THE EYE AND THE PHYSIOLOGY OF VISION. 543 



due to the contraction of a circular muscle which runs 

 through the upper and lower eyelids and so encircles the 

 open space which separates the upper from the lower lid. 



In addition to the two regular eyelids there is present in 

 man the rudiment of a third eyelid. This may be readily 

 observed as a red vertical fold at the inner corner of the 

 eye, and in some individuals reaching almost to the iris. 

 This eyelid is especially developed in many animals, such 

 as birds, and can by these animals be drawn entirely over 

 the front of the eyeball. When so fully developed it is 

 called the nictitating membrane. 



In this remnant of the nictitating membrane is a small 

 reddish elevation readily recognizable called the caruncula 

 lachrymalis, consisting of a number of sebaceous glands 

 imbedded in this fold. 



3. The Lachrymal Apparatus. To keep the surface 

 of the eyeball perfectly clear and to remove particles of 

 dust, there is provided for each eye a lachrymal apparatus 

 consisting of a tear gland and a system of ducts. 



Fig. 168. THE LACHRYMAL APPARATUS. 



1, lachrymal gland; 2, lachrymal ducts; 3, 3, the puncta lachrymalia ; 4, the nasal sac; 

 5, the nasal duct. 



The lachrymal or tear gland lies not in the inner, but in 

 the upper and outer part of the orbit under the edge of the 

 frontal bone. It is about the size of an almond. From 

 this gland (which histologically is an ordinary racemose 



