STRUCTURE OF THE RETINA. 



617 



427 



part of the retina. The ganglionic layer (2) is especially thickened, 

 the cells being from six to eight deep, bipolar and situated rather 

 obliquely. The nerve-fibre \njev (1) gradually gets thinner towards 

 the edge of the fovea, the fibres dipping in to join the inner ends of 

 the bipolar ganglion-cells. The fibre which passes from the other 

 end of each of the latter is said to bifurcate, each branch becoming 

 connected with a granule of the inner nuclear layer (Merkel). The 

 inner granules are also somewhat obliquely disposed, are large, and near 

 the bottom of the fovea, where the layers of nerve-cells and of inner 

 granules run together, they can hardly be differentiated (Hulke). The 

 outer nuclear layer (G) is occupied in the greater part of its thick- 

 ness by the very long and obliquely disposed cone-fibres ; the corre- 

 sponding nucleated enlargements are only two or three deep, and take 

 up a comparatively small portion of the layer. 



The yellow tint of the macula is deepest towards the centre : it is 

 due to a diffuse colouring matter which is seated in the interstices 

 between the elements of the several layers, except in the layer of cones 

 and the outer nuclear layer. It is soluble in water or alcohol, and 

 absorbs the blue and violet rays of the spectrum. 



Structure of the ora serrata and pars ciliaris. — At the ora 

 serrata the numerous complex layers of the retina for the most part 

 disappear, and in front of tlie ora serrata, the retina is repre- 

 sented merely by a single 

 stratum of elongated co- 

 lumnar cells with the 

 pigmentary layer external 

 to them {pars ciliaris). 

 The transition is, in 

 man, somewhat abrupt, 

 all the changes being met, 

 within a zone of about 

 ^i^ in. only in breadth. 

 Tne layer of rods ami 

 cones (fig. 427, /7) first dis- 

 appears, the cones continu- 

 ing rather further than the 

 rods, but soon ceasing ; 

 the nerve- and ganglionic- 

 layers, which were already 

 very thin near the ora, 

 here cease altogether ; 

 the inner molecular 

 layer (c), which is now 

 largely occupied by Miil- 

 lerian fibres, retains its 

 thickness up to a certain 

 point, and then suddenly terminates with a rounded off margin («')» 

 while the nuclear layers, outer and inner, (/, d), here become merged 

 into a single layer, which is continuous with the columnar cells of 

 the pars ciliaris. These (fig. 427, m), which are at first of consider- 

 able length, become gradually shorter anteriorly ; they are finely 

 granular or striated in appearance (fig. 428, 2), each with a clear oval 

 nucleus at the outer part of the cell, neax the pigmentary layer. The 



Fig. 427. — Vertical Skction through the Choroid 

 AND Retina near the Ora Serrata (Kolliker). 

 60 Diameters. 



«, lij'aloid membrane ; h, limiting membrane and 

 nervous layer of the retina ; c, ganglionic and inner 

 molecular layers with closely set Miillerian fibres ; d, 

 inner nuclear ; e, outer molecular ; /, outer nuclear 

 layer ; g, columnar layer ; h, pigment ; i, k, choroid ; 

 I, part of one of the ciliary ijrocesses ; m, pars 

 ciliaris of the retina. (The recess shown at it! is not 

 constant.) 



