EYE. 



[ 253 ] 



EYE. 



The limiting membrane (5) is an extremely 

 Fig. 216. 



Perpendicular section of a piece of the posterior part of 

 the human retina. 



a, hyaloid membrane with nuclei ; b, limiting mem- 

 brane ; c, ends of the radiating fibres, so altered as to 

 present a cellular appearance ; d, expansion of the optic 

 nerve ; e, layer of nerve-cells ; /, inner granular layer ; g, 

 intermediate or finely granular layer, in which the radia- 

 ting fibres are more distinct than elsewhere ; h, outer 

 granular layer ; i, inner division of the layer of bacilli with 

 the cones ; k, outer division, with the processes of the cones 

 and the true bacilli. 



Magnified 250 diameters. 



delicate structureless film, covering the inner 

 surface of the retina, including the entrance 

 of the optic nerve, and the punctum aureum. 



The expansion of the optic nerves forms a 

 membranous layer of extremely delicate 

 transversely radiating fibrils (fig. 217, 3), 

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

 mostly exhibiting varicosities. They contain 

 no nuclei in their course, nor do they appear 

 to contain axial fibres. They are aggregated 

 into flattened bundles, which either run 

 parallel or anastomose with each other. They 

 appear to terminate in, or rather to arise 

 from the nerve-cells of the retina ; and are 

 absent, or at least as a coherent layer, oppo- 

 site the punctum aureum. 



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 remainder of the retina is com- 

 posed of a very large number of parallel, 

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

 meter), highly refractive, radiating fibres or 

 tubes, with their axes at right angles to 

 the surface of the choroid upon which 

 their outer end rests, whilst their inner 

 triangular or branched extremities are in 

 contact with the limiting membrane. They 

 produce the striated appearance presented 

 by a section of the retina (fig. 216). They 

 are furnished at certain parts of their course 

 with expansions containing each a nucleus ; 



and the fibres are very numerous. These 

 Fig. 217. 



z. 



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), appearing 

 in the outer granular layer ; /, expanded end of the fibre 

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

 connected with a cone i ; r 1 , fibre running from the cone 

 to the cell /of the inner granular layer ; , branched ter- 

 mination of a radial fibre often present. 2. Bacilli sepa- 

 rated 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 proper radiating system. 4. 

 Two cones connected with bacilli, and fragments of the 

 fibres remaining : a, bacillus ; b, cone ; c, nucleus of 

 cone. 



Magnified 350 diameters. 



nucleated expansions being opposite each 

 other, or in the same planes, give rise to the 

 appearance of distinct granular layers men- 

 tioned above. The more internal nucleated 

 expansions are connected with the nerve- 

 cells of the retina by minute nerve-tubes. 



Their outer portions have been distin- 

 guished as the bacilli and cones, but the 

 whole probably forms one continuous system 

 of nerve-cells and tubes. 



The bacilli, regarded (fig. 217, 1 &, &', 2) as 

 distinct bodies, are cylindrical, narrow and 

 elongated; of the same breadth throughout; 

 truncated externally, and terminating inter- 

 nally in a more slender portion of the fibre; 

 they are from 1-430 to 1-330" in length and 

 1-15,000" in breadth; near the point of 

 attachment to the fibre is a transverse line. 

 They are extremely delicate, and easily 



