The histology of the eye / by John Whitaker Hulke.

  • Hulke, J. W. (John Whitaker), 1830-1895.
Date:
[1869]
Catalogue details

Licence: Public Domain Mark

Credit: The histology of the eye / by John Whitaker 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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    Journal, Nov. 1,18u9. J JJ J J The Chelonian retina agrees very closely with that of birds. Both are distinguished by bright cone-beads, and by twin-cones, the structure of which, particularly in chelonia, resembles that of lizards, and differs in the same way that this does from that of the fish's twin-cone. Each twin has certainly its own outer granule, and its separate primitiye cone-fibre, which, as in lizards, takes an obliquely radial direction from the posterior pole of the globe. The cone- beads are of three colom'S—ruby, which are the largest; and orange, passing through pale yellow into pale green: the orange and green beads are the most numerous. The intergranule layer contains large branched connective tissue corpuscles, resembling- those occurring in the same layer in the fish's retina. The Bird's retina, as I have just said, agrees in several par- ticulars with that of the chelonia. It has cones with beads of three colours, except in the case of nocturnal birds, e.g. owls, in which, as Schultze first showed, aU the beads are pale, almost colourless, a Hght yellow. It has also twin-cones, like those of reptiles. In many birds there exists a very distinct fovea, and in some H. Miiller discovered two, one at the posterior pole and the other near the ora retinae, the former being affected by the incident pencils in mono- cular vision, the latter coming into use in vision with both eyes. The primitive bacillary fibres radiate obhquely from the fovea, as in man and reptiles. The Mammalian retina is marked by the absence of twin-cones and of cone-beads. Its elements are smaller than those of the lower vertebrates. That of man has a macula lutea, in which is a distinct fovea centralis. The macula lutea occurs also in certain apes. The bat, mouse, hedgehog, and certain other animals, chiefly of nocturnal habits, have rods only; in most others cones and rods are both present, as in man. The retina is vascular; the distribution of the vessels, however, varies in different families. There are two situations where the structure of the retiaa in man and some other vertebrates which I have particularized is peculiar; these are the macula lutea and the ova retinae. The macula lutea is an oval spot, at the posterior pole, of a yellow colour: the coloration is not produced by granular pigment, as in that of the choroid, but it is a diffuse stain of the elementary tissues. In the centre of the macula is the minute pit—not a perforation— the fovea centralis. This pit is produced by the radial divergence of the primitive cone-fibres from a central point, and by the thinning and outward curving of all the retinal layers (except the bacillary) as they approach this point. In the fovea and the macula, except at its periphery, cones only occur, and they are more slender and longer than in other parts of the retina. The greater length is chiefly due to the elongation of the cone appendage. The slender- ness of the cones does not aUow their appendages to include the