Which muscle contracts to cause the shortening of the canaliculus when the eyelid closes?

The contraction that leads to the shortening of the canaliculus when the eyelid closes is primarily due to the action of the muscle of Horner. This muscle plays a crucial role in the drainage of tears from the lacrimal sac into the nasolacrimal duct. When the eyelid closes, the orbicularis oculi muscle contracts, which helps compress the lacrimal sac, and the muscle of Horner then assists in closing the canaliculus, effectively shortening it. This action is significant as it promotes the efficient drainage of tears, preventing overflow and ensuring proper tear film maintenance on the surface of the eye.

Understanding the anatomy and function of these muscles is essential, particularly in relation to their roles in the blinking mechanism and tear drainage system. In contrast, the levator palpebrae primarily functions to elevate the upper eyelid, while the medial rectus is responsible for moving the eye medially, which does not contribute to the action of the canaliculus during eyelid closure.