PISCES. 
distinctly shews the plicated retina continued 
on to the surface of the ligament, which seems 
to be covered with the nervous expansion.* 
Humours of the eye—The quantity of the 
aqueous humour in a Fish’s eye is comparatively 
very small, owing to the flat shape of the cor- 
nea and the almost perfect immobility of the 
iris. The posterior chamber is, indeed, quite 
deficient, the uvea of the iris being adherent to 
the capsule of the vitreous humour ; and even 
the anterior chamber is frequently materially 
encroached upon by the protrusion of the 
crystalline lens through the aperture of the 
pupil. As a refracting medium it is evident 
that the aqueous humour, being nearly of the 
same density as the surrounding medium, could 
have little effect in concentrating luminous 
rays, this duty being principally assigned to the 
powerful lens imniediately behind it. 
The crystalline lens in Fishes is nearly of a 
spherical form, thus presenting the converse as 
regards its refractive power of what exists in 
the eye of Birds. The size of the lens in these 
aquatic animals is very great, so that it en- 
croaches largely upon the chamber of the 
vitreous humour, extending to more than half 
way between the pupil and the back of the 
cavity of the eyeball. Its consistence is very 
great, and its nucleus so hard as to remain 
transparent even after immersion in spirit of 
wine. It is enclosed in a soft capsule, between 
which and the surface of the crystalline lens is 
a small quantity of fluid, and is fixed in a deep 
depression in the fore part of the vitreous hu- 
mour by a circular membranous zone derived 
from the hyaloid tunic, which surrounds it like 
the artificial horizon of a geographical globe. Sir 
David Brewster, in an admirable paper on the 
anatomical and optical structure of the crys- 
talline lens,t gives the following interesting 
particulars relative to its minute organization in 
the class of Fishes. Its form is that of a prolate 
spheroid, the axis of revolution being a little 
longer than the equatorial diameter. This axis 
is the axis of the eye or of vision. The body or 
substance of the lens is enclosed in an exceed- 
ingly thin and transparent membrane, called 
its capsule ; and if this be punctured, a thickish 
fluid flows from the opening; but upon re- 
moving the capsule altogether, this fluid is 
found to constitute only the outer coat of the 
lens, the substance of the lens growing denser 
and harder as we approach the centre of it. 
The body of the lens is not connected with 
the capsule by any nerves or filaments what- 
ever ; on the contrary, it floats as it were within 
the capsule, and on holding the lens in his 
hand, Sir D. Brewster observed its axis of revo- 
lution take a horizontal position whenever it 
was placed in an inclined direction. This was 
repeated several times with the same lens, 
although the experiment was tried unsuccess- 
fully with others. When the lens is taken out 
of its capsule, and the softer parts removed by 
rubbing it between the finger and thumb, a 
* Vide Preparation 1650, in the physiological 
series of the Museum of the Royal College of Sur- 
geons, London. 
+ Phil. Transact. for 1833, p. 323. 
1001 
hard nucleus is ‘obtained, which consists of 
regular transparent lamine of uniform thick- 
ness, and capable of being separated like those 
of sulphate of lime or mica. 
When the surface of any lamina has been 
examined before it has been detached, it has 
the appearance of a grooved surface like mo- 
ther-of-pearl; and in large lenses it is often 
easy to trace these apparent grooves or lines to 
the two poles of the axis of revolution, the 
fibres bounded by them being consequently 
widest at the equator, and growing narrower 
and narrower as they approach the poles. The 
maximum breadth of these fibres is about the 
5500dth part of an inch, but of course they 
become gradually attenuated as they approach 
the poles of the lens in either direction. 
Having thus determined the form and size of 
the fibres which enter into the composition of 
the crystalline lens, it remained to ascertain the 
mode in which they were fastened together so 
as to resist separation and form a continuous 
spherical surface, and this was found to be 
effected by a very curious mechanism, the con- 
tiguous fibres being united by means of teeth 
exactly like those of rack-work, the projecting 
teeth of one fibre entering into the hollows be- 
tween the teeth of the adjacent one. It was 
further found that the fibres gradually diminish 
in size towards the centre of the lens, and the 
teeth in the same proportion, so that the num- 
ber of fibres in any spherical coat or lamina 
was the same from whatever part of the lens it 
it was detached. In conclusion, Sir David 
Brewster observes, “ In’ the lens of a Cod I 
found that there were 2000 fibres in an inch at 
the equator of a spherical coat or lamina, whose 
radius was £,ths of an inch; consequently there 
must have been 2500 in the spherical surface. 
If we now suppose that the breadth of each 
fibre is five times its thickness, and that each 
tooth is equal to the thickness of the fibre, or 
that five teeth are equal in breadth to a fibre, 
we shall obtain the following results for the 
lens of a Cod four-tenths of an inch in dia- 
meter :— 
Number of fibres in each la- 
mina or spherical coat 2,500 
Number of teeth in each fibre 12,000 
Number of teeth in each sphe- 
mical Coatitiey eee! APs 31,250,000 
Number of fibres in the lens . 5,000,000 
Number of teeth in the lens . 62,500,000,000 
or, to express the result in words, the lens of a 
small Cod contains five millions of fibres and 
sixty-two thousand five hundred millions of 
teeth. A transparent lens exhibiting such a 
mechanism may well excite our astonishment 
and admiration.” 
The vitreous humour in Fishes is proportion- 
ally less abundant than in other races of Verte- 
brata,—a circumstance which is partly owing 
to the shortness of the antero-posterior dia- 
meter of the chamber of the eye-ball, and partly 
to the extent to which it is encroached upon by 
the large spherical crystalline lens; in other 
respects it presents no peculiarities worthy of 
special description. 
Muscles of the eyeball. —The eyeball of 
