*jouSl\ jSr? 186?] ^d'^^t Human Vitreous Humour. 41 
blood are those only capable of being applied to the nutrition of 
non-vascular parts; but these elements exist in solution in the 
serum of the blood in such insignificant quantity, that if the latter 
were applied directly to the lens it would at once destroy its trans- 
parency. But Nature overcomes this obstacle by causing the 
serum of the blood first to pass through a structureless membrane — 
the zone of Zinn — to insure its transparency, and afterwards among 
the meshes of a living structure to separate the formative material 
from the mass of the fluid in which it is dissolved, and thus by 
enhancing its vitality renders it capable of ministering to the repair 
of the lens. This view of the vitreous humour magnifies the 
importance of its function as a component part of the eye, and makes 
it as essential to the health of the lens in the adult as it is to its 
development in the foetus. Both in the foetus and adult the vitre- 
ous humour is to the lens what the roots are to the plant : it selects 
the crude material from the blood, elaborates it into nutriment 
within itself, and, having satisfied its own demands, conducts it 
onward to the crystalline lens. 
As the vitreous humour is one of the chief agents which contri- 
butes to the fulness of the globe of the eye, the relation which it bears 
to the healthy hardness of the latter comes next under consideration. 
But as the aqueous humour and crystalline lens also contribute a 
small share in the production of intraocular tension, these last must 
also be passed in review during the discussion. 
Intraocular tension is essentially based upon the distending 
force of the fluids of the eye, enclosed by membrane and tissue, 
which give them the character of a spherical solid body. It pre- 
supposes two conditions — first, the distending force of the fluids ; 
and, second, the reaction of the membranous sphere upon the fluids. 
Both of these forces are in constant operation in the healthy 
eye, and between them, in health, an equilibrium exists, whereby 
neither gains the ascendency. The last of these will be considered 
first. 
The contents of the eyeball — the aqueous humour, crystalline 
lens, and vitreous humour — are enclosed within a structureless 
capsule, which serves to bound and limit them, and unite them into 
a glassy sphere (Fig. 5). That segment of the sphere which cor- 
responds to the posterior hemisphere goes under the name of 
hyaloid membrane (B) ; that in the anterior hemisphere, the 
posterior elastic lamina of the cornea (A). These membranes are 
united in the ciliary body (D), which throws the two segments into 
a sphere. This union may be demonstrated by making a section 
of the tunics of the eyeball in their thickness, parallel to the course 
of the ciliary processes. In such a preparation the posterior elastic 
lamina (A), after surrounding the anterior chamber in front, as a 
homogeneous structureless membrane 1-2000 to 1-3000 of an inch 
