540 ^STHESIOLOGY. 



Its external surface is convex, and its internal concave. Wheri 

 the eye is in its natural position, only the margin of the mem- 

 brana nictitans, covered by conjunctiva, is perceptible, the rest 

 being buried in the ocular sheath ; but when, by the contraction 

 of the straight muscles, the globe presses upon the pad of fat, the 

 membrane is forced out, and covers more or less of the trans- 

 parent cornea. This movement, which is instantaneous, is for 

 the purpose of removing any offending agent from the surface of 

 the eye. In some diseases, as tetanus, this membrane is forced 

 outwards, and remains so. 



A very small reddish-yellow gland, the gland of Harder, ia 

 found near the middle of the outer surface of the membrana ; 

 it is covered by fat and fibrous tissue, and it secretes an unctuous, 

 lubricating fluid, which is poured on the internal surface of the 

 cartilage, by several small openings. All authorities do not 

 describe this gland. 



The Caeuncula lachetmalis is the smdl round body, of a 

 reddish colour, seen in the inner canthus. It consists of a num- 

 ber of follicles, and the bulbs of soft, fine hairs. Its use is to 

 direct the tears towards the puncta lachrymalia. 



LACHRYMAL APPARATUS. 



The lachrymal apparatus consists of the lachrymal gland, 

 lachrymal canals, lachrymal sac, and nasal duct. 



The Lachrymal gland, situated between the orbital process 

 and the superior straight muscle, is convex superiorly, concave 

 inferiorly, thickest and broadest anteriorly, and thinnest and 

 narrowest posteriorly. It belongs to the class of conglomerate 

 glands, is of a pale yellow colour, and consists of numerous 

 lobules, composed of sacculi and tubes held together by cellular 

 tissue. The secretion, which constitutes the tears, destined to 

 lubricate the anterior surface of the eye, escapes through numerous 

 straight ducts, hygrophthalmic canals, which open on the surface 

 of the conjunctiva underneath the upper lid, and towards the 

 outer canthus 5 it iu carried between the lids and the globe 

 towards the inner canthus, and enters two small openings, the 

 ■puncta lachrymalia, one on each side of the caruncula lachry- 

 malis. These puncta lachrymalia are the openings of the minute 

 lachrymal canals, which are formed in the substance of the eye- 

 lids, and are lined by a continuation of the conjunctiva ; they 



