The 4 demonstrable constrictions from above downward are at 1. The crico-pharyngeal fold. 2. The crossing of the aorta. 3. The crossing of the left bronchus. 4. The hiatus esophageus. There is a definite fifth narrowing of the esophageal lumen not easily demonstrated esophagoscopically and not seen during dissection, but readily shown functionally by the fact that almost all foreign bodies lodge at this point. This narrowing occurs at the superior aperture of the thorax and is probably produced by the crowding of the numerous organs which enter or leave the thorax through this orifice. _The crico-pharyngeal constriction_, as already mentioned, is produced by the tonic contraction of a specialized band of the orbicular fibers of the lowermost portion of the inferior pharyngeal constrictor muscle, called the cricopharyngeal muscle. As shown by the author it is this muscle and not the cricoid cartilage alone that causes the difficulty in the insertion of an esophagoscope. This muscle is attached laterally to the edges of the signet of the cricoid which it pulls with an incomprehensible power against the posterior wall of the hypopharynx, thus closing the mouth of the esophagus. Its other attachment is in the median posterior raphe. Between these circular fibers (the cricopharyngeal muscle) and the oblique fibers of the inferior constrictor muscle there is a weakly supported point through which the esophageal wall may herniate to form the so-called pulsion diverticulum. It is at this weak point that fatal esophagoscopic perforation by inexperienced operators is most likely to occur. _The aortic narrowing_ of the esophagus may not be noticed at all if the patient is placed in the proper sequential "high-low" position. It is only when the tube-mouth is directed against the left anterior wall that the actively pulsating aorta is felt. The bronchial narrowing of the esophagus is due to backward displacement caused by the passage of the left bronchus over the anterior wall of the esophagus at about 27 cm. from the upper teeth in the adult. The ridge is quite prominent in some patients, especially those with dilatation from stenoses lower down. The hiatal narrowing is both anatomic and spasmodic. The peculiar arrangement of the tendinous and muscular structure of the diaphragm acts on this hiatal opening in a sphincter-like fashion. There are also special bundles of muscle fibers extending from the crura of the diaphragm and surrounding the esophagus, which contribute to tonic closure in the same way that a pinch-cock closes a rubber tube. The author has called the hiatal closure the "diaphragmatic pinchcock."