When the Torah discusses those mumim – blemishes – that will disqualify an animal from being brought as a sacrifice (Vayikra 22:21-24), among those blemishes are problems with the eye. When talking about blemishes that will preclude a kohen from performing the Temple service (Vayikra 21:18-21) the Torah specifies a number of eye conditions, like someone who is tevallul be-eino.
The Mishna on today’s daf applies these to the case of a firstborn, teaching that “One whose eyelid is pierced, damaged, or slit, or if it has a cataract or a tevallul, ḥilazon (snail-shaped growth), naḥash (snake-shaped growth) or a berry that covers the eye, is disqualified.”
The Mishna asks:
“What is a tevallul? It is a white thread that bisects the iris and enters the black pupil. If it is a black thread that bisects the iris and enters the white of the eye it is not a blemish (because there are no disqualifying blemishes as regards the white of the eye).”
The black of the eye is the pupil, while the white of the eye is the sclera. Between these two is the iris, which serves to control the size and diameter of the pupil. The “blemishes” described by the Mishna are well-known conditions. What the Mishna calls the white of the eye encroaching on the black is a cyst of the iris, while the black invading the white is a coloboma of the iris.
The case of a perforated or slit eyelid can be a case where an injury occurred, but it can also be a congenital defect, like palpebral coloboma whose source is a genetic defect. According to the Rambam, the cases of ḥilazon and naḥash are both situations of pterygium, a common hereditary condition where a membrane grows on the inner part of the eye and spreads to the pupil.