mm. (1 in.) Sixth cervical
                  Antero-posterior 17 mm. (3/4 in.)
Aortic Transverse 24 mm. (1 in.) Fourth thoracic
                  Antero-posterior 19 mm. (3/4 in.)
Left-bronchial Transverse 23 mm. (1 in.) Fifth thoracic
                  Antero-posterior 17 mm. (3/4 in.)
Diaphragmatic Transverse 23 mm. (1 in+) Tenth thoracic
                  Antero-posterior 23 mm. (in.—)

For practical endoscopic purposes it is only necessary to remember that in a normal esophagus, straight and rigid tubes of 7 mm. diameter should pass freely in infants, and in adults, tubes of 10 mm.

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.