Snakes have no moveable eyelids. Instead their eyes are covered with a transparent structure termed the spectacle. Despite knowledge of the spectacle for almost 200 years, the morphology has only been described sporadically.
To provide a morphological description of the spectacle.
The spectacles of 267 snakes from 53 different families were examined using optical coherence tomography, transmission electron microscopy or histology. Morphology was described, and spectacle thickness and diameter were measured.
The spectacle consists of three layers with an appearance similar to the cornea. Despite its transparency, the spectacle holds blood vessels and nerves located in the central collagenous layer. The spectacle is attached to the surrounding skin structures and only the outermost keratin layers participate in shedding. The thickness of the spectacle varied between 0.07 and 0.24 mm. The family of vipers had the thinnest spectacle both in absolute terms and relative to spectacle diameter. The thickest spectacles, both absolute and relative, belonged to burrowing snakes. The pythons had the largest spectacle diameter.
The appearance of the spectacle is similar in all snakes; however, the thickness varies between species. This study provides information that may serve as baseline values to evaluate snakes with ocular diseases.