This study gives a comprehensive description of eyelids movement in little owl
and discusses the impact of some surrounding conditions in their kinetic performance.
The present study used the video's recording technique to record the
kinetic activity of eyelids, besides the anatomical and histological studies of the
eyelid's structure. The fundamental eyelid movements can be uniquely and reliably
characterized by their anatomical relationship that was confirmed via video
recording for their kinetic activity. The levator palpebrae muscle is considered a
main generating motor for the upper eyelid; in the little owl, this muscle splits
into multiple directions and is distinguished from the levator palpebrae superioris
(Lps) and the levator anguli oculi (Lao) muscle. That anatomical pattern of
insertion increases the movement of the upper lid. On the other side, the contraction
of depressor palpebrae inferioris (Dpi) muscle and the active upward forces
of levator palpebrae muscle help in increasing the opening of the eye's fissure.
However, the closure process is produced from the passive downward forces and
relaxation of the levator palpebrae superioris (Lps), levator anguli oculi (Lao),
and depressor palpebrae inferioris muscle, as well as the contraction of retractor
anguli oculi lateralis (Raol) and medialis (Raom) muscle. The present results also
recorded that nictitating membrane's (Nm) movement is reversely proportionate
to the level of kinetic of other eyelids. The mobility of Nm in little owl occurs
under the effect of artificial external stress. These anatomical data and sequence
video recordings have confirmed that the upper eyelid moves more compared
to other eyelids. The authors also suggest that the mobility of eyelids may get
stimulated through external pressure force of some surrounding structure like
the periorbital sheet. Also, the histological study exhibited that the structure of
two eyelids is very similar in the little owl and the variability is showing in the
number of cell layers that forms their epithelium of skin and palpebral surfaces,
the distribution of pigment granules, and degree of keratinization on their surface.
That variability in the histological characters of eyelids may counteract the
abrasive forces occurring during the opening and closing processes.