THE EYELIDS. 



821 



oculi anterior, in front, and the camera oculi posterior, behind. These are filled 

 with the aqueous humour, and, in the adult, communicate freely through the aperture 

 of the pupil, but in the -foetus are separated from each other by the membrana 

 pupillaris. The camera oculi anterior or anterior chamber is bounded in front 

 by the cornea, behind by the iris and lens, whilst peripherally it communicates 

 with the spatia anguli iridis. The camera oculi posterior or posterior chamber 

 is triangular on section, and is bounded in front by the iris, behind by the 

 circumferential part of the lens and its suspensory ligament; the base of the 

 triangle corresponds with the thick, anterior extremities of the ciliary processes. 

 It communicates with the recessus camerae posteriores and spatia zonularia. The 

 aqueous humour has a refractive index of about T336, and consists of about 98 

 per cent of water, with 14 per cent of sodium chloride, and traces of albumen. 



PALPEBE^E. 



The eyelids are two movable curtains situated in front of the bulb of the eye, 

 and named, from their positions, superior and inferior. The superior is the larger 

 and more movable, 

 being provided with a 

 special elevator muscle, 

 the m. levator palpebrce 

 superioris. The inter- 

 val between the eyelids 

 is termed the palpebral 

 fissure, and measures 

 transversely about 30 

 mm., but varies con- 

 siderably in different 

 individuals and in 

 different races. When 

 the eye is open the 

 fissure is elliptical in 

 shape, but when closed 

 it assumes the form of 

 a transverse slit, which 

 lies on a level with the 

 lower margin of the 

 cornea. The two lids 

 meet at the extremities 

 of the fissure, and form 

 the lateral and medial 

 commissures. Their FIG. 697. EYELID SLIGHTLY EVERTED TO SHOW THE CONJUNCTIVA 



free margins are flat- (enlarged), 



tened and are sur- 

 mounted by eyelashes, from the lateral commissure to a point about 5 mm. from 

 the medial commissure a point indicated by a small papilla, the papilla lacrimalis. 

 Medial to this papilla the margins are rounded and destitute of eyelashes, and 

 form the upper and lower boundaries of a triangular space, termed the lacus lacri- 

 malis, which is occupied by a small pale red body, the caruncula lacrimalis. The 

 caruncula consists of a small island of modified skin, and contains sudoriferous and 

 sebaceous glands, and fine hairs. Posteriorly the lids are lined with mucous 

 membrane, the conjunctiva, and are in contact with the bulb of the eye, except 

 near the medial commissure, where, between the bulb of the eye and the caruncula 

 lacrimalis, there intervenes a vertical fold of conjunctiva, the plica semilunaris con- 

 junctivse, which in many animals contains a plate of cartilage. 



In each eyelid there exists a framework of condensed fibrous tissue, which gives 

 consistence and shape to the lid, and is termed the tarsus. In front of the tarsus 

 are the fibres of the orbicularis oculi muscle and the integument, while embedded 

 in its posterior surface, and covered by the conjunctiva, are numerous modified 



53 & 



Margin of the upper eye- 

 ,/lid with openings of 

 ducts of tarsal glands 



Papilla lacrimalis with 

 punctum lacrimale on 

 the summit 



Plica semilunaris 



Caruncula lacrimalis 



Papilla lacrimalis 



Opening of tarsal gland 



Tarsal glands 

 shining through the 

 conjunctiva 



