Introduction
The ocular adnexa are the protective and supporting structures of the
eye, including the eyelids and their parts, the third eyelid
(nictitating membrane), orbital glands, and oculomotor muscles
(Guerra-Fuentes et al., 2014). Peripheral to the housing of the orbit,
the eye receives immediate protection and support from the eyelids
(Franz-Odendaal and Vikaryous, 2006). All reptiles have external
eyelids. Both upper and lower eyelids are present in most lizards, all
turtles, tuataras, and crocodilians. Eyelids are modified in several
species, resulting in partial fusion, as in chameleons, leaving a
circular opening equivalent to the diameter of the cornea, or complete
fusion with transparency, as in many geckos and snakes (Schwab, 2012).
In snakes, the eyelids fuse during development to form the spectacle.
Some gecko and skink species possess a secondarily derived spectacle.
Additionally, some skinks, lacertid lizards, and iguanine lizards have a
transparent lower eyelid formed by clear scales (Lawton, 2006; Schwab,
2012)\RL.
The nictitating membrane is a prominent semilunar fold of the
conjunctiva situated at the anterior canthus of the eye, between the
eyelids and the cornea (Schlegel et al., 2003). It is highly developed
in crocodilians and turtles but is absent in snakes and many lizards
(Lawton, 2006)\RL. Hiller (1995, 1998) studied the ultrastructure of
the conjunctiva and nictitating membrane of the eyelids in agamid
lizards and other reptilian species through scanning electron
microscopy. Starostová et al. (2009, 2013) examined cell size and its
effects on the scaling of metabolic rate in eyelid geckos, finding that
ontogeny of metabolic rate and red blood cell size in these species
follow different paths. Guerra-Fuentes et al. (2014) investigated the
spectacle in gymnophthalmid lizards through the looking glass.
The study of extraocular muscles has received little attention from
reptilian anatomists, as they consider these muscles’ function and
innervation patterns to be standard among vertebrates. Vertebrate
extraocular muscles consist of six muscles that typically control the
movement of the eyeball (Nomina Anatomica Veterinaria, 2012). These
muscles are named according to their insertion points and form three
pairs (antagonistic and synergistic) during eyeball movement (Cunha et
al., 2016)\RL.
Ocular movement varies among species, ranging from more than 270 degrees
of rotation in chameleons to a few degrees in snakes (El Hassni et al.,
2000). Chameleons’ highly independent eye movements, which differ from
those of other reptiles, have garnered significant interest from many
researchers. Pettigrew et al. (1999) studied the convergence of
specialized behavior, eye movements, and visual optics in the sand lance
(Teleostei) and the chameleon (Reptilia). El Hassni et al. (2000)
investigated the localization of motor neurons innervating the
extraocular muscles in the chameleon (Chamaeleo chameleon ). Ott
(2001) found that chameleons have independent eye movements but
synchronize both eyes during saccadic prey tracking\RL.
Reptilian anatomists have focused more on studying the eye structure of
certain reptilian species, such as chameleons and snakes, in detail, but
some species, like the Egyptian agama, remain neglected. The Egyptian
agama, Trapelus mutabilis , is a small diurnal lizard found in
North Sinai, Egypt (Baha el Din, 2006; Wagner et al., 2011). The hard
life of Egyptian agama has attracted the attention of the present
authors, and how these animals maintain their visual performance under
such these difficult living conditions. Studying the structure of the
eyelids helps to understand some information about the visual
performance of these animals. This study presents the anatomical,
scanning electron microscopic and histological investigations of the
ocular eyelids and muscles of the Egyptian agama, Trapelus
mutabilis . Additionally, the motion of the agama eye was monitored in
the laboratory to observe the synchronization between eyelid and eyeball
movement\RL.