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falciform process for accommodation (in Fishes). A growth 

 of mesenchyme, outside the pigment layer, forms the 

 vascular choroid coat ; and outside this the mesenchyme 

 develops the tough protective capsule of connective tissue, 

 the sclerotic, which, continuous with the cornea in front, 

 completely encloses the eye. The transparent cornea 

 (covered externally by delicate ectodermal epithelium, the 

 conjunctiva) is jelly mesenchyme formed into layered 

 tissue (corneal endothelium) by immigrant amoebocytes ; 

 and it contains a space filled with fluid (aqueous humor) 

 which becomes the anterior chamber betAveen cornea and 

 lens. 



The edge of the optic cup is inturned and overlaps the 

 lens in front, thus forming a circular rim (of choroid), the 

 iris, surrounding a central open space, the pupil. The iris, 

 muscular in front, acts as a diaphragm ; by this means the 

 pupil is dilated or contracted, and the amount of light 

 entering the eye thus regulated. Being pigmented behind, 

 the iris gives the characteristic colour to the eye. Behind 

 the iris are projections of the choroid, the ciliary processes 

 (ciliary muscles) ; here the lens is held in position by the 

 suspensory ligament around its circumference (the forward 

 portion of the hyaloid membrane which invests the vitreous 

 body). By the action of the ciliary muscles on the ligament, 

 alteration of the lens shape is effected (in accommodation). 



The retina apparently ends, near the ciliary processes, 

 in a notched ora serrata ; actually, however, it is continued 

 (non-percipient) to the edge of the pupil. In the centre of 

 the retina, near the optic nerve area or blind spot, there 

 is an elevated yellow spot (macula lutea) with a ecu) ml 

 depression (fovea centralis) ; this is the area of most acute 

 vision. The retina is complex, and has several layers ; 

 the innermost one is next the vitreous humor, and consists 

 of nerve fibres from which the optic nerve is formed ; 

 the outermost one is next the pigment layer, and is composed 

 of sensory cells carrying rods and (in the higher Craniata) 

 cones. Before the light can reach these cells, it lias to 

 penetrate the intervening layers, and it passes through 

 the basal end to the rod or cone ; this is a peculiarity of the 

 Vertebrate eye. Further, these sensory cells, though not 

 derived directly from the ectoderm, as in Invertebrates, 

 are of that origin indirectly, since they are formed from a 

 portion of the inner layer of the brain which was originally 

 continuous with the ectoderm, before the closing of the 

 medullary groove. 



The optic stalk becomes the optic nerve ; its fibres are 

 derived from the retina. The cornea, aqueous humor, lens, 



