40 
SIR DAVID BREWSTER ON THE ANATOMICAL AND OPTICAL 
septa of the posterior surface bisecting the angles formed by the septa of the anterior 
surface. 
This beautiful structure is shown in Plate V. figg. 1. and 2; fig. 1. representing the 
anterior, and fig. 2. the posterior, surface of the lens, which in quadrupeds is a real 
lens or lenticular solid, the curvature of the posterior surface having a shorter radius 
than that of the anterior surface. 
The progress of the fibres round the edges of the lens in their passage from the one 
surface to the other is shown in fig. 3, where the lens is supposed to be transparent, 
the dark continuous lines representing the three septa and the fibres of the anterior 
surface, and the dotted lines the three septa and the fibres of the posterior surface. 
From this representation it will be seen that there are three fibres having their origin 
in the anterior pole, and terminating at the extremities of the posterior septa; and 
other three having their origin in the posterior pole, and terminating in the extre- 
mities of the anterior septa, which have their parts all lying in one plane, while 
every other fibre of the lens forms a curve of contrary flexure, in order to carry it to 
its proper termination in the opposite septum. Hence it follows, that with the ex- 
ception 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. 
The arrangement of fibres shown in figg. 1. and 2. may frequently be seen, parti- 
cularly in the lenses of old animals when they are large, by examining them in their 
entire and transparent state within their capsule. In some cases I have seen them 
distinctly by looking down upon the surface of the lens ; but when they are visible 
in this way they may be seen to most advantage by looking through the surfaces 
with a small magnifier, when the lens is placed in a fluid of nearly the same refrac- 
tive power. 
In tracing the fibres to their termination in three sepia, I employed the optical 
method already described in a preceding paper ; but in general the superficial co- 
loured images are not so distinct as those in the lenses of fishes, though, like them, 
they may be transferred to wax, and are like the colours of mother-of-pearl. 
The fibres of the lenses of quadrupeds gradually diminish in size from the equator 
or margin of the lens, where they are a maximum, to their termination in the anterior 
and posterior septa. They are united together by small teeth, like those of fishes ; 
but generally speaking the teeth are smaller and less distinctly pronounced ; and in 
some lenses I have found it extremely difficult to exhibit them with the finest micro- 
scopes. As the teeth can only be seen in the indurated state of the fibres, it is pro- 
bable that their form maybe in a great measure altered or obliterated by the process 
of induration, especially when we consider that the lenses of quadrupeds are very 
much softer than those of fishes, and that the evaporation of the aqueous portion must 
produce a greater change upon the indurated albumen when it is most abundant. 
The existence of three septa was observed by Leeuwenhoek, and afterwards by 
Sattio, in the lenses of the ox, the horse, the sheep, the goat, the fox, the dog, and 
