THE EYE OE THE OX. 
225 
front, by the posterior surface of the cornea, and a small 
portion of the sclerotic; behind, by the front of the iris; e 
being’ the pupil of the eye through which the two chambers 
communicate. The posterior chamber is formed by the pos- 
terior surface of the iris, by the front of the ciliary processes, 
and lens, and a small part of the hyaloid membrane. These 
chambers contain a fluid nearly as thin as water ; and therefore 
called aqueous humour {aqua, water). 
The iris (Plate XI., fig. 2, b, 3, f, 8, 9, b,) is a circular dia- 
phragm, with a central aperture called the pupil (fig. 3, e, 8, 9, a ) ; 
it is attached, at its circumference, by means of the ciliary liga- 
ment (which is seen with the ciliary muscle at c, c, Plate XI., fig. 2) 
to the sclerotic. The iris consists of muscular fibre, of blood- 
vessels, and of colouring matter. The direction of the muscular 
fibres is mostly radiating; but at the inner margin of the iris 
are a few fibres which have a circular arrangement around the 
pupil. The vessels are numerous, but slender and delicate. 
The colouring matter is situated in irregularly-shaped cells, 
which very much obscure the microscopic structure of the iris. 
When we speak of a blue eye or a hazel eye, we always refer to 
the colour of the iris ; which is so named from its varying colours 
in different individuals {iris, a rainbow). The pupil is so named 
from pupilla, a young girl or child, as in the situation of the 
pupil, a diminutive image of the person looking at it is formed 
by reflection from the crystalline lens : amongst the Greeks, 
in the same way, the pupil was called Jcore, the daughter of 
the eye. 
The ciliary muscle (Plate XI., fig. 2, c, 3, h) is a circular band, 
attached on one side to the sclerotic, and by the other to the 
front of the ciliary processes. Its fibres radiate from the at- 
tachment to the sclerotic, some to the posterior part of the 
ciliary processes, some to the more prominent parts, where the 
processes approach the lens. 
We will now 
consider the func- 
tions of the dif- 
ferent parts of the f 
eye during the 
passage of the 
light rays. We 
see an object, be- 
cause rays of light fig. 17 . 
are reflected from 
the surface of the object, and meet the eye, the impression sub- 
sequently passing to the brain. These reflected rays diverge 
from each other, more or less, according as the object seen is at a 
less or greater distance from the eye. At a distance of eighteen 
