44 Adult Human Vitreous Humour. [^SS, JuiTS^^^^ 
As the vein, in this case, is as much subjected to the pressure of the 
vitreous as if it were in the centre of the humour, an index of the 
distending force of that body is thus obtained, which may be roughly 
estimated as equal, or slightly superior, to the lateral pressure of the 
blood in the smallest veins. 
The distending force, however, to which the aqueous capsule is 
subjected by the fluid which it encloses is conSned within the 
capsule itself; outside the capsule the distending force ceases to 
exist. This may be illustrated by the following experiment : — In a 
dying person, or in a body immediately after death, when the heart 
is either failing in power or has ceased to beat, the only obvious 
physical difference in the eye in such a case and that of a person in 
the vigour of life is, that the choroid has partially emptied itself of 
blood, and the sclerotica has become soft from the want of support 
within. In such circumstances, if pressure be made on the eyeball 
with the two forefingers, the vitreous is felt as a hard globular body 
within the eye, apparently of its normal position and size. Now, 
if the vitreous communicated any distending force beyond the 
hyaloid membrane to the outer tunics of the globe, or if the latter 
reacted by an elastic force upon the vitreous body, the tension of 
the eyeball would not diminish in the ratio of the weakness of the 
heart's impulse, but the place of the receding current of blood would 
be taken in the one case, by the distending vitreous, in the other by 
the contracting tunics. That the distending force of the fluids of 
the eye is borne entirely by the aqueous capsule and structures 
within it, is also deducible from the difference between the form of 
the eyeball in health and that which it assumes in some diseases. 
In the normal eye the aqueous capsule forms a sphere (of which 
the cornea constitutes a segment), the radius of which is somewhat 
less than that of the sclerotica, and which cuts the latter at its 
junction with the cornea. The cornea therefore is more convex 
than the sclerotica, and at the line of junction a slight depression 
exists, on account of the angle which the two structures form with 
each other. The sclerotica forms no part of the sphere, and there- 
fore can sustain no part of the distending force of the fluids of the 
eye ; for if it did, the cornea and sclerotica would be segments of a 
sphere having the same radius. But when the aqueous capsule 
becomes enlarged by a superabundance of fluid, such as occurs in 
glaucoma, the lesser sphere (the aqueous capsule) becomes enlarged, 
so that the distending force formerly borne by it comes to bear 
upon the sclerotica ; in a word, the lesser sphere merges into that 
of the sclerotica, and, as a consequence, the cornea takes on the 
curve of the latter tunic, the depression which normally exists at 
the junction of the two structures becoming obhterated. — Lancet, 
May 8thf et ante. 
