NUNNELEY, ON THE CRYSTALLINE LENS. 149 
with a knife; the difference in the density of the two ele- 
ments from which the rays of light pass in enterimg the eyes 
of fish and birds being doubtless the cause of this difference 
in the solidity of the lens in these creatures. 
The arrangement of these fibres to make up the entire lens 
is not less curious than the structure and connection of the 
individual fibres. 
Four principal types have been described by Sir David 
Brewster, to one of which, or its subdivisions, the lens of 
every aninial may be referred. 
Ist. The first is the most simple, in which a single pole 
passes through the axis of the lens to the opposite point, to 
which all the fibres converge like the meridians of a globe. 
Upon this plan is constructed the lens of all birds, of most 
fish, and of some reptiles—the frog, for instance. Sir D. 
Brewster names the frog as probably possessing the next 
form of lens, that with two septa; but I have found it dis- 
tinctly with one pole. 
2d. The second type is found, amongst mammalia, in the 
hare, rabbit, and porpoise only; in some reptiles, and in 
several fish, of which the genus Salmo affords a good illus- 
tration. In this arrangement there is a short straight lmne 
passing through the pole from which the fibres symmetrically 
diverge, and passing over the margin of the lens, reach a 
similar line on the opposite surface, but which line is placed 
at right angles to the first, so that every fibre in‘each layer 
except four have their different parts lying in different planes ; 
thus, instead of passing directly from one surface to the other, 
they proceed in a curved direction round the lens. Such a 
lens is said to have two septa at each pole. 
3d. The third type is that of all mammalia except those 
just named. In it there are three septa, diverging at angles 
of 120° from each po'e, the septa of the posterior surface 
bisecting the angles formed by the septa of the anterior sur- 
face, thus making with them angles of 60°. “There are 
three fibres having their origin in the anterior pole and ter- 
minating at the extremity of the posterior septa, and other 
three having their origin in the posterior pole and terminating 
in the extremities of the anterior septa, which have their 
parts all lying in one plane, while every other fibre of the 
lens forms a curye of contrary flexure in order to carry it to 
its proper termination in the opposite septum. Hence it 
follows, that with the exception of the six fibres originating 
in the poles, the parts of all the other fibres which constitute 
the margin or rim of the lens are not parallel to its axis.”* 
* Brewster, ‘ Phil. Trans.,’ 1836. 
