706 



THE EYE. 



glandular follicles, and named the caruncula lachrymalis. The plica 

 semilunaris is the rudiment of the third eyelid (rneinbrana nictitans) found 

 in some animals. 



Structure of the lids. The skiu covering the eyelids is thin and delicate; 

 and at the line of the eyelashes, altered in its character, it joins the conjunc- 

 tival mucous membrane which lines the inner surface of the lids. Beneath 

 the skin, and between it and the conjunctiva, the following structures are 

 successively met with, viz. : The fibres of the orbicularis muscle ; loose 

 connective tissue ; the tarsal cartilages, together with a thin fibrous mem- 

 brane, the palpebral ligament, which attaches them to the margin of the 

 orbit ; and, finally, the Meibomian glands. In the upper eyelid there is, in 

 addition, the insertion of the levator palpebrse superioris, in the form of a 

 fibrous expansion fixed, to the anterior surface of the tarsal cartilage. 



Fig. 456. 



Fig. 456. VERTICAL SECTION OF THE LEFT ORBIT AND ITS CONTENTS. 



The section has been carried through the middle of the optic foramen and optic nerve 

 obliquely as far as the back of the eyeball, and thence forward through the eyeball, 

 eyelids, &c., in an antero-posterior direction, a, the frontal bone; 6, the superior 

 maxillary bone; c, the eyebrow with the orbicularis palpebrarum, integument, &c. , 

 divided ; d, the upper, and d, the lower eyelid, partially open, showing the section of the 

 tarsal cartilages and other component parts, the eyelashes, &c. ; e, e, the reflection of 

 the conjunctiva from the upper and lower eyelids to the surface of the eyeball ; /, the 

 levator palpebraa superioris muscle; <?, the upper, #', the lower rectus muscle; h, the 

 inferior oblique muscle divided ; 1, 1, the optic nerve divided in its sheath ; 2, the 

 cornea ; 2', the sclerotic ; 3, the aqueous chamber ; 4, the crystalline lens ; 5, the 

 centre of the vitreous humour. 



The fibres of the orbicularis muscle are closely adherent to the skin by fine 

 connective tissue, entirely devoid of fat. A marginal fasciculus of its fibres 

 has been found within the line of the eyelashes, separated by the bulbs of 

 the lashes from the other fibres, and constituting the ciliary muscle of Riolan. 

 The fibres of the orbicular muscle, while adherent to the skin, glide loosely 

 on the tarsal cartilages. 



The tarsal cartilages (tarsi) are two thin elongated plates of cartilages of 

 the yellow kind placed one in each lid, and serving to give shape and firm- 

 ness to those parts. The upper cartilage, the larger, is half oval in form, 



