STRUCTURE OF THE CRYSTALLINE LENSES OF ANIMALS. 
43 
cides with that of the eye. The length of the axis is 0‘327, and its equatorial dia- 
meter 0'273 of an inch. 
In some quadrupeds I have observed an irregularity in the septa, which may have 
arisen either from an original malconformation of the lens, or from some accidental 
injury. In the lens of a horse I observed a spurious septum, one of the three being 
double. The same fact will be more particularly noticed in describing the lens of 
the elephant, in which it is more common. 
§ 4. On the Anatomical and Optical Structure of the Crystalline Lenses of Animals , 
particularly those of the IP hale, the Seal, the Bear, and the Elephant. 
From the lenses of quadrupeds in which the fibres are related to three septa, I shall 
now proceed to describe the fourth and last class of symmetrical structures, in which 
the fibres are related to four septa, placed at right angles to one another. This com- 
bination of fibres is of rare occurrence, and I have found it only in the lenses of the 
whale, the seal, and the hear. 
The character of this structure will be understood from Plate V. figg. 1 and 2, 
where fig. 1 represents the anterior surface, and fig. 2 the posterior surface, of the 
lens. The septa on the posterior surface are inclined 45° to those on the anterior 
surface, so that if the lens were transparent, the septa when seen at the same time 
would appear like the eight radii of an octagon, inclined 45° to one another. In this 
structure there are eight fibres, all the parts of which lie in one plane, passing 
through the axis of the lens ; namely, four extending from the extremities of the four 
anterior septa and terminating in the posterior pole, and other four extending from 
the extremities of the four posterior septa and terminating in the anterior pole. All 
the other fibres of the lens, except these eight, are curves of contrary flexure, which 
necessarily change their direction in passing from one septum in the one face to 
another septum in the other inclined 45° to it. 
The structure which I have now described is exhibited in the crystalline lenses of 
the whale and the seal. I found it distinctly developed in the lens of a whale, forty- 
six feet long, caught by Captain Ross in his first voyage in the Arctic regions, and also 
in a specimen of the great seal, or Phoca harhata, which Lieutenant Robertson of the 
Isabella brought home from Baffin’s Bay in the same year. 
In the lenses of other whales and seals, however, I have found a different structure, 
which is represented in Plate VI. figg. 3 and 4. This combination of fibres differs 
from that in figg. 1 and 2, in having two centres of divergence in place of one in 
each surface of the lens ; but if we conceive these two points to coincide, the two 
structures become identical. In both, the principal septa are at right angles to each 
other on the same face, and inclined 45° to those on the opposite face ; so that the 
general character of the two structures is the same. 
In the lenses of one of the bears killed by Captain Ross during his first voyage, I 
found the structure shown in figg. 2 and 3. The distance between the centres of 
g 2 
