LACHRYMAL APPARATUS I 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 sac, 

 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. 



54:2. 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 



