Chapter 1, The Reptilian Spectacle - UWSpace - University of ...

1988; Vieira et al. 2011). Thus the development of spectacles logically resembles a progression from
that of lidded reptiles rather than a regression of the process observed in mammals. No published study
of spectacle development in other reptiles, such as scincid or xantusiid lizards, has been found, so it is
unknown if they too follow a similar principle, though it seems likely that they would given the
precedent and the nature of tertiary spectacles.
1.3 Optics of the Spectacle
As the window to the outside world, the spectacle plays a crucial role in the quality of vision. This was
briefly touched upon in the description of the spectacle blood vessel layouts of the previous section
with the consideration that the blood vessels themselves might constitute an impediment to clear
vision. While such visual consequences are speculative, the fact that no vertebrate (again, other than
the Florida manatee) has non-retinal blood vessels in its visual field implies that visual clarity may be
impacted by any degree of vascularization in the optical transmissive regions of the eye. As well, the
asymmetric meshwork in the spectacle of A. nasuta that minimizes the density of vasculature in the
most acute field of vision suggests an adaptation to minimize a loss of clarity due to the vessels.
Species in which pupils constrict to near pinhole dimensions would be most likely to suffer
visually due to the increased depth of field resulting from such small apertures (Green et al. 1980)
which, in tandem with the short focal lengths of snake eyes (Sivak 1977; Howland et al. 2004), might
resolve the spectacle vessels in the retinal image. Even the horizontal slit pupil of the keen-eyed A.
nasuta is able to constrict to sub millimeter widths, but it being fortunately horizontal, vision is
thankfully saved as the increased depth of field and resolving capacity of its thin aperture would affect
only horizontal lines in the visual field, not the vertical lines of the spectacle vessels.
Spectacle blood vessels are not alone responsible for potentially limiting visual clarity. The
scale itself may acquire abrasions or collect debris during the course of the animal’s activities (Figure
1-12, next page) that could reduce retinal image contrast or result in scotomas (i.e. blind spots). As
Walls (1942) quipped: “[The] renewal of the [spectacle scale] often comes none too soon - as one
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