Chapter 2, Page 5  

NORMAL ANATOMY, EMBRYOLOGY, AND HISTOLOGY OF THE ESOPHAGUS BLOOD SUPPLY Esophageal Musculature and Sphincters As in the rest of the gastrointestinal tract, the esophagus has an inner circular and an outer longitudinal muscle coat. The upper fourth of the esophagus consists entirely of striated muscle, the next quarter contains a mixture of striated and smooth muscle, and the lower half consists entirely of nonstriated muscle (fig. 2-5). The longitudinal muscle coat arises from the cricopharyngeus muscle at the lateral aspect of the cricoid cartilage, and from two distinct bundles, one from each side of the posterior cricoid cartilage. These bundles spread laterally around the esophagus, where they interdigitate with one another in the dorsal midline. At the uppermost posterior level of the longitudinal muscle is a V-shaped gap between it and the cricopharyngeus muscle; this gap is covered anteriorly by the circular muscle (fig. 2-1). As the muscle fibers descend the esophagus, they form thick lateral masses and a thin anterior portion. The inner circular muscle coat is usually thinner than the outer longitudinal muscle coat, a histologic feature that is the opposite of the rest of the gastrointestinal tract. The direction of the circular muscle fibers of the upper and lower third of the esophagus are more elliptical than circular; in the lowermost portion the fibers have a spiral configuration, with some bundles running vertically up and down. Although both ends of the esophagus are closed under resting conditions, only the upper end has a clearly defined sphincter. This is formed by the cricopharyngeus muscle which is transversely oriented and inserted on the cricoid cartilage at the level of the sixth cervical vertebra. The tissue between the cricopharyngeus muscle and the inferior constrictors superiorly and the esophageal muscles inferiorly is relatively thin and is a potential site for esophageal diverticula. Anatomically, the lower esophageal sphincter is difficult to see. In the lower esophagus, 1 to 2 cm above the hiatus, the circular and longitudinal muscle layers gradually become thickened and extend down to the cardia. Some investigators have suggested that there is an asymmetric thickening of the circular muscle that is arranged in a spiral fashion and blends with the inner oblique muscle of the stomach in the region of the physiologic sphincter (10, 19). Contraction of these muscle fibers closes the lower esophagus.