158 
Proceedings of the Royal Irish Academy. 
is softened, and that abcesses have formed in it, then the connective 
tissue cells show some changes of a purely passive kind such as a 
granular opacity of their protoplasm, a retraction of their processes and 
a formation of vacuoles in the interior of their cell-body ; but in no case 
either of early or advanced keratitis is ever any appearance met with 
which could lead to the suspicion of a formation of pus corpuscles from 
the connective tissue cells. 
As observations on the fresh cornea are difficult in consequence of 
its great transparency and the slight difference between the refractive 
power of the connective tissue cells and that of the intercellQlar substance 
in which they lie, Cohnheim recommends that the membrane should, be- 
fore examination, be stained with chloride of gold. This most valuable 
reagent the knowledge of which, as applied to microscopy, we owe to 
Cohnheim, is used in the following way. The salt is used in solution of 
the strength of J per cent., made with distilled water, to which a few 
drops of acetic acid have been added. In this solution the perfectly fresh 
cornea is immersed for from ten to twenty minutes (the light being 
excluded), till it has acquired a distinctly yellow colour. It is then 
removed from the gold solution and placed in distilled water, to which 
enough acetic acid has been added to make it just sour to the taste, 
and submitted to the strongest sunlight which can be procured. After 
a time, varying from a few hours to some days according to the tem- 
perature and the intensity of the light, the cornea becomes of a reddish 
or purple colour from the reduction of the chloride of gold, and is fit 
for examination. The epithelium is scraped off its anterior surface, 
some radial cuts are made in its margin, so as to allow it to lie flat on 
the slide, and it is mounted in glycerine. The cornea of a small 
animal, such as a frog, may be mounted whole, that of larger animals 
has to be cut or torn into lamellse parallel to the surface. In successful 
preparations, the corpuscles are seen to be stained of a colour varying 
from red through purple to almost black. The nerve fibres also co 
their finest terminations are similarly stained, while the intercellular 
substance is left quite uncoloured. The specimens may be examined 
with the highest powers and leave nothing to be desired in the beauty 
and distinctness of the appearances. Besides showing clearly the 
shape of the cells, this method makes their nuclei, which are not at all 
visible in the fresh condition, quite distinct, and it leaves the character 
of the protoplasm as it was in the living state, its more or less coarsely 
granular appearances being preserved. With the help of the chloride 
of gold Cohnheim confirmed his observations on the uncoloured tissue, 
and found, however numerous the pus cells might be in any part, that 
nevertheless the fixed cornea corpuscles continued to exist unchanged 
from their normal condition. 
As the origin of pus from the connective tissue cells was thus ex- 
cluded, two possibilities remained by which its presence in the cornea 
could be explained. It might be derived from the so called wandering 
cells by their division ; or it might not originate in the cornea at all, 
but get into it from without. 
