THE EYELIDS AND CONJUNCTIVA. 



H43 



The palpebral fissure leads into the conjunctival sac, which, when the lids are 

 in contact, is a closed capillary space between the lids and the anterior surface of the 

 eyeball. When the cleft is open, the conjunctival space becomes an annular groove 

 of unequal depth, its height being from 22-25 mm - behind the upper lid and only 

 about half as much behind the lower, and being shallowest at the inner angle. 

 That part of the sac which covers the posterior surface of the lids constitutes the 

 palpebral conjunctiva and that reflected onto the eye ball is the bulbar con- 

 junctiva, while the bottom of the groove, where these two portions are continuous, 

 is known as the fornix conjunctive, the superior and inferior being distinguished. 



The lachrymal lake (lacus lacrimalis) is the shallow bay into which the con- 

 junctival sac is prolonged for about 5 mm. between the medial ends of the eyelids. It 

 contains an irregularly oval or comet-shaped elevation, the lachrymal caruncle. 

 The latter (caruncula lac- 



FIG. 1201. 



Skin 



Subcutaneous tissue 



Orbicularis palpebrarum 

 Fat 



Tarsal muscle 



Levator palpebrae 



superiorts 



Blood-vessel 



Accessory gland 



rimalis) consists of an is- 

 let of modified skin from 

 which project usually 

 about a dozen minute 

 and scarcely visible hairs, 

 provided with large seba- 

 ceous and smaller sweat 

 glands and embedded in 

 a cushion of fatty tissue. 

 Just to the outer side of 

 the caruncle, a vertical 

 crescentic fold, the plica 

 semilunaris, indicates 

 the limit of the bulbar 

 conjunctiva. The fold is 

 of interest as probably 

 representing in a very 

 rudimentary way the 

 nictitating membrane, or 

 third eyelid, of the lower 

 animals. The semilunar 

 fold frequently contains 

 a minute plate of hyaline 

 cartilage as the vestige of 

 the stronger bar in the 

 nictitating membrane. 

 Likewise the small group 

 of alveoli sometimes 

 found within the base of 

 the fold is regarded as 

 the homologue of the 

 Harderian gland of the 

 lower types. The points 

 at which the slightly 

 curved boundaries of the 

 lachrymal lake pass into 

 the more arched edges of the eyelids are emphasized by little elevations, the lach- 

 rymal papillae, each of which is pierced by a minute aperature, the punctum 

 lacrimalis, that marks the beginning of the canals by which the tears are normally 

 carried off from the conjunctival sac. 



Structure of the Eyelids. The eyelid comprises five layers which, from 

 without inward, are: (i) the skin, (2) the subcutaneous tissue, (3) the muscular 

 layer, (4) the tarso-fascial layer and (5) the conjunctiva. 



The skin covering the outer surface of the eyelids is distinguished by its unusual 

 delicacy, being thin and beset with very fine downy and widely scattered hairs, pro- 

 vided with sebaceous follicles ; small sweat glands also occur. It presents numerous 



Duct in tarsal plate 



Artery of tarsal arch 



Meibomian duct 



Glands of Moll 



Cilia 



Ciliary bundle 

 Vertical section of upper eyelid of child. X 15. 



