THE FIBROUS TUNIC 11 



in the fibrous tunic. It is at the Umbus that an isolated eyeball will 

 rupture, if it is squeezed until it bursts. 



The outer surface of the fibrous mass (substantia propria) of the 

 cornea is covered by a stratified epithelium which is much like that lining 

 the mouth, and lacks the dead, homy outer layers which are present on 

 the general epidermis of the body. This corneal epithelium (Fig. 4b, e; 

 Fig. 5, ce) is continuous, at the limbus, with the thicker and less regular 

 epithelium of the conjunctiva (Fig. 5, ec). The conjunctiva (Fig. 5, co) 

 represents head skin which at the margins of the upper and lower eyelids 

 is doubled back on itself up underneath them to form their linings, and 

 is continuous over the front of the eyeball, with which it is fused to form 

 the 'conjunctiva fixa' — the loose folds in the culs-de-sac up under the 

 lids being the 'conjunctiva libera'. The connective-tissue dermis of the 

 conjunctiva fixa can hardly be distinguished from the loose connective 

 tissue which clings to the sclera; but at any rate it stops at the limbus 

 and only the epidermis appears to continue over the cornea. 



Actually the dermis belonging to the corneal-epithelium part of the 

 conjunctiva is represented by some of the outermost layers of the sub- 

 stantia propria — no one can say just how many, in the case of the human 

 eye. The very outermost layer, just beneath the epithelium, is devoid of 

 cells and stains a little differently. It is known as Bowman's membrane 

 (Fig. 4b, b) ; but it scarcely deserves recognition and in the lower animals 

 cannot ordinarily be made out at all as a distinct part of the substantia 

 propria. 



The corneal epithelium is richly supplied with pain-sensory nerve end- 

 ings and apparently with no others, and is remarkable for the speed with 

 which it can grow to repair or replace itself if injured. There is some 

 reason to think that it is normally nourished from entirely outside the 

 body — from the tears, which contain appreciable amounts of nutrient 

 substances. 



The inner surface of the cornea is lined by a thin pavement of cells 

 usually called the endothelium of the cornea. (It more properly deserves 

 to be considered a portion of the mesothelium of the anterior chamber, 

 however, since it is continuous with the anterior covering of the iris and 

 the term 'endothelium' is outmoded as applying to mesodermal epithelia 

 generally). Between the substantia propria and the mesothelium, and 

 secreted by the latter as its basement membrane, is the thin, homo- 

 geneous, elastic 'membrane of Descemet' (Fig. 4b, d). 



