THE EYE AND ITS APPENDAGES. 360 



resistant mass gives form and support to the softer tissues of the lids 

 and partly covers the Meibomian glands embedded within its sub- 

 stance. 



The Meibomian or tarsal glands constitute a series of about 

 thirty elongated tubulo-acinous structures embedded within the 

 substance of the tarsal plate, nearer the anterior than the poste- 

 rior surface. Each gland consists of a long vertical duct, whose 

 general course is perpendicular to the margin of the lid ; into this 

 canal numerous short lateral tubular acini open. Since the ex- 

 tremities of the glands occupy the outer arched border of the tarsus, 

 these structures are longest in the middle of the lid and progressively 

 shorten towards either end. The ducts open on the straight pal- 

 pebral border as a row of minute orifices situated parallel to, 

 but at some little distance from, the sharply-defined inner palpebral 

 border. In their histology the Meibomian glands so closely re- 

 semble the sebaceous follicles of the skin that they must be re- 

 garded as modifications of these structures ; their secretion consists 

 of a fatty substance similar to the sebum lubricating the integu- 

 ment. The ducts of these glands, about . i mm. in diameter, are lined 

 by an epithelium possessing the character of the surrounding epi- 

 dermis, while the acini (.08-. 15 mm.) contain several layers of poly- 

 hedral cells, most of which are in various stages of fatty degen- 

 eration. In the upper part of the tarsus, especially in the nasal 

 half, additional branched tubular glands lie partially surrounded 

 by the fibrous tissue ; these structures correspond in composition to 

 the tear-glands, and are known as the accessory lachrymal glands. 



The conjunctiva constitutes the innermost layer and surface of 

 the lid, being continuous at the base of the lid with the bulbar con- 

 junctiva and at its palpebral border with the integument. The con- 

 junctiva consists of the epithelium covering the free surface and 

 the connective-tissue matrix, or tunica propria. The epithe- 

 lium covering the inner surface of the lid is stratified columnar ; 

 at the margin of the lid the columnar epithelium passes over into 

 the squamous cells of the epidermis. The surface of the conjunctiva 

 covering the tarsal plates is smooth, but beyond its epithelium forms 

 irregular pockets, which in section somewhat resemble glands. 



Numerous lymphoid cells within the reticulated tunica propria, 

 in certain localities, strongly suggest the presence of diffuse aden- 

 oid tissue ; the amount of such lymphoid tissue is subject to 

 much individual variation ; it is, however, usually best marked in the 

 retrotarsal portions of the conjunctiva. Circumscribed lymph- 

 follicles are occasionally observed, although these structures are less 

 constant in man than in many of the lower animals dog, cat, sheep, 

 or ox. 



24 



