orbital opening

or·bi·tal o·pen·ing

the somewhat quadrangular anterior entrance to the orbit that forms the base of the pyramidal orbital cavity. It is bounded by the sharp supraorbital, infraorbital, and lateral orbital margins and a less obvious medial margin on each side of the upper nose.
References in periodicals archive ?
Three plates surround a slightly oval orbital opening (orb), which is 6.
In many actinolepidoids and phlyctaenioids the PtO plate is rather large, bounding the orbital opening from behind.
In Eukaia the infraorbital sensory line (ioc) is developed on the SO plate as a short segment near the orbital opening, whereas in Holonema it crosses the entire SO plate (Miles 1971, figs 32, 33).