PIGMENTARY LAYER OF THE RETINA. 



51 



In addition to the eleoients which belong specially to the layers above described, 

 there are certain other structures which are common to nearly all the layers, 

 passing through the thickness of the retina from the inner almost to the outer 

 surface, and, although not actually of the nature of connective tissue, serving 

 the same kind of purpose, namely to bind together and support the more 

 delicate nervous structures of the membrane. These mstentacular fibres or fibres 

 of Miiller (figs. 48, 58), commence at the inner surface of the retina by a broad 

 conical hollow base or foot, which may be forked (fig. 58), and often contains 

 a spheroidal body, staining with haematoxylin (pseudo-nucleus). The bases of 



.l.e. 



Fig. 58. A FIBRE OF MiJLLER FROM THE POCr's RETINA, SHOWN 



BY GOLGI'S METHOD. (R. y Cajal.) Highly magnified. 



1, nerve-fibre layer ; 2, ganglionic layer ; 3, inner molecular 

 layer; 4, inner nuclear layer ; 5, outer molecular layer ; 6, outer 

 nuclear layer ; m.l.e., membrana limitans externa ; 'in.l.i., mem- 

 brana limitans interim ; b, nucleus of the fibre ; a, process extend- 

 ing from nucleated part into inner molecular layer. 



adjoining fibres are united together at their edges 

 (fig. 59), so as to give, in vertical sections of the 

 retina, the appearance of a distinct boundary line 

 (fig. 48) ; this has been named membrana limitans 

 interna, but, as may be inferred from the above 

 description, it is in no way a continuous or inde- 

 pendent membrane. The Miillerian fibres pass through 

 the nerve- and ganglionic layers, either with a smooth 

 contour, or with but two or three well-marked lateral 

 projections from which fine lamellar processes extend 

 amongst the elements of these layers : gradually 

 diminishing in size they then traverse the inner 

 molecular layer. In the mammalian retina the fibres 

 may be marked by slight projections in passing 

 through this layer. In the inner nuclear layer they 

 again give off delicate flattened processes from their 

 sides, which pass round the inner granules and 

 serve to support them. Moreover, each Miillerian 

 fibre is here characterised by the presence of a 

 clear oval or elliptical nucleus (already mentioned 

 in the description of the inner nuclear layer), con- 

 taining a nucleolus, and situated at one side of, and in close adherence to the fibre 

 to which it belongs (fig. 58, b). On reaching the outer nuclear layer (after passing 

 through the outer molecular) the fibres of Miiller break up into fibrils and thin 

 lamellae, and in this form they pass outwards through the layer, between the outer 

 granules and the rod- and cone-fibres, enclosing these structures, filling up the 

 intervals between the granules and forming partial sheaths for them. At the level 

 of the bases or central ends of the cones and rods, the numerous offsets terminate 

 along a definite line which marks the boundary between the outer nuclear 

 layer and the layer of rods and cones, and has been termed membrana limitans 

 externa. This also, like the m. I. interna, is in no way a continuous membrane, nor 

 is it isolable from the Miillerian fibres ; indeed, numerous fine fibrillar offsets of 

 these pass a short distance beyond the so-called limiting membrane, and closely 

 invest the bases of the inner segments of the rods and cones. 



The Miillerian fibres exhibit a fine striation. They swell up and become 



E 2 



