422 PHILOSOPHY OF ZOOLOGY. 



intimately united with it, so that the lens is divided into 

 two unequal hemispheres. Its central surface is coated, as 

 in the higher classes of animals, with the coloured mucous 

 pigment which has been denominated pigmentum ni- 

 grum. In the cephalopoda, however, it is of a purplish- 

 red colour. 



The optic nerve, after cnteiing the sclerotica, expands 

 into a large ganglion, from the peripheral surface of which, 

 issue numerous nervous filaments. These pierce the cho- 

 roidcs by as many holes, and go to form, by their reunion, 

 the retina. This important membrane extends to the cili- 

 ary zone, and, like it, appears to unite itself with the groove 

 of the lens. 



The vitreous humour is contained in a peculiar vesicle, 

 having the lens seated in a concavity on its external sur- 

 face. The lens divides easily into two parts, the line of 

 separation being the groove which receives the ciliary 

 ligament. Each portion consists of a number of concen- 

 tric layers of variable thickness, composed of radiated fibres, 

 becoming less and less distinct towards the centre, near 

 which the laminated and radiated appearances cease to be 

 perceptible. An imperfect representation of this struc- 

 ture is given by Sir E. Home, probably from preparations 

 by Mr John Hunter, in the Phil. Trans, vol. Ixxxiv. 

 tab. 5. p. 26. 



The conjunctiva supplies the place of a cornea, and 

 covers directly the crystalline lens, as there is no aqueous 

 humour. This membrane, in some, is continuous with the 

 skin, but in others, there are imperfect eye-lids formed by 

 its duplicaturc, previous to passing over the lens. The 

 skin, at the opening of the pupil, foi'med by the scle- 

 rotica, in the absence of an uvea and iris, is strength- 

 .cned by a membrane which appears to be muscular, and 



