REPTILES 



389 



annular pad. there is a region on the anterior surface (except in BoicW) 

 where the subcapsular epithelial cells instead of being cuboidal are 

 elongated to form an anterior pad (Fig. 468). In most diurnal types 

 the whole structure is pigmented yellow (Rabl. 1898 ; Hess, 1912 ; 

 Walls, 1931). The zonule consists of two systems of fibres, one running 

 from the anterior surface of the ciliary roll to the anterior surface of 

 the lens, the other from the posterior surface of the ciliary body to the 



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Ketixa of the Grass Sxake. T ik 



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The pure-cone retina of a dinrnal snake. 1. optic ner\-e fibre layer ; 2, 



ganglion cell layer ; .3, inner plexiform layer ; 4, inner nuclear layer ; .■>, outer 



plexiform la,\-er ; 6, outer nuclear layer ; 7. external limiting membrane ; 8, 



cones ; 9, pigmentary epithelium ( ;■ 330) (Katharine Tansley). 



posterior surface of the lens ; except in the boa. Emcrates, there are 

 no intermediate fibres attaching to the equatorial region between these 

 two systems. Accommodation is effected by a unique mechanism cpiite 

 different from that seen in other Reptiles. ^ 



The fundus oculi seen ophthalmoscopically presents a remarkably 

 constant picture (Johnston, 1927) (Plate X. Figs. 1 and 2). The back- 

 ground is grey mottled with spots, usually white (as in the corn-snake. 

 Coluber guftatus) or red (as in the Boidse). and the semi-opaque nerve 

 fibres radiating uniformly from the optic disc are consi)icuous. Occa- 



1 p. 64S. 



