CHAP. XIX.] OKGANS OF SPECIAL SENSE. 



241 



(the Meibomian glands a ), the ducts of which open on the edge 

 of the eyelid. The secretion of these glands is provided to 

 prevent adhesion of the eyelids. 



Arranged in a double or triple row at the margin of the lids 

 are the eyelashes ; those of the upper lid, more numerous and 

 longer than the lower, curve upwards ; those of the lower lid 

 curve downwards, so that they do not interlace in closing the 

 lids. The upper lid is attached to a small muscle which is 

 called the elevator of the upper lid; and arranged as a sphincter 

 around both lids is the orbicularis palpebrarum muscle, which 

 closes the eyelids, and is the direct antagonist of the elevator of 

 the upper lid. 



The slit between the edges of the lids is called the palpe- 

 bral fissure. It is the size of this fissure which causes the 

 appearance of large and small eyes, as the size of the 

 eyeball itself varies but little. The outer angle of this fissure 

 is called the external canthus ; the inner angle, the internal 

 canthus. 



The eyelids obviously serve for the protection of the eye ; 

 movable shades which by their closure exclude light, par- 

 ticles of dust, and other 

 injurious substances. 



The lachrymal gland is 

 a compound gland, closely 

 resembling the salivary 

 glands in structure. It 

 secretes the tears, and is 

 lodged in a depression at 

 the outer angle of the 

 orbit. It is about the size 

 and shape of an almond. 

 Its ducts run obliquely 

 beneath the conjunctiva, 

 and open by a series of 

 minute orifices upon the 

 upper surface of the eye. 



After passing over the surface of the eyeball, the tears are carried 

 away through minute openings in the inner angle of the eye into 



1 By inverting the eyelids, these glands may be seen through the conjunctiva 

 lying in parallel rows. 



FIG. 139. THE LACHRYMAL APPARATUS. 



