456 



ADAPTATIONS TO MEDIA AND SUBSTRATES 



It is also possible that the snakes, being under the necessity of lubricating 

 the apposed surfaces of the cornea and the spectacle, elected the Hard- 

 erian gland simply because of the superiority of oil over water as a 

 lubricant. 



In both snakes and spectacled lizards, an outer layer of the spectacle 

 is periodically replaced whenever the skin is shed. This renewal of the 

 surface often comes none too soon — as one appreciates on observing the 

 sadly scratched and dull appearance of the spectacle of a garter snake 



Fig. 154 — The ophidian eye and its accommodation. 



a, eye of European grass snake, Natrix natrix, in vertical section, x 22. Redrawn from 

 Schwarz-Karsten, modified from original preparations. 



am- accommodatory muscle, forming sphincter-like mass near root of iris; ap- anterior pad of 

 lens; t- brain; c- cornea; c^- ciliary body (devoid of muscle); cr- cranium; c^- canal of 

 Schlemm; hg- Harderian gland; to- infraocular scale; s- sclera (entirely fibrous — no cartilage 

 or bone) ; sm- sphincter muscle of pupil; so- supraocular scale; sp- spectacle; z- zonule. 



b, anterior segment of Coluber <esculapii, in relaxation, x 5. Redrawn from Beer. 



c, same as b; in accommodation under electrical stimulation; note forward movement of lens 

 and decrease of eyeball diameter at limbus (the dome of the cornea has been cut away, 

 but this does not alter the course of ophidian accommodation). 



