1 06 PRACTICAL HISTOLOGY. 



reddish purple, and lying between the laminae of the cornea. Trace some processes communi- 

 cating with the cells in the same plane, and others with processes of those cells that lie above 

 and below them. {Indicate this in one half of 'PL XXV., Fig. I.) 



Terminations of the Corneal Nerves. The nerve-plexus in the cornea can be well studied 

 in this section. Observe the primary nerve-plexus in the corneal tissue, and from it trace 

 bundles of nerves running vertically to form the sub-epithelial plexus. It is difficult to find 

 the very fine fibrils which proceed from this plexus to end as an epithelial plexus between 

 the cells of the anterior epithelium. 



Horizontal Section of the Cornea. EXAMINATION (L). Observe the branched corneal 

 corpuscles seen on the flat. They are deeply stained, so they stand out distinctly. The 

 primary nerve-plexus in the cornea will be only just distinguishable. 



(H). Cornea Corpuscles. Note their flattened characters, their branched processes, and 

 their frequent anastomoses with processes from adjoining cells. Each one consists of a 

 ' granular ' plate with a nucleus (PI. XXV., Fig. 2). 



Nerve-Plexus. Observe a meshwork of fine nerve-fibrils, with perhaps varicosities on them. 

 It may be that a bundle of nerve-fibrils, constituting an axis-cylinder, and surrounded with a 

 primitive sheath, may be found. 



THE CELL-SPACES OF THE CORNEA. 



PREPARATION. Pith a frog, and scrape away the epithelium from the anterior surface of 

 the cornea. Drop into the eye a few drops of a two per cent, solution of silver nitrate. Leave 

 the silver to act until the cornea becomes of a greyish-white colour throughout, which usually 

 takes from fifteen to twenty minutes. Excise the eyeball, and cut out the cornea, and make 

 one or two cuts in its margin, and lay it flat on a slide in a drop of glycerine, and expose it to 

 the light until it becomes brown in colour. Similar preparations may be made from a mammal, 

 but of course the animal must be deeply anaesthetised to avoid all pain. 



EXAMINATION (L and H). The cement-substance is stained brown, and in it is left a 

 series of clear branched and anastomosing spaces- the cell-spaces or lymph-canalicular spaces. 

 They correspond exactly in shape to the corneal corpuscles, which partially fill them (PL XXV., 

 Fig. 3). The spaces appear to be empty, because the cells filling them are not affected by the 

 silver ; but if a section be stained with logwood the nuclei of the corpuscles are brought into 

 view. Double-staining with silver and then gold brings the corpuscles into view. The silver 

 gives the negative picture and the gold preparations already described represent the positive 

 picture. 



The Fibrous Tissue of the Cornea. PREPARATION. Place a small piece of the cornea of 

 any newly-killed animal in a solution of picric acid for twenty-four hours. Tease a small 

 piece in water or glycerine. 



EXAMINATION (H). Observe the bundles of wavy fibrous tissue stained yellow. This 

 preparation need not be kept. 



