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HANDBOOK OF PHYSIOLOGY. 



disappear in the fully developed lens except at the outside. The super- 

 ficial fibres are softer. The fibres are really six-sided prisms when seen 

 in section, and fit exactly together with little connecting material. The 

 capsule is a homogeneous transparent elastic membrane. The hardest 



FIQ. 400. A. Section of the retina, choroid, and part of the sclerotic, moderately magnified, a, 

 membrana limitansinterna: b, nerve-fibre layer traversed by Mailer's sustentacular fibres (of the 

 connective tissue system); c, ganglion cell layer; d, molecular layer; e, internal granular layer; /, 

 intergranular layer; gr, external granular layer; h. membrana limitans externa, running along the 

 lower part of i, the layer of rods and cones; fc, pigment cell layer formerly described as part of the 

 choroid; Z, m, internal and external vascular portions of the choroid, the first containing capillaries, 

 the second larger blood-vessels, cut in tranverse sections; n, sclerotic. (W. Pye.) 



portion of the lens is that which is most internal. It forms the so- 

 called nucleus of the lens (Fig. 403, 1). 



Optical Apparatus. 



The eye may be compared to the camera used by photographers, and 

 the transparent media correspond to the lens which is screwed into the 

 front part. In the photographic camera images of external objects are 

 thrown upon a ground-glass screen at the back of a box, the interior of 

 which is painted black. In the eye the camera proper is represented by 

 the eye-ball with its choroidal pigment, and the screen by the retina. 

 In the case of the camera the screen is enabled to receive clear images 

 of objects at different distances, by an apparatus for focussing, and the 

 convex lens too can be screwed in and out. The corresponding contri- 

 vance in the eye will be described under the head of Accommodation. 



The essential constituents of the optical apparatus of the eye may be 

 thus enumerated: (1) A nervous structure (the retina) to be stimulated 



