REPTILES 



383 



only rod-fovese in terrestrial Vertebrates (Fig. 

 463). 1 The optic nerve, like that of Crocodilians, 

 is slender and simjole in architecture without a 

 septal system. 



THE OCULAR ADNEXA resemble closely 

 those of the lizard, but the tendon of the nicti- 

 tating membrane slips round a sling formed 

 by the unusually large two-headed retractor 

 bulbi nniscle, to find insertion into the orbital 

 wall. The lacrimal gland is lacking but a 

 simple harderian gland is present. In contrast 

 to that of the lizard, the orbit is enclosed with 

 sturdy temporal arches. 



Fig. 465. — The Poster- 

 ior Segment of the 

 Globe of Sphesodox. 



B and R, the two heads 

 of the retractor bulbi 

 muscle ; A', tendon of 

 nictitans ; ON, optic 

 nerve (after Franz). 



THE OPHIDIAN EYE 



THE OPHiDiA (snakes or SERPENTS), Hmbless reptiles having no 

 pectoral and never more than a hint of a pelvic girdle, are of widespread 

 distribution j)articularly in the trojDics ; most are terrestrial, a few 

 amphibious, and many habitually marine. Although many genera 

 exist, the eyes of all snakes are very alike — apart from the Typhlopidae, 

 degenerate creatures generally smaller than earthworms and sub- 

 terranean in habit which have vestigial eyes.^ 



Curiously, however, the ophidian eye is extremely unlike that of 

 all other Reptiles in almost every particular. There is no scleral 

 cartilage or ossicles ; the iris vasculature forms an indiscriminate iietivork 

 and its striated muscnlature, ectodertnal in other Reptiles, is replaced by 

 mesodermal fibres derived from the ciliary region ; the ciliary venous sinus 

 is corneal in location ; the lens possesses sutures and an anterior annular 

 pad, and since it is divorced from the ciliary body, a new method of 

 accommodation has been invented depending on pressure transmitted to 

 the vitreous ; the retina has no conus papillaris but a membrana vasculosa 

 retince ; the visual elements are distinctive and varied in their type ; and 

 the thick optic nerve is fascicular, each bundle being provided with an 

 axial core of ependymcd cells. 



It would at first sight seem strange that the eyes of snakes should 

 be unique and so profoundly different from those of other Reptiles, 

 particularly lizards from wliich the Ophidia are directly derived. It 

 would appear, indeed, as was suggested by Walls (1942) and maintained 

 by Bellairs and Underwood (1951), that the first snakes, derived from 

 burrowing lizards, lived a nocturnal existence underground during 



1 Compare the ill-formed temporal fovea^ of the deep-sea Teleosts, Bathyirocies 

 and Balhi/Iar/us which also contain rods, p. 310. 



2 p. 731. 



