34 
Structure of the 
[Monthly Microscopical 
Journal, July 1, 1869. 
away, tlie residue, inclusive of the zonula Zinnii, hyaloid membrane, 
and posterior capsule of the crystalline lens, does not amount to 
one grain. 
The fluid which occupies the meshes of the vitreous tissue gives 
to the humour its solidity, as well as its pellucid and highly trans- 
parent quality. It has a consistence intermediate between water 
and the serum of the blood. Of the organic constituents of the 
blood, the only one which it contains, and that even in very small 
amount, is albumen. According to Berzelius, the healthy vitreous 
humour has the following chemical composition : — 
Chloride of sodium, with a small quantity of extractive 
matter soluble in alcohol . , 
Matter soluble in water 
Albumen 
Water 
100-00 
Millon and Wohler have discovered traces of urea in the fluid of the 
vitreous humour.* 
The fluid of the vitreous humour is chiefly secreted by the ciliary 
processes ; but pathological observation renders it probable that the 
vessels of the retina take some share in the same process.f Inter- 
vening between the substance of the humour and the ciliary pro- 
cesses is the zone of Zinn — a part of the containing envelope of 
the vitreous humour, — which is a structureless membrane, giving 
off internally the peculiar structure of the vitreous humour and 
externally strong fibres, which bind it firmly down to the ciliary 
processes. J The two structures are dovetailed into each other — a 
highly favourable relationship for the secretion of the vitreous fluid. 
Through the structureless membrane of the zone of Zinn the 
vitreous fluid filters in its way to the humour ; but, like all other 
cases in which secretion takes place through membranes without 
the intervention of ceUs, the vitreous fluid consists merely of the 
permeable constituents of the blood. The zone of Zinn seems likely 
to exercise some influence over the transparency and decoloration 
of the vitreous fluid during hfe, for after death the power to resist 
the coloured constituents of the blood is lost, in consequence of 
which the vitreous body soon becomes coloured greenish-yellow. A 
moderate amount of heat also relaxes this membrane so as to facili- 
tate the transudation of fluids through its walls, and consequently 
inflammatory action of the neighbouring structures will probably 
* Carpenter's ' Human Physiology,' 4th edit., p. 54. 
t Bowman : ' Lectures on the Parts concerned in the Operations on the Eye/ 
p. 125. 
X The structure of the zone of Zinn, as here stated, will be demonstrated in a 
course of lectures on the Eye, which it is my intention to deliver in Glasgow during 
the ensuing winter. 
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0-02 
0-16 
98-40 
