422 LACHRYMAL APPARATUS :—SENSE OF VISION. 
tears, that we become conscious of its existence. It is ordi- 
narily drawn-off as fast as it is formed, by a curious apparatus — 
situated at the inner border of the eye. If the edges of the — 
lids be carefully examined, there will be seen upon each of © 
them, close to the inner corner of the eye, a minute spot 
which is the entrance to a small canal termed the lachrymal — 
duct. The two ducts, one commencing at the corner of the 
upper lid and the other at that of the lower, soon unite into one 
canal, which swells into a sort of reservoir, the lachrymal sae, 
_ that lies upon the side of the upper part of the nose ; and 
from this reservoir a canal passes down through the bones of 
the nose into its cavity. By this apparatus, the fluid which 
is poured by the lachrymal gland over the exterior of the eye, 
is drawn-off at the interior after washing its surface ; whence 
it is carried into the nose, to be got rid of by the current of © 
air that passes through its cavity in breathing. The edges 
of the lids meet in such a manner, when they are closed, as: 
to form a sort of gutter or channel, along which the lachrymal 
secretion flows from their outer to 
their inner corner during sleep. 
542, Having thus described the 
structure of the Eye, and the general — 
actions of the parts by which it 
is adapted to the performance of 
its remarkable function, we shall — 
examine into the details of this 
function ; in other words, into the 
nature of vision. 
543. The rays of light which 
diverge from the several points of 
any object, and fall upon the front 
of the cornea, are refracted by its 
convex surface whilst passing 
through it to the eye, and are made 
to converge slightly. They are 
brought more closely together by 
the crystalline lens, which they 
reach after passing through the 
pupil ; and its refracting influence, 
together with that produced by the vitreous humor, is such 
‘as to cause the rays that issued from each point to meet 
Fig. 210. 
