266 



THE EYE IN EVOLUTION 



two layers to prodvice a similar configuration may occur in some fishes as an 

 adaptation to protect the eye when the animal is crawling in mud or sand 

 (bottom-fishes, lung-fishes, cat-fishes) or to prevent desiccation in forms which 

 leave the water for air (lung-fishes, eels, mud-skippers, some gobies, etc.). An 

 entirely different configuration — the secondary spectacle — is formed by the 

 development of a transjMrent area in the lids, either a transparent window in a 

 moveable lower lid, as in a few chelonians and some lizards, or by the edge-to- 

 edge fusion of the two lids which have become transparent to form a fixed 

 spectacle as is seen among Fishes in anchovies and in many Reptiles 

 (snakes and some lizards) ; it is this that gives the characteristic glassy 

 stare to the eyes of snakes and most lizards. In this case the cornea is comprised 

 of all its constituent layers and between it and the fused lids there is a true cavity 



Figs. 278 and 279. 



Spectacles." 



Fig. 278. — The primary spectacle of 

 Cyclostomes and aquatic Amphi- 

 bians. 



E, the surface epithelium forming 

 the dermal cornea ; C, scleral cornea ; 

 M, mucoid tissue between the two. 



Fig. 279. — The secondary spectacle as 

 seen jiarticularly in Reptiles. 



E, the " spectacle " formed by 

 fusion of the lids which are transparent; 

 C, the cornea ; S, the conjunctival sac 

 lined throughout by epithelium, proxi- 

 mally corneal and distally palpebral. 



(the conjunctival space) lined by epithelium, the distal part of which represents 

 the palpebral conjunctiva, the proximal the corneal epithelium (Figs. 279 and 

 470) (Hein, 1913 ; Franz. 1934 ; Walls, 1942). 



The uveal tract of the lamprey is also primitive in its characteristics. 

 A single artery penetrates the sclera beneath the optic nerve, which 

 divides into four vessels, one for each quadrant ; these break up into 

 a choriocapillaris overlying the retina, but instead of the efferent blood 

 being drained away by veins, the outer half of the choroid is composed 

 of a continuous lake of blood (the subscleral sinus) which in turn 

 leads hy four apertures traversing the sclera into a complex system of 

 extra-ocular venous sinuses surrounding the outer aspect of the sclera 

 (Figs. 276-7). In the posterior half of the globe between the subscleral 

 venous =<inus and the sclera there is in some species (Petromyzon 

 marinus) a peculiar epichoroidal tissue composed of large pigmented 



