CHAPTER XXI 

 ORGANS OF SPECIAL SENSE THE EYE 



In a preceding chapter we noted that every sensory nerve 

 ends in a special bulb, or corpuscle, and some of these were 

 shown in Figures 117, 118 and 162. The endings there rep- 

 resented were very simple, having only simple functions to 

 perform, like that of receiving touch, heat or pain stimuli. 

 There are other sensory nerves, particularly some of those 

 coming out of the brain directly, which have very elaborate 

 endings, usually spoken of as organs of special sense. The 

 most important of these are the eye and the ear. 



EXTERNAL PARTS OF THE EYE 



The eye (Fig. 165) is set into a cavity in the skull, the eye 

 socket, which is lined with a layer of fatty tissue serving as a 

 cushion. The eyeball is not perfectly round, since over the 

 colored portion at the front the sphere bulges slightly, although 

 not enough to be noticed ordinarily. 



Externally the eyes are protected by the lids, which are 

 folds of the skin of the face supplied with muscles to permit 

 movement in winking; Fig. 166. In the process of moving 

 the lids two muscles especially are concerned: one, the so- 

 called orbicular, is a circular muscle running around in both the 

 upper and the lower lids; the other, the leva tor, is in the 

 upper lid, and runs upward from the edge and raises it after 

 the orbicular has closed the eye. The lower lid moves very 

 little, and is not drawn downward by any special muscle. 



At the inner corner of each eye is a little whitish mass of 

 tissue which is of doubtful value in man; in birds and some rep- 

 tiles, a corresponding structure is a third eyelid, which is trans- 

 parent and can be closed while the others remain motionless. 



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