462 HISTOLOGY 



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

 cutaneous tissue there are bundles of the striated orbicularis palpebrarum 

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

 the roots of the cilia, is called the musculus ciliaris Riolani. Posterior to 

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

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

 part, associated with smooth muscle fibers, are inserted into the upper 

 border of the tarsus and form 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 (or Meibomian) glands, which open along the palpebral border. 

 Each of them cnsists of a wide excretory duct, surrounded on all sides 

 by small acini, which empty into the duct through short stalks. In 

 structure they resemble sebaceous glands. At the upper end of the 

 tarsus and partly enclosed in its substance, there are branched tubular 

 accessory lachyrmal 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 gradu- 

 ally changes to that of the conjunctiva, which has several basal layers of 

 cuboidal cells and a superficial layer of short columnar cells. The latter 

 are covered by a thin cuticula, and goblet cells are found among them. 

 The transition from the superficial squamous cells to the columnar form 

 may occur at the posterior edge of the lid, or quite high on the conjunc- 

 tival surface. Toward the arch where the palpebral conjunctiva becomes 

 continuous with that of the bulb, the epithelium is so folded that in sec- 

 tions it may seem to form glands. 



The conjunctiva bulbi is similar to that of the lid. Its outer epithe- 

 lial cells, however, become squamous toward the cornea and over the 

 exposed portion of the eye, and its basal cells contain pigment. The 

 yellow appearance of the exposed portion, often most pronounced near 

 the medial border of the cornea, and known as pinguecula, is said not to 

 be due to fat or to an epithelial pigment; it accompanies a thickening of 

 the connective tissue layer. The tunica propria forms well-marked 

 papillae near the cornea. Its lymphocytes may form nodules, as many as 

 twenty having been found in the human conjunctiva bulbi. Occasional 



