Snl\ ^^^^^ Human Vitreous Humour. 43 
The vitreous tissue in the posterior segment, and the posterior 
elastic lamina of the cornea in the anterior segment, confer upon 
the aqueous capsule the property of elasticity in a high degree, so 
that it is ahle to resume its original shape after it has been altered 
by any of the physiological actions of the eye. The elasticity of the 
vitreous tissue has been already adverted to (vol. ii. 1868, p. 378). 
Further evidence that it is possessed of this physical property might 
be multiplied. I will only mention one other example. Place a 
fragment of vitreous between two sHps of glass, previously rendered 
opaque by a weak solution of nitrate of silver, to enable its action 
to be more readily observed. On pressing the glasses in contact 
the vitreous tissue is observed to expand, and on removing the 
pressure it instantly regains its former size. The hyaloid membrane 
is not extensible, though it bears a greater strain than might be at 
first supposed. The elasticity of the limiting membrane of the 
anterior hemisphere is a well-established fact.* The elastic struc- 
tures of the aqueous capsule of the eye contract upon the contained 
fluid, and give it the character of a solid sphere. In the healthy 
state of parts it is always full, offering in front a smooth surface for 
the perfect refraction of the rays of light, and behind for the close 
adaptation of the retina which is spread out upon it. 
But not only is the aqueous capsule, in health, always full, but 
the fluid which it encloses subjects it to a distending force, against 
which it reacts in an equal degree. The distending force to which 
the anterior hemisphere is subjected is often illustrated in practice 
in cases of ulceration of the cornea. The posterior elastic lamina, 
in these cases, in losing the support of the lamellated tissue in front, 
may often be observed to protrude through the bottom of the ulcer 
in the form of a vesicle. This is caused by the distending force of 
the fluid within. The distending force of the vitreous body is illus- 
trated by the pressure effects on the central vein of the retina. In 
the normal state of parts the central artery and vein of the retina, 
as they pass over the optic disc, are raised above its surface, and are 
in direct contact with the hyaloid membrane, even projecting some- 
what into the substance of the vitreous. In such circumstances the 
artery does not show any signs of pulsation, but the vein does in 
many cases ; and in all the latter is flattened by the pressure of the 
vitreous, and pulsation is easily provoked in it. Now, the pulsation 
of blood-vessels, in any case arises from the resistance which the 
walls of the vessels or outlying structures give to the distending 
force of the blood. While, in the eye, therefore, the distending 
force of the vitreous humour is insufiicient to compress the arteria 
centrahs retinae to that degree which will give rise to pulsation, it 
is sufiicient to flatten the vena centralis retinae, and to give rise to 
pulsation, or an intermittent current of blood over the optic disc. 
* Bowman : Op. cit, p. 19, 
