( lis ) 
III. — On the Structure of the Eye of the Sword- 
fish (Xiphias gladius, Lin,) 
By Robert Edmond Grant, M. D., F. R. S. E., 
F.L.S., M.W.S., &c. 
Member of the Medico-Chirurgical Society of Edinburgh, Fellow of 
the Royal College of Physicians of Edinburgh, &c. 
{Read 21 th January 1827.) 
From the great density and obscurity of the medium 
through which fishes receive the rays of light, compared 
with those of the atmosphere, their organs of vision neces- 
sarily present considerable differences in form and structure, 
from those of land animals. Predatious fishes, which fre- 
quent great depths, have very large eyes, from the general 
darkness of the ocean, and probably from the diminished 
energy of the nervous system in animals so low in the scale. 
The eyes of the common swordfish, here described, were 
four times as large as those of an ox, although the bulk of 
the fish did not equal one half of the bulk of that quadru- 
ped. Blainville has observed, that fishes which inhabit 
the open sea have the eyes larger and better developed 
than those which frequent the shore. As the refractile 
power of the aqueous humour of the eye is the same as that 
of water in which fishes constantly reside, this humour is 
unnecessary, and nearly absent, in that class of animals, 
VOL. VI. H 
