DB KNOX on the Comparative Anatomy of the Eye. 67 



I ought now to speak of the mode in which the optic nerve 

 enters the eye-ball, which could not have been done previous to 

 describing the anatomy of the choroid and marsupium. It will 

 be unnecessary to detail the more usual facts, as they must be al- 

 ready well known to those whom I have the honour of address- 

 ing. I shall merely observe, that, in the mammalia generally, the 

 optic nerve passes through the choroid by innumerable little fo- 

 ramina, or, perhaps, by short canals, to expand into the pulpy 

 membrane of the retina, always supposing that the retina is an 

 expansion of the optic nerve *. The change relative to the en- 

 trance of the optic nerve commences in the ruminantia, for we 

 may observe, that, even in the sheep, the point at which the op- 

 tic nerve expands into the retina is not circular, but somewhat 

 semicircular. In the eye of the American deer f, the circular 

 mode of entrance has entirely disappeared, and the nerve pre- 

 sents, on entering the eye-ball, a small segment of a very large 

 arch. In the eye of the fallow deer, the line by which the nerve 

 enters the eye is nearly straight, and greatly lengthened. Last- 

 ly, In that of the bird, it is completely so. In the latter class of 

 animals, the nerve escapes on all sides through the reflected cho- 

 roid ; in others, it merely passes through the choroid, stretched 

 over the entrance of the nerve, when such happens to be the 

 case . I have, throughout this paper, alluded often to the ana- 

 tomy of the eye of the deer ; it approaches very nearly that of 



Some anatomists think that the optic nerve is merely distributed on the reti- 

 na. There are several analogies in favour of this opinion, and even direct ocular in- 

 spection in the eyes of some fishes. 



j- The animal was said to have been sent to this country by his Excellency the 

 Governor-General of Canada. 



* It is probable, that, in all animals, the choroid passes directly over the entrance 

 of the optic nerve ; but it would be extremely difficult to demonstrate this, owing to 

 the excessive tenuity and transparency of the membranous expansion. 



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