692 THE ORGANS OF SENSE. 
rhodopsin, already referred to, is found only in the outer segments of the rods, the terminal 
parts of which extend into the layer of pigmented epithelium. 
8. Layer of pigmented epithelium (stratum pigmenti).—This consists of a single 
stratum of cells which, on surface view, are hexagonal (Fig. 502), their outer flattened sur- 
faces being firmly attached to the chorioid. When seen in profile the outer part of each 
cell contains a large oval nucieus and is devoid of pigment, while the 
inner portion is filled with pigment and extends as a series of thread- 
like processes amongst the outer segments of the rods and cones. 
When the eye is kept i in the dark the pigment accumulates near the 
outer part of the cell, but, when exposed to light, it streams in 
between the rods and cones. 
It will be seen from the foregoing description that there is no 
direct continuity between the nervous elements which form the 
different layers of the retina. In the inner molecular layer there is 
merely an interlacement between the dendrites of the ganglionic 
layer and the arborisations of the cells of the inner granular layer, 
and a similar interlacement in the outer molecular layer between 
ee the rod and cone elements and the processes of the outer granules. 
tenner riawed Sustentacular fibres of the retina (fibre Miilleri).—These 
from the surface). support the nervous structures and extend from within outwards 
through the thickness of the retina as far as the bases of the rods 
and cones (Fig. 500, M). “They begin at the inner surface of the nerve fibre layer in 
single, or forked, expanded bases, by the apposition of which a delicate membrane, the 
membrana limitans interna, is formed. In the ganglionic layer they give off a few lateral 
branches, and, on passing through the inner nuclear layer, supply lateral ramifications 
amongst the inner granules for their support ; in this part of each fibre there is seen an 
oval nucleus. In the outer nuclear layer they break up into a network of fibrils which 
surround the rod and cone fibres, and end externally at the bases of the rods and cones in 
a delicate membrane, the membrana limitans externa. 
Structure of the macula lutea and fovea centralis.—The yellow colour 
of the macula is due to the presence of pigment in the inner layers of the retina. At the 
circumference of the macula the nerve fibre layer is greatly thinned and the rods are few 
in number ; the ganglionic layer, on the other hand, is much thickened and may contain 
from seven to nine strata of cells, while the outer granular layer is also thicker and its 
bipolar cells have an oblique direction. At the fovea centralis the retina is much thinned, 
since here its nerve fibre and ganglionic layers are absent and its other strata creatly 
attenuated. The stratum pigmenti, on the other hand, is thicker and its pigmentation 
more pronounced. The cone nuclei are situated some distance internal to the outer 
limiting membrane, and thus the thin inner and outer granular layers are in apposition. 
There are no rods, and the cones, closely crowded together, are narrower and _ their 
outer segments more elongated than elsewhere, so that the line of their bases, indi- 
cated by the membrana limitans externa, presents a convexity directed forwards. The 
fovea centralis and macula lutea are spoken of by the physiologist as the “region of 
distinct vision.” 
Structure of the ora serrata.—Here the nervous layers of the retina suddenly 
cease ; the layer of rods and cones first failing, to be immediately followed by the disappear- 
ance of the other nervous strata. In front of the ora serrata the retina is prolonged over the 
ciliary processes in the form of two layers of cells: (a) an inner layer of columnar epithelium, 
and (4) an outer, consisting of the stratum pigmenti, the two forming the pars ciliaris 
retine. The same two layers are prolonged over the back of the iris, where both are pig- 
mented and form the pars iridica retinze. 
Vessels of the retina.—The retina is supplied by the arteria centralis retine, 
a branch of the ophthalmic artery, which pierces the sheath of the optic nerve about three- 
quarters of an inch behind the eyeball and makes its appearance in the centre of the optic 
disc. Here it divides into an upper and a lower branch, and each of these again bifurcates 
into an internal, or nasal, and an external, or temporal, branch. The reoulen 1@ four 
branches ramify towards the periphery of the retina, and are named the superior and 
inferior temporal and the superior and inferior nasal arteries. ‘The temporal arteries 
pass outwards above and below the macula lutea, to which they give small branches ; 
these do not, however, extend as far as the fovea centralis, which is devoid of blood-vessels. 
The macula also receives two small arteries (superior and inferior macular) directly from 
the porus opticus. The larger vessels run in the nerve fibre layer near the membrana 
limitans interna and form two capillary networks—an inner, in the nerve fibre layer, and 
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