358 NORMAL HISTOLOGY. 



longed 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 consists of an islet of modified skin from which project usually about a 

 dozen minute and scarcely visible hairs, provided with large sebaceous and 

 smaller sweat-glands and embedded in a cushion of fatty tissue. Just to the 

 outer side of the caruncle, lies a vertical crescentic fold, the plica semilu- 

 naris, which frequently contains a minute plate of hyaline cartilage as the 

 vestige of the stronger bar in the nictitating membrane, which the fold repre- 

 sents. Likewise the small group of alveoli sometimes found within the base 

 of the fold is regarded as the homologue of the Harderian gland. Where the 

 boundaries of the lachrymal lake pass into the edges of the eyelids are little 

 elevations, the lachrymal papilla, each of which is pierced by a minute 

 aperture, the punctum lacrimalis, that marks the beginning of the canals by 

 which the tears are normally carried off from the conjunctival sac. 



The Eyelids. The eyelid comprises five layers which, from without 

 inwards, are: (i) the skin, (2) the subcutaneous tissue, (3) the muscular 

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



The skin covering the outer surface of the eyelids is characterized by 

 its unusual delicacy, being thin and beset with very fine downy and widely 

 scattered hairs, provided with sebaceous follicles ; small sweat-glands also occur. 



The subcutaneous tissue is distinguished by the entire absence of fat, 

 its loose texture and great extensibility and elasticity. In consequence of 

 these properties, it sometimes becomes the seat of extensive swelling after 

 edema or hemorrhage. 



The muscular layer, for the most part the annular bundles of the 

 orbicularis palpebrarum, is so blended with the subcutaneous tissue as to be 

 practically embedded within the latter. In vertical sections of the eyelid, 

 (Fig. 409) the circularly arranged muscular bundles show as transversely 

 cut groups of muscle-fibres enclosed by condensations of the surrounding 

 areolar tissue. A distinct annular tract, the ciliary bundle or muscle of 

 Riolan, lies close to the free border of the lid, chiefly between the tarsal 

 plate and the hair-follicles, in part often also between the conjunctiva and 

 the tarsus. In the upper lid, in addition to the circular bundles of the orbic- 

 ularis palpebrarum, the terminal strands of the longitudinal fibres from the 

 levator palpebrae superioris descend along the deeper surface of the first- 

 named muscle. Some of these penetrate between the circular bundles and 

 end in the deeper layer of the skin ; others descend more vertically to find 

 their insertion in the upper border of the tarsal plate. Under the name 

 tarsal muscles or muscles of Mutter, are described the uncertain bundles of 

 involuntary muscle found in the vicinity of the convex borders of the tarsi. 

 Those within the upper lid arise from the tendon and intermingle with the 

 fibres of the levator palpebrarum and insert either into the upper border of 

 the tarsal plate or into the adjacent fibrous tissue. In the lower lid, they 

 are less numerous and regular, and extend from the fornix conjunctivas to 

 the adjacent border of the tarsus. 



The tarso-fascial layer is represented next the margins of the lids 

 by the tarsal plates and beyond the latter by a dense fascial sheet. 



The tarsal plates are two crescentic lamellae of dense fibrous tissue, 

 one in each lid, that occupy the margins of the eyelids, to the maintenance 

 of whose form they largely contribute. The upper tarsus is the larger. The 

 plates are approximately i mm. in thickness and consist of densely felted 

 fibrous tissue. They are blended in front and below with the subcutaneous 

 tissue, above with the orbital fascia and the insertion of the lid-muscles, 



