284 THE CEPHALOPODA. $ 247. 
The Optic nerves enter the posterior part of the orbit through a kind of 
Foramen opticum, after which they swell] into a large kidney-shaped ganglion 
in which a portion of the nerve-fibres are completely interlaced with those 
from the opposite side. Leaving this Ganglion opticum, the nerve 
divides into numerous filaments which traverse the cribriform sclerotica, 
and then unite with the other elements of the Retina. The external layer 
of the retina is composed of these filaments; beneath it, is a pigment layer 
of a reddish-brown color, and pierced by numerous fibres given off rectan- 
gularly from the external layer. The internal layer is composed of gran- 
ules, among which the fibres of the optic nerve probably terminate. The 
external layer is continuous as a thin membrane upon the ciliary body, 
and even upon the septum of lens.“ 
The two optic ganglia are enveloped by a peculiar white substance com- 
posed of fat-cells, which, perhaps, serves only as a fat-cushion.” The 
eye is moved by several straight and oblique muscles, which arise from the 
cartilaginous portion of the orbit, and are inserted, usually, upon the mid- 
dle of the bulb. 
But with Nautilus, the eyes differ in many respects from those of the 
other Cephalopoda. They are supported upon a muscular stalk and pro- 
ject from the head; while with the other Cephalopoda, excepting Loligopsis, 
they are sunken deeply in the head.“® From the rudimentary lower lid a 
narrow furrow passes over the anterior surface of the eye even to the small; 
circular pupil. As yet, neither cornea nor lens has here been found.“ 
9 For the interlacement of the nerve-fibres in the 
Ganglion opticum, —see, especially, Wharton 
Jones, and John Power, loc. cit. 
10 The intimate, very complicated structure of the 
retina, has been described principally by Trevi- 
ranus (loc. cit. p. 155), Wharton Jones (loc. cit.), 
and Paccini (Nuove ricerche microscop. sulla tes- 
situra intima della retina nell’ Uomo, nei Vertebrati, 
nei Cefalopodi e negli Insetti. Bologna, 1845, p. 55, 
fig. 13, 14). The mysterious phenomenon, that, 
according to the older anatomists, the surface of 
the retina exposed to the light is covered with a 
pigment-layer, rests only on an imperfect knowl- 
edge of the structure of this organ, as has been 
shown by Wharton Jones (loc. cit.), and Valen- 
tin (Repert. f. Anat. II. 1837, p. 109). 
11 See Krohn, and Wharton Jones, hoc. cit. 
12 Mayer (Analekt. &c. p. 53) regards this sub- 
stance as a semi-adipose gland with several excre- 
tory ducts,— a kind of Jachrymal gland the product 
of which is poured into the conjunctival sac ; but 
Kélliker (Entwick. d. Ceph. p. 103) could find 
nothing glandular in its structure. 
13 For the pedunculated eyes of Loligopsis, see 
Rathké, in the Mém.d. St. Petersburg, loc. cit. 
Pl. I., and Férussac, loc. cit. 
14 The eyes of Nautilus having been studied for 
a long time upon dead specimens, it may be sus- 
pected that the E 1able ities ob- 
served by Owen, and Valenciennes, are referabic 
to the want of fresh specimens. It is, at first, 
singular that Owen (On the Nautilus, p. 39, Pl. I. 
Vv. w., or Isis, p. 52, Taf. I. 1, fig. 1, v. w., or Ann. 
des Sc. Nat. p. 139, Pl. I. fig. 1, v. w.) speaks of a 
ridge, and Valenciennes (loc. cit. p. 289, Pl. IX. 
fig. 1, No. 3) of a furrow, running from the border. 
of the lower lid to the pupil. As the cornea is 
wanting, it might almost be supposed, from examin- 
ing Valenciennes’ figure (Pl. VIII. fig. 2, P.), that 
Nautilus belonged to the Oigopsidés of D’Or- 
bigny, except, that with this animal, instead of a 
complete absence of the anterior part of the ocular 
capsule, there exists only a fissure, regarded by one 
of the authors in question asa ridge, and by the 
other as a furrow. The lens, not perceived by 
either Owen, or Valenciennes, escaped perhaps 
through this fissure, after having been detached 
by maceration. 
As for the pigment layer, spoken of by Owen, 
as situated uron the concave surface of the retina 
of Nautilus, this problem will be explained, from 
researches upcn fresh specimens in the same way, 
as with the ott 2r Cephalopoda. 
