CORNEA 75 



back corneal surfaces. Descemet's membrane is optically con- 

 tinuous with, and is indistinguishable from, the substance anterior 

 to it. The dichotomously branching normal nerve-fibres are 

 readily visible in the optical section of the cornea. Reference 

 has already been made (Fig. 25) to the visibility of the lining 

 endothelial cells when the posterior surface is viewed in the axis 

 of specular reflection. If the anterior surface is viewed in the 

 axis of specular reflection no cell-structure is visible, the surface 

 acting as a mirror being in this case the covering layer of tear- 

 fluid ; moreover, the cellular layer on the anterior surface is 

 multiple. Near and at the limbus the epithelial surface line 

 becomes very slightly raised from the underlying transparent 

 fibrous tissue, by the intervention of a narrow optical gap, so 

 that it is discrete (4, Fig. 3). The most superficial vessels which 

 spread over the cornea in the condition of " pannus " lie in the 

 plane of this gap. On the peripheral side of the limbus, i.e., 

 over the sclera, this gap is widened by the presence of the lax 

 areolar tissue which lies between the epithelium and the sclera 

 throughout the bulbar conjunctiva. 



Reference has been made to the internal illumination of the 

 cornea faintly but clearly against a dark background by the simple 

 means of " sclerotic scatter " (Figs. 33 and 34). In Fig. 44, A 

 shows the detection of a small nebula (at 2) of the cornea, the beam 

 of light being focused on to the sclera near the limbus (at 1) ; (3) is 

 the crescent of light in the sclerotic diametrically opposite the 

 region of imj^act of the beam and after it has passed across the 

 cornea. The region of the nebula itself is shown in B, under 

 higher magnification in the direct illumination of the wide slit- 

 beam. The nebula is a superficial one and bears with it vessels 

 continuous with the conjunctival vessels. Under still higher 

 magnification in C, the plane of these superficial vessels is seen 

 by direct illumination (1) at its true level in the proximal lateral 

 face of the block, other vessels of the same group being visible by 

 retroillumination (2) against the light-patch on the iris behind. 

 The plane of the main cellular infiltration which is giving rise to 

 this nebula is finally defined with exactness in the thin optical 

 section D. 



