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."

_Direction of the Esophagus_.--The esophagus enters the chest in a
decidedly backward as well as downward direction, parallel to that of
the trachea, following the curves of the cervical and upper dorsal
spine. Below the left bronchus the esophagus turn