616 



THE EYE. 



up iuto fine fibrils and tliin transparent lamellie, and in this form they 

 pass outwards through the layer, between the outer granules and the 

 rod and cone-fibres, more or less enclosing these structures, and some- 

 times forming almost complete, delicate sheaths for them. At the level 

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

 unite around those structures along a definite line which marks the 

 boundary between the outer nuclear layer and the layer of rods and 

 cones, and has been termed memhrana VtmUans cxicrna. This also, like 

 the m. L i/ifcnia, is in no way a continuous membrane, nor is it isolable 

 from the Miillcriau fibres ; indeed, numerous fine fibrillar off"sets 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 sometimes exhibit a fine striation. They swell 

 up and become indistinct on treatment witli acetic acid and dilute 

 alkalies, but much more slowly than connective tissue fibrils ; more- 

 over, they are not dissolved by boiling in water. They are much less 

 developed in the central and posterior part of the retina than in the 

 peripheral and anterior ; towards the ora serrata they are very distinct 

 and closely set. 



Structure of the Macula lutea and fovea centralis (fig. 426). — 

 The peculiarities in structure which these present have been for the 

 most part incidentally noticed in the preceding description of the retina 

 layers. Thus in the first place no rods are met with, and the cones are 



Fig. 426. 



Fig. 426. — Vertical Section through the Macul.v Lutea and Fovea Centralis ; 

 DiAURAMMATic (after Max Scliultze). 



1, nerve layer ; 2, ganglionic layer ; 3, inner molecular ; 4, inner nuclear ; and 5, 

 outer molecular layers ; 6, outer nuclear layer, the inner part with only cone-fibres form- 

 ing the so-called external fibrous layer ; 7, cones and rods. 



much longer and narrower, especially opposite the fovea centralis, than 

 elsewhere. All the other layers are very much thinned at the fovea, 

 but towards its margin most of them are thicker than at any other 



