46 
resting on this layer are still less elongated and smaller; and 
it is in this region, in the middle depth of the epithelium, 
that it is most difficult to distinguish the component elements 
as they lie in position. Carrying the examination still 
ontowards ward the surface, large cells are again met with, 
superimposed on which are others of a flatter form ; and at 
last the squamous layer is reached which covers all. 
By subjecting the dyed epithelium to a strong solution of 
the bichromate of potash, its elements may be completely 
isolated, and then there are seen floating numerous smaller 
corpuscles than could be well distinguished when the parts 
are in position. Among these are apparently free nuclei, and 
others in minute corpuscles of spheroidal or less regular form ; 
also, bluntly spindle-shaped cells, containing sometimes a 
single nucleus, sometimes two, and occasionally a nucleus in 
process of division. These spindle-shaped cells have their 
position between the processes of the columnar stratum and 
among the digitated cells which are next to it. 
Here and there in the columnar stratum an appearance of 
great importance is seen, namely, cells in various stages of 
decadence. Cell-walls, shrivelled and narrow, without any 
appearance of a nucleus, occur, which might be supposed to 
have been torn and deprived of the nucleus in manipulation 
were it not that the other stages of old age are observed. 
Columnal cells retaining their general form are noticed, 
which have the nucleus replaced by an air cavity with gene- 
rally the appearance of thickening of the Avail round it, as if 
from desiccation of the cell contents : numerous others may 
be seen in which, Avhile the cell contents seem yet unaltered, 
the nucleus is partially replaced by an air cavity lying close 
to the remaining portion. Other cells beside the columnar 
may sometimes be found undergoing the same degeneration; 
and in the sheep these air cavities are specially numerous in 
the squamous stratum of the corneal epithelium. 
The appearances noAv described seem to prove conclusively 
tAvo points ; firstly, that the corneal epithelial cells are not 
developed in regular succession from the deepest part and 
pushed outAvards as they groAv ; and, secondly, that they are 
not all cast off at the surface, but that some of them degene- 
rate and are absorbed in a deep position. 
The source of origin of the columnar stratum is not to be 
found on its deep surface ; and the digitated cells are certainly 
not altered and more fully developed columnar cells. But it 
is alleged by Dr. Schneider, as quoted by Dr. Sharpey , 1 that 
the elongated cells of the deepest layer of the corneal 
1 Quain’s ‘ Anatomy,’ seventh edition, p. lvi. 
