Once the bolus of food passes through the oro- and laryn-gopharynx, it enters the esophagus. This muscular tube, located behind the windpipe, is collapsed when empty. The esophagus is about 10 inches long, starts at the bottom of the laryngopharynx, and ends at the opening of the stomach called the cardiac sphincter. A sphincter is a circular arrangement of

Nasopharynx Oropharynx Hypopharynx
Figure 5.3 The structures of the pharynx and esophagus are illustrated here. The pharynx is composed of the nasopharynx, oropharynx, and the hypopharynx. The esophagus lies at the base of the hypopharynx and connects the pharynx to the stomach.

muscle, usually smooth muscle cells, that is used to open or close a tube. Here, this sphincter loosely controls entry into the stomach. The esophagus passes through the diaphragm located just above the stomach. This muscle assists the cardiac sphincter in limiting access to the stomach.

The esophagus has four layers. The epithelial lining is made up of stratified squamous cells, as in the mouth and pharynx. The mucosa and submucosa of the esophagus folds along the length of the tube when it is empty. The muscularis has two layers of muscle, the inner circular and the outer longitudinal, but not all of the muscle is the same type. The first third of the esophagus has skeletal muscle in the muscu-laris layer, the last third has smooth muscle, and the middle portion has a mixture that gradually goes from skeletal to smooth muscle cells. The esophagus has an adventitia that blends the tube covering with the surrounding tissue and holds the esophagus in place in the throat.

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