APPENDAGES OF THE EYE. 515 



1402. Membrana nictitans, third eyelid (Fig. 126). The 

 third eyelid is a fibrous, crescent-shaped organ situated at the mesal 

 canthus between the eyelids proper and the eyeball. Both of its 

 surfaces are covered by a continuation of the conjunctiva. Its 

 cephalic edge is free and dark bordered, its caudal edge is flexibly 

 attached to the eyeball (Fig. 126). 



The office of the membrana is to keep the eye free from dust. 

 The generalization made by Chauveau. A, is, "That it is most 

 developed in animals that are unable to use their cephalic limbs 

 for removing foreign particles from the eye." 



Demonstration. The movement of the membrana depends upon 

 that of the eyeball, not upon the action of special muscles. There 

 are, however, a few striated muscular fibers in the band of connec- 

 tive tissue passing from the rectus ventralis to the membrana as 

 shown in Fig. 126. If the membrana is not visible, press upon the 

 cornea so as to force the eyeball farther into the orbital fossa, and 

 it will appear. It may be made to entirely cover the cornea. To 

 cause it to disappear, cut the masseter muscle and force a scalpel 

 handle into the orbital fossa so as to push the eyeball cephalad. 



1403. Lachrymal Apparatus. This consists of the lachrymal 

 or tear glands and their ducts. The glands are, in general struc- 

 ture, like the salivary glands ( 788, 789). They are situated near 

 the lateral canthi of the lids. 



Demonstration. (A.) Glandula lachrymalis, lachrymal gland 

 proper. Nip the orbital process of the frontal and malar bones, cut 

 the soft parts connecting the malar process, and turn the end dor- 

 sad. The lachrymal gland will cling to the reflected part. It ap- 

 pears like the molar gland ( 785), and is so formed as to mold 

 itself to the eyeball upon which it naturally rests. 



(B) Glandula Harderi. Harder' s gland, situated on the convex 

 or ectal surface of the membrana nictitans, extends from near its 

 attached border over about one third its width (Fig. 126). It is 

 very apparent after the removal of the eyeball ( 1415). 



It is found only in animals possessing the third eyelid (Milne- 

 Edwards, A, XII, 121). 



Lachrymal Canal. On the free edge of each eyelid, about 3 

 mm. from the nasal canthus, is the opening of a lachrymal canal. 

 These canals collect the tears and convey them to the lachrymal 

 duct. 



Demonstration. The openings of the lachrymal canals may 



