The Correlation Between the Eyelids Characteristics and Their Kinetic Performance in Two Wild Birds.

The present avian anatomists have shown a renewed interest in looking at whether the structure of the avian eyelid is accommodated by the kinetic achievement for each eyelid. The current work utilised both histological and anatomical methods to explore the eyelid's structural association with their kinetic, utilising the hoopoe and cattle egret as natural models. The third lid moved only without implicating other lids. Wrinkles on the edge of the hoopoe's eyelid became less pronounced during the opening phase, in contrast to, the edge of the egret's eyelid. The elevator muscle was visible only in the hoopoe, while both birds possessed the retractor anguli oculi muscles. The two later muscles collaborate with the depressor muscle during the closure phase. Two types of collagen I and III were detected within the stroma of the eyelids of both bird species; the elastic fibres was observed; few were in the hoopoe's eyelids. The eyelid edge of the hoopoe has more of the elastic fibres than those in its eyelid skin. The hoopoe's eyelid's epithelial layers contained more cytokeratin (AE1/AE3) than the egret. In conclusion, the anatomical traits of the hoopoe eyelids contribute to its greater motion compared to the egret eyelids.