Horner's muscle and the lacrimal system.

Anatomical dissections were performed on eight fixed cadavar orbits to observe the relationship of the pretarsal, preseptal, and Horner's muscle to the canaliculus. The attachments of Horner's muscle and the deep preseptal muscle to the lacrimal diaphragm reflected their important role in the lacrimal diaphragm's function. In addition, the muscles involved in lacrimal drainage were mechanically pulled with a forceps to induce canalicular pressures. Shortening and compression of the canaliculus by the pretarsal, preseptal, and lateral Horner's muscle increased canalicular pressure, whereas Horner's muscle and the deep preseptal muscle induced negative lacrimal pressures. A positive-to-negative pressure gradient in the lacrimal drainage system was proposed as the main force in lacrimal tear transport.