CHAP, in.] SIGHT. 57 



is in turn succeeded by the layer of pigment epithelium, beyond 

 which we come at once upon the limiting membrane of the choroid, 

 the so-called membrane of Bruch ( 713). 



As we shall see, there is a functional connection, even if not 

 actual continuity of structure, between the optic fibres on the 

 inside and the rods and cones on the outside. As we shall see, 

 the processes through which the rays of light are able to give rise 

 to visual impulses begin in the region of the rods and cones; 

 the rays of light have to pass through the whole or nearly the 

 whole of the thickness of the retina before they begin their work ; 

 their work begun in the rods and cones is carried back through 

 the thickness of the retina to the optic fibres and gathered up 

 from the layer of optic fibres to the optic disc and so to the optic 

 nerve. It is a necessity therefore that all the several elements 

 of the retina should be very transparent. 



We have already called the retina a piece of the brain ; and 

 even the brief statement which we have just made helps, from the 

 likeness to cerebral structures which it suggests, to justify such a 

 view. It is not surprising to find that in the retina as in the brain 

 purely nervous elements are mixed with neuroglial elements, and 

 that further, as in the brain, great difficulty is often met with in 

 determining exactly which structure is really nervous and func- 

 tional, and which merely neuroglial and sustentative. The broad 

 distinctions however are easy. 



739. Supporting or Neuroglial Elements. We apply the term 

 neuroglial to all those elements of the retina which, though not 

 nervous in nature, are derived from epithelial cells and not of 

 mesoblastic origin. The inner surface of the retina, that which 

 lies in contact with the hyaloid membrane investing the vitreous 

 humour, is defined by a thin transparent membrane of a cuticular 

 nature, the inner limiting membrane (Fig. 145, m.l.i.}. Between 

 the outer nuclear layer and the layer of rods and cones lies a 

 somewhat similar thin transparent membrane, the outer limiting 

 membrane (m.l.e.), pierced with holes for the passage of tlie inner 

 parts of the bodies of the rods and cones, the greater part of the 

 bodies of which lie outside the membrane plunged, as we shall 

 see, in the pigment epithelium. These two membranes correspond 

 to the inner and outer surface of the inner (anterior) wall of the 

 retinal cup, which we have said ( 703) alone furnishes the retina, 

 proper. The rods and cones therefore may be said to project 

 beyond the retina, and indeed, when the development of the 

 retina is traced out, we find that the rods and cones do really 

 grow out from the inner wall of the retinal cup and thrust them- 

 selves into the outer wall, which is wholly transformed into the 

 pigment epithelium. 



Stretching radially from the inner to the outer limiting 

 membrane in all regions of the retina, and therefore seen in a 

 vertical section as vertically disposed structures, are certain 



