EYE. 



C 312 ] 



EYE. 



granular layer (h) ; 8, a narrow outer limit- 

 ing membrane ; and 9, the layer of cones 

 (*'), and bacilli (k). The pigment-cells 

 of the choroid membrane are also some- 

 times considered a layer of the retina. 



Fig. 210. 



Perpendicular section of a piece of the posterior part 

 of the human retina. 



a, hyaloid membrane with nuclei ; 6, internal limiting 

 membrane ; c, ends of the radiating fibres, so altered 

 aa to present a cellular appearance; d, layer of 

 optic nerve-fibres; e, layer 01 nerve-cells; f, inner 

 granular layer: g, intermediate or finely granular 

 layer, in which the radiating fibres are more distinct 

 than elsewhere; h, outer granular layer; t, inner 

 division of the layer of bacilli, with the cones ; k, outer 

 division, with the processes of the cones and the true 

 bacilli. 



Magnified 350 diameters. 



The internal limiting membrane (b) is an 

 extremely delicate structureless film, cover- 

 ing the inner surface of the retina, including 

 the entrance of the optic nerve, and the 

 macula lutea. 



The expansion of the optic nerve forms 

 a membranous layer of extremely delicate 

 transversely radiating fibrils (fig. 211,5), 

 from 1-24,000 to 1-12,000" in diameter, 

 and mostly exhibiting varicosities. They 

 contain no nuclei, and appear to consist of 

 the axial fibres only. They are aggregated 

 into 'flattened bundles which either run 

 parallel or anastomose with each other. 

 They are absent, or at least as a coherent 

 layer, opposite the macula lutea. 



The layer of nerve-cells (e) consists of 

 ordinary nerve-cells, pyriform, roundish, or 

 angular, with pale processes ; they vary in 

 diameter from 1-3000 to 1-750". 



The molecular layer lies between the 

 ganglion-cell layer (e) and the inner granular 

 layer (/). The outer limiting membrane 

 is situated between the outer granular 

 layer (h) and the layer of cones and bacilli 

 0> 



The remainder of the retina is mainly 

 composed of a very large number of parallel, 

 very slender (1-60,000 to 1-20,000" diame- 

 ter), highlv refractive, radiating fibres or 

 tubes (Muller's fibres), with their axes at 

 right angles to the surface of the choroid, 

 upon which their outer ends rest, whilst 

 their inner, triangular or branched ex- 

 tremities are in contact with the limiting 

 membrane. They produce the striated 

 appearance presented by a section of the 

 retina (fig. 210). They are furnished at 

 certain parts of their course with expan- 

 sions containing each a nucleus j and the 

 fibres are very numerous. These nucleated 

 expansions being opposite each other, or in 

 the same planes, give rise to the appearance 

 of distinct granular layers mentioned above. 

 The more internal nucleated expansions are 



Fig. 211. 



Elements of the human retina. 1. "Radial fibres with 

 bacilli : k, bacillus, connected with the fibre (r) by its 

 inner acute end ; h, nucleated expansion (cell), appear- 

 ing in the outer granular layer ; I, expanded end of the 

 fibre, resting upon the limiting membrane m; k 1 , a 

 bacillus connected with a cone i ; r', fibre running from 

 the cone to the cell./ of the inner granular layer ; , 

 branched termination of a radial fibre, often present. 

 2. Bacilli separated from the fibres, broken and curved, 

 &c. 3. Fibrils from the expansion of the human optic 

 nerve : a a, larger, b, smaller, fibrils with varicosities ; 

 c, undulating pale fibres belonging probably to the pro- 

 per radiating system. 4. Two cones connected with 

 bacilli, and fragments of the fibres remaining : a, ba- 

 cillus ; b, cone ; c, nucleus of cone. 



Magnified 350 diameters. 



