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The longitudinal fibres ascend from the medulla oblongata and leave the pons by emerging from its upper surface as fibres of the two crura cerebri.

Interior of dorsal valve. c, c’, cardinal process; b’, b’, hinge-plate; s, dental sockets; l, loop; q, crura; a, a’, adductor impressions; c, accessory divaricator; b, peduncle muscles; ss, septum.

Through the crura pass the splanchnic nerves, and in addition to these the left crus is pierced by the vena azygos minor.

B, basis of the crura; P, pons; Mo, medulla oblongata; Ce, cerebellum; Sc, spinal cord.

Anatomical studies have corroborated this finding by demonstrating a definite sphincteric mechanism consisting of muscle bands springing from the crura of the diaphragm and surrounding the esophagus at the under surface of the hiatus.