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XI. On the Chameleon's Retina ; a further Contribution to the Miimte Anatomy of the 

 Retina of Reptiles. By J. W. Hulke, F.R.C.S., Assistant Surgeon to the Middlesex 

 and Royal London Ophthalmic Hospitals. Communicated by William Bowman, 

 F.R.S. 



Received June 1, — Read June 15, 1865. 



The chameleon's retina is remarkable in two respects: 1st, it has 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 (i. 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 surface 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 part 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. Wern. Soc. vol. v. p. 2, and Edinb. Phil. Journal, Oct. 1823, p. 358. 

 t MiJLLEE, H., Wiirzb. Naturwiss. Zeitschr. 1862, Bd. III. Heft 1. 8. 20. 



