On the chameleon's retina : a further contribution to the minute anatomy of the retina of reptiles / by J. W. Hulke.

  • Hulke, J. W.
Date:
[1866]
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Credit: On the chameleon's retina : a further contribution to the minute anatomy of the retina of reptiles / by J. W. Hulke. Source: Wellcome Collection.

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    XI. On the Chameleon's Retina; a further Contrihutimi to the Minute Anatomy of the Eetina of Reptiles. By J. W. Hulke, F.R.C.S., Assistant Surgeon to the Middlesex and Royal London Ophthalmic Hospitals. Communicated hy William Bowman, F.R.S. Received June 1,—Read June 15, 1865. The chameleon's retina is remarkable in two respects: 1st, it lias a yellow spot and fovea centralis*; and 2ndly, the fibres which connect the cones with the inner layers cross the connective-tissue radial fibres instead of running parallel to themf. The yellow spot lies in the centre of the retina, at the posterior pole of the globe. The fovea is a deep circular pit. Its bottom (^. e. centre) appears as a dark-brown dot. encircled by a lighter brown ring corresponding to the sides of the pit, and this in turn is surrounded by a colourless zone, the outer edge of which passes gradually into the colour of the surrounding parts (Plate XIX. fig. 1). A flat subconical pecten, rising above the inner sm-face of the retina, marks the entrance of the optic nerve, 1', at the temporal side of the fovea. The nerve pierces the sclerotic very obliquely, and its bundles, escaping from the base of the pecten in the level of the ganglionic layer of the retina, are distributed on the inner retinal surface in such a manner that those only which belong to the fovea run directly towards this from the pecten, while all other fibres for parts on the nasal side of the pecten arch above and below the fovea in increasingly open curves. Minute Structure. 1. Bacillary Layer.—Cones only occur in this layer (figs. 2 & 3). They consist of an inner and an outer segment, separated by a bright transverse line. The outer segment is a very slender, strongly-refracting shaft; it tapers slightly towards its outer end; its length increases from the periphery to the centre of the retina, but is always much less than that of the inner segment. It is completely buried in the choroid, which also sheaths the outer part of the inner segment. This latter segment, although much larger than the outer, is inconspicuous in fresh specimens in consequence of its low refraction. At the centre of the retina its length is very considerable, while its breadth scarcely exceeds that of the outer segment; these proportions give the cones in this i)art the appearance of tall slender pillars; but at the periphery the inner segment is stout, and the cones resemble tall flasks. In chromic acid preparations the outer part of the flask-shaped cone-body is finely granulated, while the inner is more homogeneous, and often exhibits * Knox, Mem. Wcrn. Soc. vol. v. p. 2, and Edinb. Phil. Journal, Oct. 1823, p. 358. t MiiLLER, H., Wiirzb. :Natunviss. Zeitschr. 18G2, Bd. III. Heft 1. S. 20.