198 Dr Knox on the Limits of the Retina 



over the anterior surface of the lens. Contrary to what has 

 been asserted, I found the iris just described to be perfectly 

 moveable in the living Sepia, and the pupil it formed contract- 

 ed and dilated precisely as in other animals. The pupil is 

 linear and horizontal, and nearly equal in breadth through- 

 out ; but it expanded somewhat in the living animal on being 

 removed from a strong to a weaker light, and on the death of 

 the animal, which happened unexpectedly by merely pouring 

 over it a quantity of spring-water, the pupil suddenly dilated 

 so as to become circular, and continued so as long as pre- 

 served. 



This seems to me the true anatomy of the eye of the cuttle- 

 fish, as deduced from preparations which had been for some 

 time immersed in spirits. The number examined was eight, 

 viz. two of the calmar, and six of the smaller species of Sepia, 

 frequently met with in the Frith of Forth ; but as the latter 

 were brought to me alive, it may be worth while to mention 

 the precise appearances of the eye-ball when laid open in a 

 perfectly fresh state. This may be done very briefly. I 

 found the vitreous humour, hyaloid membrane, and lens, co- 

 lourless, and eminently transparent ; nothing indicated the 

 presence of that darkish coloured stripe which we have shown 

 to traverse the lens from side to side, or of those wedge-like 

 cavities which we know to exist betwixt its hemispheres. The 

 pigmentum covers every portion of the great central chamber 

 of the eye-ball occupied by the vitreous humour, and is of a 

 dark purplish colour; the antero- posterior diameter of the eye 

 is short. 



Having thus described what I consider as the true anatomy 

 of the retina and ciliary processes in these animals, and cor- 

 rected the highly erroneous notions hitherto entertained rela- 

 tive to the pupil, it will now be expected that I should offer 

 some opinion as to the mode in which I suppose vision to be 

 performed in the cuttle-fish ; but this is a subject the consi- 

 deration of which is attended with great difficulty. A part of 

 the difficulty may be got over by supposing that the eye of 

 the cuttle-fish is adapted not for the distinct perception of ob- 

 jects, but for the more general sensation of light, whereby it 

 mav regulate its course, and be guided to those depths in 



