608 



THE EYE. 



Fig. 420. 



The f(jllowiDg are the designations of the layers, from within out- 

 wards : — 



1. The layer of nerve-fibres (nerve-layer). 



2. The layer of nerve-cells (ganglionic layer). 



3. The inner molecular layer. 



4. The inner nuclear layer. 



5. Tlie outer molecular layer (internuclear). 



6. The outer nuclear layer. 



7. The layer of rods and cones (columnar layer, Jacob's membrane). 



8. The layer of hexagonal pigment cells (pigmentary layer). 



In addition to these eight strata two excessively delicate membranes 

 are described — the one, memdrcom liniifans inierna, bounding the retina 

 on its inner surface, next to the hyaloid membrane of the vitreous 

 humour ; the other, mcmhwia Ihnitans cxtcr?io, lying between the outer 

 nuclear layer and the layer of rods and cones; but, as will be afterwards 

 explained, these so-called " membranes " are merely the boundary lines 

 of the sustentacular tissue of the retina. The accompanying figure, 

 from Max Schultze, represents (somewhat diagrammatically) the general 



arrangement and structure of the layers. 



1. Layer of nerve-fibres. — The 

 optic nerve (fig. 418, ^ h) passes at the 

 porus opticus directly through the thick- 

 ness of the retina to reach its inner 

 suriace, on which it spreads out in form 

 of a membrane (fig. 420) towards the ora 

 serrata. Its fibres, which are destitute 

 of a primitive sheath and vary much in 

 size, lose their medullary sheath on 

 reaching the retina, consisting there of 

 axis-cylinder only (Bowman). They 

 are collected into small bundles, which, 

 compressed laterally, intercommunicate 

 and form a delicate web with narrow 

 elongated meshes. At the yellow spot 

 this layer is wanting (fig. 420, l), else- 

 where it forms a continuous stratum, 

 gradually diminishing in thickness in 

 front, interrupted only by the enlarged 

 ends of the fibres of Miiller to be after- ^ 

 wards described (p. 615). The nerve- 

 bundles, as well as the cells of the next 

 layer, are partially covered and sup- 

 ported by flattened connective tissue 

 corpuscles. 



2. Ganglionic layer. — Immediately 

 external to the nerve-fibre layer is a 

 stratum of nerve-cells (figs. 419, 422, 2), 

 of a spheroidal or pyriform figure, and 

 having in the fresh condition a pellucid 

 aspect. Each cell has a single un- 

 branched process extending obliquely 



from its rounded inner extremity amongst the fibres of the preceding 

 layer, with one of whichit is no doubt continuous. From the opposite 



'■^ 



^^::i;MLjAV. . 



"mC^m 



Fig. 420. — Part of the Layek of 



Nerve-Fibres on the Inner 



Surface of the Retina (Kol- 



liker). Slightly Magnified. 



a, coUiciilus opticus ; h, yellow 



spot ; c, rf, bundles of nerve fibres 



passing round tliis ; e, e, bundles 



radiating in all directions from the 



point of entrance of the nerve. 



