EYELIDS. 375 



lamellae throughout the stroma. They extend into the epithelium and 

 there form a very delicate plexus with free intercellular endings. 



EYELIDS. 



The eyelids or palpebrae (Fig. 427) are covered with thin skin pro- 

 vided with fine lanugo hairs; small sweat glands extend into the corium. 

 The latter contains pigmented connective tissue cells, which are rare else- 

 where in the corium. The subcutaneous tissue is very loose, having 

 many elastic fibers and few or no fat cells. Near the edge of the lid there 

 are two or three rows of large hairs, the eyelashes or cilia, the roots of which 

 extend obliquely, deep into the corium. Since they are shed in from 100 

 to 150 days they occur in various stages of development. They are pro- 

 vided with small sebaceous glands, and the ciliary glands [of Moll] open 

 close beside or into their sheaths. The ciliary glands are modified sweat 

 glands with simpler coils which may show successive constrictions; "a 

 branching of the tubules has been observed." 



The central portion of the eyelids is muscular. Beneath the sub- 

 cutaneous tissue there are striated bundles of the orbicularis palpebrarum 

 extending lengthwise of the lid. A subdivision of this muscle found behind 

 the roots of the cilia is called the muscuhis ciliaris Riolani. Posterior to 

 the orbicularis muscle are found the terminal radiations of the tendon of 

 the levator palpebrae. A part of these are lost in connective tissue; another 

 part associated with smooth muscle fibers, is inserted into the upper border 

 of the tarsus and forms the superior tarsal muscle. This occurs in the upper 

 lid, but correspondingly in the lower lid the radiations from the inferior 

 rectus muscle contain smooth muscle fibers, forming the inferior tarsal 

 muscle. 



The inner portion of the lids consists of the conjunctival epithelium 

 and the underlying connective tissue including the tarsus. This is a plate 

 of dense connective tissue which gives firmness to the lid. It begins at the 

 free edges and extends over the adjacent two-thirds of the lid close to the 

 conjunctiva. Imbedded in its substance in either lid there are about 30 

 tarsal glands [Meibomian], which consist of a wide excretory duct opening 

 along the palpebral border and of small acini with short stalks which enter 

 it from all sides. In structure they resemble sebaceous glands. At the 

 upper end of the tarsus and partly enclosed in its substance, there are 

 branched tubular accessory lachrymal glands. They occur chiefly in the 

 medial (nasal) half of the lid. 



The tunica propria of the palpebral conjunctiva contains plasma and 

 lymphoid cells; the latter invade the epithelium beneath which in some 

 animals they form nodules. The stratified epithelium of the skin gradually 



