowing, the smooth muscle may contract to sweep remaining material into the stomach (secondary peristalsis). This is also a protective mechanism to strip any acid that may have washed back into the esophagus from the stomach.