308 Dr. R. Dyce on the Hye of the Mackerel. 
ordinary form, and with scarcely any diminution in size, if left 
unheeded to dry; whereas, in most other fish, the eye shrivels 
up, unless some distending medium (as cotton) is used to keep 
it in shape. 
Again, in every fish which I have examined, the optic nerve 
penetrates the sclerotic coat by a round aperture, the coat closely 
encircling the nerve (see Pl. VII.: eye of Haddock, Whiting, 
and Cod); but in the Mackerel there is in this unyielding 
sclerotic coat a portion, as it were, cut out from the back of 
the eye, extending from near the opposite edges of the cornea, 
thus leaving, when in its quiescent state, an elliptical space, like 
two narrow cones joimed at their bases( <>); but if the sides 
of the eye are pressed close, as they must be by the muscles when 
the focal distance is to be changed, it then becomes a mere line 
or slit. 
There is a still further peculiarity in the Mackerel, viz. a small 
semicircular notch on the nasal side of this linear slit, in which the 
optic nerve lies secure from pressure in its passage through it. 
(See Plate.) It is, I believe, admitted that the adjusting power 
in the eye, in order to obtain distinct vision at different distances, 
is mainly dependent upon the flexibility of the sclerotic coat, 
which allows of its being compressed by the muscles, and thus, 
by the pressure of the humours, increasing the convexity of the 
cornea, while it also brings the retina closer to the posterior 
surface of the lens. Hence, in the greater number of fish, the 
sclerotic is soft and dexible. yet sufficiently firm to ania its 
spherical shape. In a very few fish it is as hard as bone—in 
the sword-fish (Xiphius gladius), for example—and nearly in- 
flexible ; yet all of them possess the same adjusting-power. 
Amongst these the Mackerel has this peculiar formation: in 
this fish the eye would be nearly inflexible, from its hard carti- 
laginous nature ; but by the very simple, yet beautiful, arrange- 
ment which I have described (of the elliptical slit), compression 
may be effected to any useful extent—thus accommodating the 
form of the eye to distances. The provision made to prevent 
pressure upon the optic nerve by this notch in the hard un- 
yielding sclerotic will also be noticed. This notch clearly de- 
monstrates not merely that, in this fish at least, the sclerotic 1s 
an investing membrane to preserve the form of the eye, but that 
compression is produced to suit the focal distance; and if the 
compression were so great as to close the gap left in the sclerotic, 
it would, but for this notch, destroy, for a time at least, the 
optic nerve. 
This singular and beautiful arrangement appears remarkably 
adapted to the habits of this fish. It is well known to be a very 
strong and rapid fish; it must therefore greatly facilitate it, in 
