On the anatomy of the fovea centralis of the human retina / by J. W. Hulke.

  • Hulke, J. W.
Date:
[1867]
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Licence: Public Domain Mark

Credit: On the anatomy of the fovea centralis of the human retina / by J. W. Hulke. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    fibres agree with that which obtains in other parts of the retina, only their branches are with difficulty traceable through the granular band lying between the cone-fibre plexus and the inner granule-layer, in which most of them appear to end. They are also struc- turally connected with the finely areolated tissue composmg the granular layer, and, further, with the interstitial tissue which pervades all the layers. Blood- Vessels.—In none of my sections have I found blood-vessels at the centre of the fovea; at the margin, however, capillaries occur, and small arteries are not uncommon within the limits of the macula. The vessels nowhere penetrate beyond the outer surface of the inner granule-layer. Deductions. 1. Since the total of the effects of light upon living tissue will be greater as the .extent of tissue traversed by it is greater, and since the relative common sensitiveness of a surface varies with the number of distinct sentient elements it contains, it follows that the greater length of the cones and rods, and their greater slenderness, Avhich allows a larger number of them to the superficial unit, are in harmony with the greater sensi- tiveness of the retina at the macula lutea. Inasmuch, however, as the foveal cones are stouter than the rods, a superficial unit at the centre of the fovea contains fewer sentient {i. e. percipient) elements than the same unit near the periphery of the macula lutea; and on this ground the sensitiveness of the retina at the fovea should be less than that of the retina near the periphery of the macula. On the other hand, the extreme thinness of the inner layers of the retina at the centre of the fovea, places the bacillaiy layer here most favourably for receiving incident light. 2. The division of the rods and cones into an outer and an inner segment is natm-al. The facts in support of this are, the presence of the division in perfectly fresh specimens; its sharpness and constant occurrence at a definite place; the constantly rectilinear figure of the outer, and the curvilinear figure of the inner segment; the diffierent refrac- tive powers of the segments; and their diff'erent behaviour towards staining and chemical solutions. 3. From these structural differences it is a fair inference that the segments have dif- ferent physiological meanings. The higher refractive power, straight sides, and slender cylindrical or prismatic figure of the outer segment may be adaptations for confining within the segment light incident upon its end, and for preventing the lateral escape of light througli the sides of the segment into neighbouring cones and rods. These considerations incline me to adopt the opinion that this segment has an optical function, an opinion which derives further support from the fact that, in those animals in whicli the segment is so wide a cylinder that a ray might be incident upon the inner surface of its sides at a small enough angle not to be reflected but to pass out, the segment is insulated by a sheath of black pigment. The inner segments of the cones and rods arc the specially modified peripheral termi-