THE EYE. 837 



The cones are conical or flask-shaped, their broad ends resting upon the mem- 

 brana limitans externa, the narrow pointed extremity being turned to the choroid. 

 Like the rods, they are made up of two portions, outer and inner; the outer por- 

 tion is a short conical process, which, like the outer segment of the rods, presents 

 transverse striae. The inner portion resembles the inner portion of the rods in 

 structure, presenting a superficial striated and deeper granular part ; but differs 

 from it in size, being bulged out laterally and presenting a flask shape. The 

 chemical and optical characters of the two portions are identical with those of 

 the rods. 



10. The Pigmentary Layer, or Tapetum Nigrum. The most external layer of 

 the retina, formerly regarded as a part of the choroid, consists of a single layer of 

 hexagonal epithelial cells, loaded with pigment-granules. They are smooth exter- 

 nally, where they are in contact with the choroid, but internally they are prolonged 

 into fine, straight processes, which extend between the rods, this being especially 

 the case when the eye is exposed to light. In the eyes of albinos, the cells of the 

 pigmentary layer are present, but they contain no coloring-matter. In many of 

 the mammals also, as in the horse, and many of the carnivora, there is no pigment 

 in the cells of this layer, and the choroid possesses a beautiful iridescent lustre, 

 which is termed the tapetum lucidum. 



Supporting Frameivork of the Retina. Almost all these layers of the retina 

 are connected together by a supporting framework, formed by the fibres of Mutter, 

 or radiating fibres, from which the membrana limitans interna et externa are 

 derived. These fibres are found stretched between the two limiting layers, " as 

 columns between a floor and a ceiling," and passing through all the nervous layers 

 except Jacob's membrane. Each commences on the inner surface of the retina by 

 a conical hollow base, which sometimes contains a spheroidal body, stained- deeply 

 with hoematoxylin, the edges of the bases of adjoining fibres being united and 

 thus forming a boundary line, which is the membrana limitans interna. As they 

 pass through the nerve-fibre and ganglionic layers they give off few lateral branches ; 

 in the inner nuclear layer they give off numerous lateral processes "for the support 

 of the inner granules, while in the outer nuclear layer they form a network around 

 the rod and cone-fibrils, and unite to form the external limiting membrane at the 

 bases of the rods and cones. In the inner nuclear layer each fibre of Miiller pre- 

 sents a clear oval nucleus, which is sometimes situated at the side of, sometimes 

 altogether within, the fibre. 



DESCRIPTION OF FIG. 448. 



I. Section of the dog's retina, a, Cone-fibre. 6, Rod-fibre and nucleus, e, d, Bipolar cells (inner granules) 

 with vertical ramification of outer processes destined to receive the enlarged ends of rod-fibres. 

 e, Bipolars with flattened ramification for ends of cone-fibres. /, Giant bipolar with flattened ramifica- 

 tion, ff, Cell sending a neuron or nerve-fibre process to the outer molecular layer, h, Amacrine cell 

 with diffuse arborization in inner molecular layer, i, Nerve-fibrils patsing to outer molecular layer. 

 j, Centrifugal fibres passing from nerve-fibre layer to inner molecular layer, m, Nerve-fibril passing 

 into inner molecular layer, n, Ganglionic cells. 



II. Horizontal or basal cells of the outer molecular layer of the dog's retina. A, Small cell with dense arbori- 

 zation. R, Large cell, lying in inner nuclear layer, but with its processes branching in the outer 

 molecular, a, Its horizontal neuron, c, Medium-sized cell of the same character. 



III. Cells from the retina of the ox. a, Rod-bipolars with vertical arborizations. 6, c, d e, Cone-bipolars with 



horizontal ramification of outer process, h, Cells lying on the outer surface of the outer molecular 

 layer, and ramifying within it. i,j,m, Amacrine cells within the substance of the inner molecular 

 layer. 



IV. Neurons or axis-cylinder processes belonging to horizontal cells of the outer molecular layer, one of them, 



6, ending in a close ramification at a. 



V. Nervous elements connected with the inner molecular layer of the ox's retina. A, Amacrine cell, will 

 long processes ramifying in the outermost stratum. B, Large amacrine with thick processes ramifying 

 in second stratum, c, Flattened amacrine with long and fine processes ramifying mainly in the hrst 

 and fifth strata. D, Amacrine with radiating tuft of fibrils destined for third stratum^ E, Large 

 air 



es 



ramifying"uifimateiy"in first 'stratum." drA"rnediYim-s"ized ganglion-cell ramifying in fourth stratum. 

 e, Giant-cell, branching in third stratum. /, A bistratified cell ramifying in second and fourth st 

 VI. Amacrines and ganglion-cells from the dog. A, Amacrine with radiating tuft. B, Large amacrine pass; 



to third stratum, c and G, Small amacrines with radiations in second stratum. F, Small amacrine 

 passing to third stratum. D, Amacrine with diffuse arborization. E, Amacrine belonging to fourth 

 stratum, a, d, e, g. Small ganglion-cells, ramifying in various strata. b,f, Large ganglion-cells show- 

 ing two different characters of arborization, i, Bistratified cell. 



VII. Amacrines and ganglion-cells from the dog. A, B, c. Small amacrines ramifying in middle of molecu a 

 layer. 6, d, q, h, i, Small ganglion-cells showing various kinds of arborization. /, A larger cell, similar 

 in character to <j, but with longer branch, a, c, e, Giant-cells with thick branches ramifying in the 

 first, second, and third layers. L, L, Ends of bipolars branching over ganglion-cells. 



