372 



THE EYE IN EVOLUTION 



Tortoise 



The pupil is circular and immobile both to light and drugs although 

 its sphincter is powerful ; this muscle is essentially accommodative 

 in function (Fritzberg, 1912). The ciliary body separates abruptly from 

 the sclera to approach the lens leaving the angle of the anterior 

 chamber deep and cleft-like ; the angle is traversed by the loose 

 pectinate ligament linking the iris with the cornea, while deep in the 

 cleft lies the ciliary venous sinus. The ciliary body has some 60 well- 

 marked ciliary processes which abut against the lens in accommodation. 

 The striated musculature resembles that of the lacertilian eye with 

 the ventral transversalis muscle usually well-developed (Briicke, 1846 ; 

 Mercanti, 1883 ; Hess, 1912 ; Fritzberg, 1912) ; the latter is absent 

 in some forms {Testudo, Konig, 1934). The vascular arrangements of 

 the uveal tract are of the usual reptilian type (Fritzberg, 1912). 



The lens is extremely soft and almost fluid in consistency, probably 

 the most readily moulded in the vertebrate phylum, and while it takes 

 the form of a flat ellipse in land tortoises, it is of necessity almost 

 spherical in sea turtles ; the annular pad is small. 



The fundus oculi of Chelonians as seen ophthalmoscopically is 

 singularly primitive and uniform (Plate VII, Fig. 3). The background 

 is orange-red and from the circular disc readily visible nerve fibres 

 radiate to the periphery, sometimes, as in the snapping turtle, Chelydra 

 serpentina, almost completely obscuring the background. The disc is 

 without a conus and is white, apart from a brownish patch of pigment 

 in the Murray turtle, Chelodina longicollis, in which the nerve fibres 

 are few and faintly marked. 



The fundus of the Bvirgoma soft-shelled turtle, Emyda granosa, is unique 

 (Plate VII, Fig. 4). The background is of brownish pink with red dots, and the 

 large white disc is surrounded by a red choroidal ring outside which the nerve 

 fibres radiate giving the ai3i:)earance of a solar corona (Johnson, 1927). 



Histologically the retina does not reach the high degree of defini- 

 tion in its architecture found in the lizard ; throughout its extent the 

 different layers are by no means exclusively segregated but their 

 elements tend to be intermingled (Figs. 449-452). ^ In the early stages 

 of development an avascular glial cone may appear- on the optic disc 

 in some turtles ^ but this always disappears in the adult ; the retina is 

 thus entirely avascular depending only on the choroid for its nourish- 

 ment. The visual cells show a vast predominance of cones, either single 

 or double, the former and one element of the latter containing an oil- 

 droplet, orange, yellow or ruby -red in colour. Cells with a cone -like 

 structure but resembling rods in the heaviness of the outer segment 



1 See Hulke (1864), Heinemann (1877), Chievitz (1889), W. Krause (1893), Putter 

 (1912). 



2 In the sea-turtle, Chelonia, the snapping turtle, Chelydra, the painted turtle, 



Ch^-;icmys, etc. 



