PARTS FOR THE PROTECTION OF THE EYEBALL. 



813 



and size toward the folds. This portion of the membrane presents large capillary blood- 

 vessels and lymphatics and is covered with a layer of cells of flattened epithelium. The 

 sclerotic portion is thinner, less vascular, and has no papillse. It is covered by conical 

 and rounded epithelial cells, which present from two to four layers. Over the cornea, 

 the epithelium of the sclerotic portion is continued in delicate, transparent layers, without 

 a distinct basement-membrane. 



The Lachrymal Apparatus. The eyeball is constantly bathed in a thin, watery fluid 

 which is secreted by the lachrymal gland, is spread over the globe by the movements of 

 the lids and of the eyeball, and is prevented, under ordinary conditions, from overflowing 

 upon the cheek, by the Meibomian secretion. The excess of this fluid is collected into 

 the lachrymal sac and is carried into the nose by the nasal duct. The lachrymal gland, 

 the lachrymal canals, duct, and sac, and the nasal duct, constitute the lachrymal appa- 

 ratus. 



The lachrymal gland is an ovoid, flattened gland of the racemose variety, resembling 

 the salivary glands in its general structure. It is about the size of a small almond and is 

 lodged in a shallow depression in the bones of the orbit at its upper and outer portion. 

 It is closely attached to the periosteum by its upper surface and is moulded below to the 

 convexity of the globe. Its anterior portion is separated from the rest by a well-marked 

 groove, is comparatively thin, and adheres to the upper lid. It presents from six to eight 

 (usually seven) ducts, which form a row of openings into the conjunctival fold. Five or 

 six of these orifices are situated above the outer canthus and two or three open below. 

 In its minute structure, this gland presents no points of special physiological interest as 

 distinguished from the ordinary racemose glands. It receives nervous filaments from the 

 fifth cranial nerve and the sympathetic. 



FIG. 256. Lachrymal and Meibomian glands. (Sappey.) 



1, 1, internal wall of the orbit; 2, 2, internal portion of the orbicularis palpebrarura ; 3. 3, attachment of this muscle to 

 the orbit; 4, orifice for the passage of the nasal artery; 5, muscle of Homer; C, 6. posterior surface of the eyelids, 

 with the Meibomian glands; 7, 7, 8, 8, 9, 9, 10, lachrymal gland and ducts; 11, openings of the lachrymal ducta. 



The apparatus by which the excess of tears is conducted into the nose begins by two 

 little points, situated on the margin of the upper and the lower lid, near the inner canthu*, 

 called the puncta lacrymalia, which present each a minute orifice. These orifices open 

 respectively into the upper and the lower lachrymal canals, which together surround the 

 caruncula lacrymalis. At the inner angle, just beyond the ciinincula, tlio two canals 

 join, to empty into the lachrymal sac, which is the dilated upper extremity of the nasal 

 duct. The duct is about half an inch in length and empties into the inferior meatus of the 

 nose, taking a direction nearly vertical, and inclined slightly outward and backward. This 



