THE STRUCTUKE OF THE EYEBALL 



01 



out within the hollow cup formed by the solera and choroid is a soft delicate 

 membrane of nervous tissue, the retina, which is connected with the optic 

 nerve. The spherical cavity thus formed is entirely filled by three trans- 

 parent structures, the lens, the aqueous humour and the vitreous humour. 

 The lens is a biconvex body of high refractive index, which is situated 

 symmetrically behind the opening in the iris, being held in place by the 

 suspensory ligaments. The aqueous is the fluid which fills the cavity in front 

 of the lens, while the semi- solid vitreous fills the cavity behind it. The eye 

 is therefore a solid organ having considerable rigidity. 



DEVELOPMENT OF THE EYE. The period at which the development of the 

 eye commences in the embryo follows rapidly after the invagination of the epiblast 

 to form the central nervous system, namely at about the first week in the human 

 foatus. It shows itself by a bulging outward of a pair of buds from the nervous layer 

 towards the sides of the head. During its advance each bud becomes folded on 

 itself to form a hollow cup which remains in connection with the central nervous 

 system through a hollow tube, the future optic nerve. As the optic cup approaches, 

 the epiblast becomes thickened, and this portion sinks inwards till it comes to lie in 

 the mouth of the optic cup. The epiblast now becomes folded over it, and the edges 

 coalesce, leaving the thickened mass as a nearly spherical body (the future crystalline 



CENTRAL CANAL 



CONJUNCTIVA 



6"* MONTH 



FlG. 250. Diagram to show the different stages in the development of the eye. 



