452 ADAPTATIONS TO MEDIA AND SUBSTRATES 



chamber and its basement (Descemet's) membrane, together with per- 

 haps some connective tissue outside of the latter and homologous with 

 the iris stroma; and an outer addition of skin whose epidermis became 

 the corneal epithelium and whose dermis merged and fused with the 

 outer surface of the original dural (sclerotic) tunic, which the retinal 

 cup had carried outward with it when it grew away from the side of the 

 brain (see p. 119). 



Before this second addition was made, the eyeball had been required to 

 remain below the level of the skin and to look out through a flat window 

 therein. This is still the situation in lampreys (Fig. 103, p. 258), which 

 are too primitive ever to have produced a conjunctiva — a conjoining of 

 skin and eyeball. The field of vision is restricted just as is that of a man 

 who looks through a closed window. If he opens the window and puts 

 his head out, he can see much more. 



The higher fishes could not open the window, but they could bulge it 

 outward — make a bay-window of it, so to say. Friction on the eyeball 

 being then intolerable, it was expedient to fuse the window onto the eye, 

 retaining rotability by simultaneously producing, around the window, a 

 deep circular infolding of flexible, membranous skin so that slack could 

 be allowed to permit of turning the eyeball. 



Thus the conjunctiva came into existence. The addition to the cornea 

 was coincidental, and not produced for its own sake. The circular fold 

 of skin overlapping the cornea all the way around proceeded to come in 

 handy, as when the eye of a ray, for instance, is hauled back into the 

 orbit by the retractor bulbi muscle, and the skin puckers together over 

 the eye and protects it. Land animals found that a much neater arrange- 

 ment was possible, by extending the superior and inferior margins of the 

 fold to form permanent upper and lower lids. All of them, in their em- 

 bryonic development, still form their lids from a circular, at first con- 

 tinuous, fold. These lids being opaque and shutting off vision whenever 

 they are closed, some animals have added a third, almost or completely 

 transparent lid, made by folding the conjunctiva in the nasal corner of 

 the eye and pulling this fold — the nictitating membrane — laterally over 

 the cornea by special means (Fig. 142, p. 420). 



The triple origin of the definitive vertebrate cornea cannot ordinarily 

 be made out in a histological preparation. The human cornea, under 

 the slit-lamp microscope, does show a superficial extra-clear layer under 

 the epithelium which may represent the dermal contribution to the sub- 

 stantia propria. The connective-tissue fiber-bundles are here somewhat 



