ORGANS OF VISION. 591 



amples analogous in simplicity of structure, and in a pedicellate mode 

 of support and attachment to the head. Moreover, as the Pearly 

 Nautilus, like the latter group of mollusks, is also attached to a heavy 

 shell, and participates with them in the deprivation of the ordinary 

 locomotive instruments of the Cephalopods, the anatomist whose re- 

 marks we quote hence deduces the more immediate principle of their 

 reciprocal inferiority with respect to their visual organ, observing that 

 it would little avail an animal to discern distant objects when it could 

 neither overtake them if necessary for food, nor avoid them if inimical 

 to its existence. 



(1584.) The eyes of Nautilus (fig. 284, m) are not contained in 

 orbits, but are attached each by a pedicle to the side of the head, im- 

 mediately below the posterior lobes of the hood. The ball of the eye 

 is about eight lines in diameter ; and although contracted and wrinkled 

 in the specimen examined, it appeared to have been naturally of a 

 globular form, rather flattened anteriorly. The pupil was a circular 

 aperture, less than a line in diameter, situated in the centre of the 

 anterior surface of the eye. This small size of the pupil in Nautilus, 

 which contrasts so remarkably with the magnitude of that aperture in 

 the Dibranchiate Cephalopods, Professor Owen suggests is most pro- 

 bably dependent on the great degree of mobility conferred upon the eye 

 of the Nautilus in consequence of its attachment to a muscular pedicle, 

 which enables it to be brought to bear with ease in a variety of direc- 

 tions ; whilst in the higher Cephalopoda, corresponding motions of the 

 head and body, on account of the more fixed condition of the eye in 

 them, would have been perpetually required, had not the range of vision 

 been extended to the utmost by enlarging the pupillary aperture. 



(1585.) The principal tunic of the eye is a tough exterior membrane 

 or sclerotic (fig. 291), thickest posteriorly, where it is continued from 

 the pedicle, and becoming gradually thinner to the margins of the pupil. 

 The optic nerves, after leaving the optic ganglions (2), traverse the 

 centre of the ocular pedicles, and, entering the eye, spread out into a 

 tough pulpy mass which extends as far forwards as the semidiameter 

 of the globe. This nervous tissue, as well as the whole interior of the 

 cavity, is covered with a black pigment which is apparently interposed 

 between the impinging rays of light and the sentient membrane. The 

 contents of the eye-ball, of whatever nature they had been, had escaped 

 by the pupil. If the eye had ever contained a crystalline lens, that 

 body must have been very small ; as otherwise, from the well-known 

 effect of ardent spirits in coagulating it, it would have been readily 

 perceived. What adds, however, to the probability of this eye being 

 destitute of a crystalline humour is the total absence of ciliary plicae, or 

 any structure analogous to them. In some parts of the cavity a mem- 

 brane could be distinguished which had enveloped the fluid contents of 

 the eye ; but it had entirely disappeared at the pupil, which had in 



