74 PHYSIOLOGY OF THE HUMAN BODY. 
of the eye, 10, and also the posterior, 11, or that part behind the iris 
and in front of the crystalline lens, 12. The crystalline lens is a humour 
like a thick, transparent jelly, enclosed in a transparent bag, and attached 
to the choroid, so as to keep it in its place immediately behind the pupil. 
As a whole, it forms a double-convex lens (Oprics, p. 28), but with the 
posterior surface more convex than the anterior. Filling the whole cavity 
behind the crystalline lens is the vitreous? humour, a glassy, transparent 
fluid, of the consistence of thin jelly. The optic nerve enters immediately 
behind the eye, 2 (fig. 61), and is spread out into the retina. 
All the movements of the eye are performed by six muscles: four 
attached directly opposite to each other, above, below, and at each side, 
9, 13, 10, and 11—12; and two oblique muscles, which turn the eyes 
towards any of the corners. The 
act of thus pulling the eye forward 
is effected by these oblique muscles, 
7, 8, passing through pulleys, as 6, 
in fig. 61. A very important 
appendage of the eye is the eyelid, 
which serves to protect the eye. 
when closed, and also to sweep off 
any dust or impurity, by coming 
Fig. 61. down over the eye, which it can do 
so rapidly as not to interfere with 
_ the sight. And in order to prevent a disagreeable rubbing in doing 
this, there is a fluid known as tears (when in more than an ordinary 
quantity), being continually poured on the upper part of the eye 
from the lachrymal? gland. This fluid is carried off (except when so 
abundant as to overflow the eyes) by a little canal which communicates 
with the nose. The entrance to it may be seen on the inner edge of the 
under eyelid, almost in the inner corner. 
When diverging (Optics, p. 26) rays of light, from any point, fall 
on the eye, those that fall on the opaque sclerotic, or white part of 
the eye, are lost. The cornea being transparent, the rays that fall on it 
pass 0n ; then those that fall on the coloured iris are absorbed, while some 
pass through the pupil. After traversing the aqueous humour, which 
probably has little effect in changing their direction, the rays strike on 
_ the crystalline lens; the latter, on account of its convexity and comparative 
density, has strong refracting power, and after passing through it and the 
vitreous humour (the function of which is chiefly to support the retina), 
the rays are drawn to a point, or focus, on the retina. When rays from 
all the points of a body are thus thrown on the retina, the impression 
conveyed to the brain is a complete image of the object. 

1 From Latin vitrum, glass. 2 From Latin lachryma, a tear. 
