174 



THE EYE AND ITS APPENDAGES. 



The canal of Petit^ is formed between the laminae of the hyaloid 

 membrane, and surrounds the edge of the lens. 



The zone of Zinn, is a circle of plaitings upon the vitreous 

 humour, which dove-tail with the ciliary processes. 



The crystalline lens^ is a double convex lens, whose posterior 

 surface is more convex than its anterior. In childhood it is spherical, 

 and in old age it is flattened. It is transparent and soft, and surrounded 

 by a capsule which contains a thin fluid, called liquor Morgag7ii. 

 It consists of concentric laminse, having a tendency to divide into 

 three equal segments. 



The aqueous humour^^ is a transparent fluid, occupying the an- 

 terior and posterior chambers of the eye, bathing the posterior sur- 

 face of the cornea, and the anterior surface of the capsule of the 

 lens. It is from four to five grains in quantity. This is said to be 

 secreted by the membrane of Demours, which lines the whole of the 

 anterior and posterior chambers. This membrane can only be 

 traced upon the posterior surface of the cornea. 



The eyebroivs are two elevations of the skin covered with short, 

 stiff' hairs, protecting the eye from too strong a light, and from the 

 perspiration flowing down the forehead ; they materially affect the 

 expression of countenance. The space between the two is called 

 glabella. • 



The eyelids or palpebrce, consist of skin, muscular fibres, car- 

 tilage, and mucous membrane ; and when in contact, leave a trian- 

 gular canal, which passes transversely between them. Their edges 

 are covered with cilia^ or eyelashes, which are short, stiff*, curved 

 hairs, protecting the eyeball. 



Tarsal cartilages. — Are two in number. That of the upper lid is 

 larger than that of the lower ; they are crescentic in shape, and 



attached by the external 



Fig. 151. 



and internal palpebral 

 ligaments. 



Conjunctiva. — Is a 

 soft mucous membrane 

 lining the lids, and cover- 

 ing the anterior surface 

 of the ball, forming a 

 fold called plica semi- 

 lunaris^ near the inner 

 canthus. It is vascular, 

 and well supplied with 

 nerves. 



Meibomian glands, 

 — Are twenty or thirty 

 small yellow thread-like 

 glands, between the con- 

 junctiva and the pos- 



