VISION. 



417 



stances which may happen to get introduced, is 

 distributed over the outer surface of the eye by 

 means of the eye-lids. Each lid is supported 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, im- 

 mediately 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. The eye-lashes are curved in op- 

 posite directions, so as not to interfere with each 

 other when the eye-lids are closed. Their utility in 

 guarding the eye against the entrance of various 

 substances, such as hairs, dust, or perspiration, and 

 also in shading the eye from too strong impres- 

 sions of light, is sufficiently apparent. The eye- 

 lids, in closing, meet first at the outer corner of 

 the eye ; and their junction proceeds along the 

 line of their edges, towards the inner angle, till 



r" 



VOL. II. 



See Fig. 46, vol. i. p. 

 E K 



125. 



