56 STRUCTURE OF THE RETINA. [BOOK in. 



nerve suddenly lose their medulla and pass on to the retina 

 as naked axis cylinders supported by neuroglia ; owing to the 

 loss of medulla the thickness of the nerve is greatly diminished. 



The medullaless fibres pass on to the level of the anterior, 

 inner surface of the retina, forming the optic disc or optic 

 papilla, in the centre of which lie the central artery and vein. 

 From the optic disc the fibres radiate in curves, giving a pattern 

 not unlike that known as ' engine-turned,' over the retina as far 

 as the ora serrata, and form on the anterior, inner side of the 

 retina, next to the vitreous humour, a layer which we shall 

 describe presently as ' the layer of optic fibres.' Thus the optic 

 nerve spreads itself out as a thin film lining the interior of 

 the retinal cup next to the vitreous humour ; and the other 

 structures of the retina, in which the fibres end, lie outside or 

 behind this film, between it and the choroid. 



738. The Layers of the Retina. Vertical sections of the 

 retina, which has an average thickness of about '15 mm., shew 

 that it is made up of a series of layers superimposed, the 

 one on the other ; and the broad features of the layers are 

 very much the same over the whole extent of the retina except 

 at one part, the macula lutea containing the depression called 

 the fovea centralis. The structure of this part differs materially 

 from the rest of the retina, and we must consider it by itself , but 

 we may treat of all the rest of the retina surrounding the optic 

 disc as one. 



The layer of optic fibres (Fig. 145, I.) lies, as we have said, 

 next to the vitreous humour and forms what we may henceforward 

 call the innermost layer. Next to this comes a layer in which 

 relatively large branched nerve cells are present ; this is the 

 layer of ganglionic corpuscles (Fig. 145, II.). It is succeeded 

 by a peculiar layer, very closely- resembling the molecular layer 

 of the cerebellum ( 648) and the ground substance of the cortex 

 ( 649), and hence called the molecular layer or reticular layer 

 (Fig. 145, III.), or to distinguish it from another somewhat 

 similar layer, the inner molecular layer or inner reticular layer. 

 Next comes a layer characterized by the presence of conspicuous 

 nuclei closely packed together (Fig. 145, IV.), and still farther 

 outwards lies a similar but somewhat different second layer '(Fig. 

 145, VI.) of closely packed nuclei. The first is called the inner 

 nuclear layer, or sometimes the " inner granular layer," the second 

 the outer nuclear layer, or sometimes "the outer granular layer." 

 The two layers in question are separated from each other by a 

 layer (Fig. 145, V.) often very thin, which since in some of its 

 features it resembles the inner molecular layer, is called the 

 outer molecular layer, or " outer reticular layer;" it is sometimes 

 called the " fenestrated layer or membrane." Outside the outer 

 nuclear layer comes the remarkable layer of rods and cones (Fig. 

 145, VII.) which is the last of the layers of the retina proper; this 



