PRIMARY SPECTACLES 



451 



form a cul-de-sac and comes forward again to fuse onto the anterior 

 surface of the eyeball. 



The epidermis of this special conjunctival skin continues over the 

 cornea, slightly changed in the direction of a greater regularity of cell- 

 arrangement, to form the corneal epithelium. If we seek the dermis be- 

 longing thereto, we find it not — for it has become the outer layers of the 

 substantia propria of the cornea (Fig. 151a, b). This augmentation of the 

 cornea, by the fusion to it of a layer of skin, was not a part of the orig- 

 inal plan of the vertebrate eye at all. The original cornea was composed 

 entirely of fibrous connective tissue and was simply the skin-ward por- 

 tion, kept transparent throughout Ufe, of the dural envelope enclosing 

 the retinal cup. It quickly received two additions during early vertebrate 

 evolution — an inner one contributed by the mesothelium of the anterior 



Fig. 151 — Comparative morphology of the cornea, the conjunaiva, 

 and the three categories of speaacles. 



a, the primitive situation as exhibited by the lampreys. The primitive cornea pc is a con- 

 tinuation of the sclera, sc, and moves freely beneath a primary speaacle, which is merely 

 a transparent window in the head skin sk- b, in other fishes, the skin has fused with the 

 primitive cornea to form the definitive cornea, dc. Between a distinct margin m and the 

 eyeball, a deep sulcus s creates a fold of skin which forms a 'conjunctiva fixa' where it 

 joins the eyeball and a 'conjunaiva libera' where it lies free to permit rotation of the 

 eyeball. A line of demarcation can still be made out between the primitive cornea and its 

 new addition, and a secondary splitting of the cornea along this line will create a secondary 

 spectacle, anatomically similar to the type shown at a. c, the situation in land animals; 

 lids (/) have formed, so that the conjunaiva is differentiated into a conjunctiva palpebrae 

 (cp), conjunaiva libera (c/), and conjunaiva fixa (c/). Beneath the lids are deep fornices 

 or culs-de-sac (cds). The cornea, c, now shows no evidence of the dual origin of its sub- 

 stantia propria, d, the tertiary spectacle, sp, as seen in snakes and in some lizards and 

 fishes, has been created by the edge-to-edge fusion of horizontal or vertical lids. Between the 

 speaacle and the cornea c there is now a blind intraconjunaival space, ics, derived from the 

 culs-de-sac of the lidded ancestor. This space is lined throughout by epithelium (stippled). 



