SEC. 6. THE STKUCTUKE OF THE RETINA. 



737. We have now to inquire how the rays of light thrown 

 on to the retina, by means of the dioptric mechanism, in the form 

 of an optical image give rise to visual sensations and so to a 

 perception of the object sending forth the rays. For this purpose 

 we must turn to the structure of the retina, including with it the 

 pigment epithelium derived from the outer, as the retina proper is 

 from the inner, layer of the retinal cup. 



The optic nerve, as we have already said, is not so much 

 an ordinary nerve as a strand of white matter extending from 

 the brain ; and several of its features shew this. Its outer 

 wrapping is not an ordinary perineural sheath but a prolonga- 

 tion of dura mater, and within this lies a ' pial' sheath, a con- 

 tinuation of the pia mater. Between the two may be recognised 

 a membrane corresponding to the arachnoid membrane with 

 scanty sub-dural and sub-arachnoid spaces. The pial sheath 

 sends supporting septa into the interior of the nerve, and at 

 about 15 or 20 mm. from the eyeball a large process of the 

 pial sheath passing obliquely forwards carries the central retinal 

 artery and vein into the middle of the nerve, and thence onwards 

 along the axis of the nerve to the retina. 



The nerve fibres, for the most part of very small diameter 

 2 p, though a few (possibly "pupil" fibres) are larger, 5 (j, or 10 /*, 

 are up to the eyeball medullated fibres, but as in the brain and 

 spinal cord possess no neurilemma. They are supported by neu- 

 roglia very similar to that of the spinal cord, and continuous with 

 the pial septa, which like those of the spinal cord are irregular, 

 the arrangement, so common in an ordinary nerve, of definite 

 longitudinal bundles, each- with its own sheath, being absent. 



The number of fibres in the whole nerve has been calculated 

 to be about 500,000 ; but higher estimates have been made. 



Where the nerve joins the eyeball the dural sheath becomes 

 continuous with the sclerotic coat and the pial sheath with the 

 choroid coat, fine bundles from the sclerotic and also to some extent 

 from the choroid passing transversely into the nerve and forming 

 a network, the "lamina cribrosa." At this level the fibres of the 



