Canthus (pl. canthi, palpebral commissures) is either corner of the eye where the upper and lower eyelids meet. More specifically, the medial and lateral canthi would be described as the medial and lateral ends/angles of the palpebral fissure. (Edit - Where as medial and lateral are the appropriate positional terms for most of the body, the preferred terminology for the eye is inner and outer for the canthus. E.g. Inner Canthus or Outer Canthus.)
The lateral palpebral commissure (commissura palpebrarum lateralis; external canthus) is more acute than the medial, and the eyelids here lie in close contact with the bulb of the eye.
The medial palpebral commissure (commissura palpebrarum medialis; internal canthus) is prolonged for a short distance toward the nose, and the two eyelids are separated by a triangular space, the lacus lacrimalis.
^Genetic Hearing Loss from UTMB, Dept. of Otolaryngology. DATE: March 17, 2004. RESIDENT PHYSICIAN: Jing Shen. FACULTY PHYSICIAN: Ronald W. Deskin, MD. SERIES EDITORS: Francis B. Quinn, Jr., MD and Matthew W. Ryan, MD.