244 
Histology of the Eye. 
[Monthly Microscopical 
Jouniat, Nov. 1, 1869. 
in a deeper level than those of the rods ; while in birds, in some 
reptiles, and in the batrachians, they are immediately betrayed by 
their bright-coloured beads. 
But there are many things which cannot be made out in the 
fresh retina, or which can only be recognized by a practised observer 
already familiar with their characters when they have been arti- 
ficially hardened and stained. The fresh retina is also too soft to 
allow us to cut vertical sections sufficiently thin without greatly dis- 
turbing the tissues. The most useful agents are chromic and osmic 
acids. Of the former acid, solutions of about a half per cent, are 
most useful ; they have a pale straw tint ; small eyes may be placed 
in them entire, but large ones should be cut in two before immer- 
sion. After remaining during three or four days in this solution, 
the retina will be hard enough to allow sections to be cut sufficiently 
thin for study with -^^-mch. object-glass* The usefulness of chromic 
acid lies chiefly in its hardening the retina well, with little alteration 
in the shapes of most of its elementary tissues, and enabling us 
to cut our sections in any given direction we choose — for instance, 
through the fovea, or tangental to it. But it has the disadvantage 
of distorting the elements by distending them, when the solution is 
too weak, or by shrinking them when it is too concentrated. It 
also renders them granular and proportionately opaque. Sections 
so prepared may be still stained with carmine. 
Osmic acid is, in some respects, more useful than chromic. It 
was first brought into notice by Max Schultze of Bonn, whose 
labours have thrown much light on retinal histology. Solutions of 
from a quarter to a half per cent, are best. It not only blackens 
the transparent nervous tissues, making them distinct, but it enables 
us, with a couple of fine needles, to split the retina in vertical 
planes, which afford us beautiful sections much thinner and clearer 
than any that the most practised hand can cut with the sharpest 
knife. Another advantage is, that it does not make the tissues so 
granular as chromic acid ; but it has this drawback, that with it we 
cannot run the section in any direction we choose. It is of greater 
service in those vetebrates whose retinae are devoid of blood-vessels, 
because their presence seriously interferes with clean cleavage. The 
retina, stained and hardened by osmic acid may be kept for use in 
distdled water without undergoing any further change during 
several weeks. It is best mounted in glycerine for microscopic 
examination. 
To return from this digression to the description of the retinal 
layers ; in the outermost or bacillary there are two sorts of elements, 
distinguished as rods and cones. 
Every rod and every cone consists of two segments — an outer 
one, the bacillus or shaft ; and an inner one, the appendage or body. 
The shafts of both rods and cones are highly refracting conspicuous 
