iigo A MANUAL OF ANATOMY 



The plica semilunaris is a vertical, semilunar fold of the con- 

 junctiva which is situated on the outer side of the caruncle, its 

 concave margin being directed outwards. It corresponds to the 

 membrana nictitans, or third eyelid, of some animals. 



Development of the Eyelids and Meibomian Glands. — The eyelids make their 

 appearance as two folds of skin, above and below the developing eyeball. 

 Each fold contains some mesodermic tissue which gives rise to the connective- 

 tissue element and tarsal plate of the lid. The ectoderm of the posterior sur- 

 faces of the lids acquires the characters of mucous membrane, and forms the 

 conjunctival epithelium. In the course of the third month the eyelids grow 

 together and unite along their margins, a space being thereby enclosed be- 

 tween thelids and the front of the developing eyeball. The union affects the 

 epithelium only, and persists until near the end of intra-uterine life. 



During the period of fusion of the eyelids the Meibomian glands and the cilia 

 or eyelashes are formed. The Meibomian glands are developed from the epi- 

 thelium along the line of fusion of the lids. Solid rods of epithelial cells are 

 formed which grow into the mesodermic tissue of the two lids and give off 

 lateral processes. These solid rods beqome hollow, and so form the Meibomian 

 glands. Some of the epithelial rods give rise to the glands of Moll. A short 

 time before birth the eyelids become separated, and the palpebral fissure 

 is thereby formed. 



The plica semilunaris is developed as a vertical fold of conjunctiva near the 

 inner canthus of the eye, external to the caruncle, but it attains little develop- 

 ment in man. 



The caruncle is developed from that portion of the margin of the lower 

 eyelid which intervenes between the inferior punctum lacrimale and the inner 

 canthus. The Meibomian glands in this region become modified, and the 

 tissue containing these modified glands becomes raised, and forms the reddish, 

 spongy caruncle. 



Lachrymal Apparatus. — The constituent parts of the lachrjonal 

 apparatus are as follows: 



1. The lachrymal gland. 3. The lachrymal sac. 



2. The lachrymal canaliculi. 4. The nasal duct. 



The lachrymal gland will be found described on p. 1146. 



The lachrymal canaliculi are two in number, superior and inferior. 

 They commence at the puncta lacrimalia on the summit of the 

 papillae lacrimales, which latter are situated on the ciliary margins 

 of the eyelids, close to the lacus lacrimalis. The superior canali- 

 culus at first ascends vertically for about yV inch, after which it 

 makes a sudden bend, and passes inwards and downwards to the 

 lachrymal sac. The inferior canaliculus at first descends vertically 

 for about ^^ inch, after which it makes a sudden bend and passes 

 almost horizontally inwards to the lachrymal sac. The two canali- 

 culi open into the lachrymal sac, either close together or by a common 

 orifice, and their mucous membrane is hned with stratified squamous 

 epithelium. The two slips of the tensor tarsi muscle are closely 

 related to the horizontal portions of the canaliculi. 



The lachrymal sac is the dilated upper part of the passage by 

 which the tears are conveyed from the lachrymal canaliculi to the 

 inferior meatus of the nose. It occupies the lachrymal groove of 

 the lachrymal bone and nasal process of the superior maxilla. 



