The Lower Conjunctival Sac of the eye is the space that is bound by the conjunctival membrane which is between the palpebral and bulbar conjunctiva, in which lacrimal fluid is secreted; it is a closed space when eye is closed; when eye is open, the sac is open anteriorly through the palpebral fissure "between the eyelids".
The looseness of the conjunctiva at these points is what makes the movement of our eyelids and eyeballs possible.