Dr. R. Dyce on the Eye of the Mackerel. 307 
Eozoon Canadense, not only (as I have the best authority for 
stating) without having seen a section of that fossil thin enough 
to show it, but even (I have good reason to believe) without 
having examined the like structure in any Nummuline shell, so 
as to be able to recognize it when seen. I trust, therefore, 
that the scientific public will now hold me absolved from the 
necessity of taking any further notice of his pertinacious at- 
tempts to throw discredit upon my observations. 
I remain, Gentlemen, 
Your obedient Servant, 
WiiiaM B. Carpenter. 
March 21, 1866. 
XXXVI.—WNotes on some Peculiarities in the Eye of the Mackerel. 
By Rosert Dyce, M.D., F.R.S. Edin., Professor of Midwifery, 
University of Aberdeen. 
[Plate VII.] 
In the following observations it is not my intention to enter 
into any minute detail of the anatomical structure of the eye, 
but merely to notice some very striking and interesting pecu- 
liarities, different from the eye of any other fish which I have 
met with. 
The eye of a fish, like the eye of all vertebrate animals, is 
constructed upon principles essentially similar, and presents 
the same coats and lenses as are met with in the human 
eye, and, generally speaking, arranged similarly. It, however, 
differs in many points of structure from that of terrestrial 
Vertebrata, its organization being, of course, adapted to the 
denser medium in which the fish resides, and so adapted as to 
bring the rays of light to converge at a shorter focus upon the 
retina. It is hence more globular or spherical, has always a 
very flattened form of bulb and a shorter axis, and is always 
covered by an investing fibrous membrane called the sclerotic coat. 
This is more or less thick and elastic; it is not, however, uni- 
formly thick, being more so at the back of the eye than im front 
towards the cornea, in order, it is believed, to preserve the flat- 
ness of the cornea—an arrangement rendered necessary in all 
swimming animals, as well as fish, who reside constantly in 
water, and who receive the rays of light through so dense a 
refractive medium. The sclerotic also varies in thickness in 
different fish ; in the larger fish it is very thick, while in the 
generality of ordinary-sized fish it is very thin, soft, elastic, and 
flexible. In the Mackerel, however, instead of being soft, it is 
uniformly firm, nay, entirely cartilaginous, and would be inflexible 
but for its peculiar construction,—so much so that it retains its 
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