Between the bulbar and the palpebral conjunctiva there are two loose, redundant portions forming recesses that project back toward the equator of the globe. These recesses are called the upper and lower fornices, or conjunctival sacs; it is the looseness of the conjunctiva at these points that makes movements of lids and eyeball possible.
https://www.britannica.com/science/human-eye#ref531476

Gornji svod vežnjače (fornix conjunctivae superior) i donji svod vežnjače (formix conjuctivae inferior) predstavljaju zajedno jedan kružan, dubok, a uzan sluzokožni špag, koji obrazuje vežnjača prebacujući se sa zadnje strane kapaka na prednji deo očne jabučice. Svodovi vežnjače omogućuju nesmetane pokrete kapaka i očne jabučice.
"Anatomija čoveka - glava i vrat, za studente medicine" (Prof. dr S. Jovanović; Prof. dr N.A. Jeličić)