liH Dr Knox on the Limits of the Retina 



peared to me on a first examination quite extraordinary, and 

 induced me to revise the subject with the greatest care. 



I am far from thinking that I have removed the difficul- 

 ties which the anatomy of the eye presents in the cuttle-fish ; 

 but I entertain hopes that this brief memoir may induce 

 those to resume the scalpel, who are better qualified for the in- 

 vestigation. 



Some imagine that the retina in the cuttle-fish terminates 

 anteriorly in a number of delicate striae, which may with pro- 

 priety be compared to the ciliary processes of vertebral ani- 

 mals ; which striae are firmly fixed all round the crystalline 

 lens, passing in betwixt the segments into which the lens of 

 these animals is readily divisible. Now, were this the case, 

 little difficulty could occur in explaining the mode by which 

 the rays of light reach the retina in the eye of the cuttle-fish, 

 for the anterior surface of these so named ciliarv processes, 

 being covered by parts of no great depth, the rays of light 

 might impinge directly on this expansion of the retina. But 

 this is by no means accordant with the views I have adopted 

 of the anatomy of the part. I shall endeavour to describe 

 the distribution of the membranes of the eye of the loligo, 

 such as they have appeared to me after the most careful dis- 

 section. 



The interior of the eye-ball is filled by the lens anteriorly, 

 and bv the vitreous humour, and its capsule posteriorly. The 

 hyaloid capsule, which is very delicate, does not form septa 

 as in the eyes of vertebral animals, for, on being punctured, 

 the whole of the vitreous humour suddenly escapes ; the hu- 

 mour is colourless, and perfectly transparent. When its re- 

 taining capsule is detached from the posterior surface of the 

 lens, to which it adheres very slightly, it retains the coloured 

 impressions of the ciliary processes. Between the capsule of 

 the vitreous humour, and the retina, there is a thick layer of 

 a pigmentum of a very dark purplish colour, and sometimes 

 even blackish, which, covering the whole of the inner aspect of 

 the retina, renders it difficult to imagine how the rays of light 

 reach and affect the sensitive membrane, in the way in which 

 they usually do, viz. by traversing the humours. The retina 

 is formed by the expansion of the optic nerves, but the mode 



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