VISION. 



467 



low of the orbit, and the ducts (d) proceeding 

 from it open upon the inner side of the upper 

 eye-Hd (e). This fluid, the uses of which are 

 obviously to wash away dust, or other irritating 

 substances which may happen to get introduced, 

 is distributed over the outer surface of the eye 

 by means of the eye-lids. Each lid is sup- 



ported by an elastic plate of cartilage, shaped 

 like a crescent, and covered by integuments. 

 An orbicular muscle, the fibres of which run in 

 a circular direction, immediately underneath the 

 skin, all round the eye*, is provided for closing 

 them. The upper eye-lid is raised by a separate 

 muscle, contained within the orbit, immediately 

 above the upper straight muscle of the eye-ball. 



* See Fig. 46, vol. i, p. 136. 



