176 PHILOSOPHY OF ZOOLOGY. 
The Vitreous Humour occupies the posterior and late- 
ral parts of the cavity. It is convex behind, and concave 
before. It is invested by a delicate thin transparent mem- 
brane, termed the Hyaloid. At its anterior surface, it may 
be separated into two laminze, between which, air may be in- 
troduced. This is the bullular canal of Petit. The in- 
terior of this humour is divided by the same kind of mem- 
brane, into numerous cells. ‘These are filled with a fluid, 
of the consistence of the white of an egg, which does not 
readily escape from them, even when the external mem- 
brane is punctured, This fluid consists chiefly of water, 
with a small quantity of albumen and gelatine. 
The Crystalline Humour, or Lens, as it is frequently 
called, occupies the centre of the cavity of the eye. It 
is doubly convex, its posterior side resting in a concavi- 
ty of the vitreous humour. Itis enclosed in a membranace- 
ous capsule, which is soft and transparent, and with which 
it is but loosely connected. The lens itself is denser at the 
centre than towards the circumference. It becomes indurat- 
ed by boiling and by alcohol, and then exhibits its peculiar 
structure. It consists of an infinite number of concentric 
laminze, formed from delicate fibres which proceed from 
two centres, situated at the two extremities of the axis. 
At each axis, there is the appearance of a membrane dis- 
posed in rays, from which the fibres originate. ‘These 
rays vary in number, in different animals, from two to five. 
The rays of the one axis, are placed opposite the intersti- 
ces of the other. The matter of the lens coagulates by 
boiling, and consists of water and albuminous matter, with 
a small quantity of cellular substance. 
‘The Aqueous Humour occupies the remaining part of 
the cavity of the eye, in front of the crystalline. It is di- 
vided by the iris into two chambers. The anterior, which is 
the largest, occupies the space between that membrane and 
