ORGANS OF SPECIAL SENSE THE EYE 341 



The lashes on the edges of the lids protect the eyes some- 

 what from dust. The inner surface of the lids is really nothing 

 but the external skin folded back underneath, but it is pro- 

 vided with special glands which are not present in the ordinary 

 surface of the skin. These Meibomian glands are arranged in 

 little clusters, with their openings near the edges of the lids 

 (Fig. 166) and are more abundant in the upper than in the 

 lower lid. Their secretion is oily and by mixing with the 

 more watery product of the tear glands prevents the rapid 

 drying of the latter as it is spread over the eye in a thin 

 layer when one winks. 



A tear, or lachrymal, gland is located just above the 

 outer corner of each eye. It is about three-quarters of an 

 inch long, one-quarter of an inch wide, and the material which 

 it secretes is poured under the upper lid through six or seven 

 tiny canals. This lachrymal secretion is largely water, with 

 a little salt, and the smearing of the secretion over the eye- 

 balls in winking prevents the delicate membranes of the eye 

 from becoming dry, and consequently more or less opaque. 



The liquid material secreted by the several glands about 

 the eye runs away through two tiny openings in the inner 

 corner of each eye into canals, which unite into the lachrymal 

 duct, leading to the nose chamber. A downward current is 

 produced in these canals by cilia which project into them on 

 all sides. 



Muscles which Move the Eye. The movement of the eye is 

 effected by six different muscles, all of which are attached to 

 the eyeball at different points back of the lids: Fig, 165. Four 

 of these, one above, one below, one outside and one inside, are 

 called rectus muscles and pass from their attachments to the 

 eyeball directly back to a point in the deepest part of the 

 socket. The one next the nose is called the internal rectus; 

 the outer, the external rectus; the upper is the superior, and the 

 lower the inferior rectus. The other two muscles are called the 

 obliques. The inferior oblique is attached to the lower sur~ 



