200 



THE SPECIAL SENSES. 



FIG. 47. FRONT VIEW op RIGHT EYE. 

 (Natural Size.) 



1. The Lachrymal, or tear gland, lying be- 

 neath the upper eyelid. 



2. The Nasal Duct is shown by the dotted 

 line. The * marks the orifice iu the lower 

 lid. 



The central black spot is the pupil; sur- 

 rounding it is the iris ; and the triangular 

 white spaces are the visible portion of the 

 sclerotic. 



54. The Eyelids. The eyelids are two movable cur- 

 tains, or folds, which, when shut, cover the front part of 



the orbit, and hide the 

 eye from view. The up- 

 per lid is the larger, has a 

 curved margin, and moves 

 freety, while the lower lid 

 is comparatively short 

 and straight, and has but 

 a slight degree of motion 

 (Fig. 47). Skin covers 

 the exterior of the lids, 

 while a fine mucous 

 membrane lines their in- 

 ner surface, and is like- 

 wise spread out over the 

 entire front of the eye- 

 ball. This membrane, 

 which is called the Conjunctiva, is highly sensitive, and 

 thus plays an important part in protecting the eye against 

 the lodgment of sand, ashes, chaff, and other foreign parti- 

 cles that are blown about in the air. This sensitive mem- 

 brane will not endure the presence of these particles. If 

 any find access, it causes a constant winking, a flow of 

 tears, and other signs of irritation, until it is removed. 



55. The long, silky eyelashes, which garnish the edges 

 of the lids, act like a sieve to prevent the entry of dust and 

 the like; and together with the lids, they regulate the 

 amount of light which is permitted to enter the eye, so that 

 it is shielded from a sudden flood or glare of light. The little 

 points seen in the figure just within the line of the lashes, 

 especially on the lower lid, represent the months of numer- 

 ous little sebaceous glands (Fig. 48, D,D), such as are always 



54. What are the eyelids? The upper lid? The lower one? The mucous 

 membrane of the eye ? 



55. The eyelashes? The little points within the line of the lashes ? Of what 

 use are these glands ? 



