CRYSTALLINE. CHAMBERS OF THE EYE. MUSCLES. 157 



it folds back on a line nearly corresponding to the border of 

 the crystalline, and is continuous with the ciliary zone of 

 Zinn, or the ciliary processes of the vitreous body; this zone 

 embraces the border of the crystalline, around which it forms 

 the cajial of Petit, and adheres intimately to its capsule. 



Crystalline. This is the name given to a double convex 

 lens, more curved posteriorly than anteriorly, translucent, 

 and placed vertically in the axis of the eye, so that the axis 

 of the lens corresponds to the centre of the pupil. The 

 crystalline is formed of superposed layers, which are less con- 

 sistent outside than towards the centre; it is contained in a 

 capsule, which applies itself closely without adhering to it. 

 The greater or less convexity of the surfaces of the crystalline 

 modifies the power of the eye, determining whether the vision 

 is long or short, i.e. presbyopic or myopic. The opacity of 

 the lens, or of its capsule, forms the disease called cataract. 

 We have already stated that its edge is set into the zone of 

 Zinn, to which its capsule adheres. 



Anterior and posterior chambers of the eye. Formerly a 

 certain space was supposed to exist between the crystalline 

 and the iris ; this was called the posterior chamber of the eye. 

 We now know that the posterior surface of the iris is in 

 direct contact with the anterior surface of the crystalline, and 

 the posterior chamber is only an imaginary space. The 

 interval which divides the iris from the cornea is the anterior 

 chamber, which is filled with a fluid called the aqueous humor, 

 translucent like the vitreous but less dense ; it is secreted by 

 the ciliary processes. 



Muscles of the eye. Conjunctiva. The globe of the eye is 

 situated in the anterior portion of the orbit, beyond which 

 it extends, and its axis, which is on the plane of that of the 

 orbit, is directed inwards towards the centre of the base of the 

 cranium. The eye is fixed in the orbit by an aponeurotic 

 capsule, the optic nerve, and by six muscles which turn it 

 in every direction. A mucous membrane, the conjunctiva, 

 so named because it unites the eye to the lids, spreads over 

 the anterior portion of the globe, as is proved by the injec- 

 tion of its vessels in some ophthalmic affections, and then 

 folds back on itself, and lines the internal surface of the eye- 



