OPTIC NERVES. 
optic nerve; but the more obvious effect of 
this organization is to increase the surface of 
the nervous material. A similar contrivance is 
at times resorted to in the nervous centres, as is 
exemplified in the cerebral hemispheres ; these 
masses in the higher and more intelligent 
animals being covered with large convolutions 
and deep sulci, while in the lower classes they 
are smooth and consequently possess a super- 
ficies of limited extent. 
Optic nerve in that form of monstrosity known 
by the varied appellations of “ Cyclops,” 
“ Fetus a trompe,” “ Monops,” “ Rinence- 
phale,” Sc. §c. 
The abnormal anatomy of the optic nerve is 
not in strictness comprised within the scope 
of the present article, but nevertheless a brief 
_ description of the above malformation will pro- 
bably not be considered out of place. 
A single eye placed in the middle line of the 
forehead, and in general a trunk or proboscis 
growing immediately above this solitary organ 
of vision, constitute the most striking apparent 
anomalies in monsters of this class. The 
writer is indebted to Dr. Johnson, Master 
of the Dublin Lying-in Hospital, for permis- 
sion to dissect a specimen of this species of 
monstrosity in the human subject, and he 
has obtained through the kindness of Dr. 
William Wilson Campbell, (formerly assistant 
‘in the same establishment,) the particulars of 
another similar case exa- Fig. 427. 
_ mined by that gentleman in 
| the year1834. ‘The annexed 
' wood-cut (fig. 427) gives a 
faithful representation of the 
| optic nerve in the prepara- 
_ tions dissected by the writer 
-and Dr. Campbell; and it 
ees exactly with the ap- 
arances found by Geoffroy 
aint-Hilaire, in the cases 
which fell under his obser- 
vation. The tractus optici 
present a very natural ap- 
in front, and from it, only writer.) : i 
a single optic nerve pro- 4% 4 tractus opti~ 
ceeds ; Bi ppance ditectly nated na al > 
f s to the back of the gie optic nerves d, 
ball, where it penetrates sclerotic coat of "the 
e sclerotic and terminates eye perforated bythe 
in the retina. optic nerve. 
_ Fig. 428 represents the encephalon, optic 
hherve, and organ of vision in a kitten at the full 
iod of gestation, (the subject of the same 
of monstrosity,) which lately came into 
he writer's possession : the preparation is pre- 
served in the Museum of the Richmond Hos- 
ime School, Dublin. In all essential particu- 
this specimen bears the closest resemblance 
jto the human monsters of which the dissection 
has just been described. 
The fundamental defect in these monstrous 
777 
\ Fig. 428. 
a d 
Brain and organ of vision of 
b @ Cyclops katten, at the full 
i period of gestation. Seen from 
Wi Mil— qd below. ( Natural size.) 
wail 
a, organ of vision, single, 
and of great dimensions; b, b, 
cerebral hemispheres seen from 
below ; d, d, tubereula quadri- 
gemina ; c, optic nerve, single, 
and of great size. 
fetuses consists in the total absence of the 
organ of smell, in consequence of which de- 
ficiency the symmetrical organs at either side 
become united in the middle line and actually 
engrafted upon each other: the ¢wo eyes are 
conjoined so as to form but a single organ of 
vision, and the very same metamorphosis occurs 
-in the two orbits, the two optic foramina, the 
two optic nerves, &c. 
That this is the rationale can scarcely admit 
of doubt, since in some parts of the organs 
the fusion remains incomplete ; thus two crys- 
talline lenses still exist in the interior of the 
solitary eye-ball: a double set of muscles with 
their corresponding nerves are provided for the 
globe of the eye; and four eye-lids protect the 
organ in front, causing the aperture of the lids 
to assume a quadrangular form. 
General developement of the optic nerves in the 
higher classes of animals. 
Fish.—In fish as a general rule these nerves © 
are highly developed, and exhibit a marked 
preponderance in size when contrasted. with 
the corresponding nerves in many animals 
holding a more exalted position upon the scale. 
This may be explained by the nature of the 
medium which the fish inhabits; for some of 
the light incident on the surface of the water is 
reflected, and another part, after penetrating the 
water, becomes absorbed, in consequence of the 
continual disturbance to which the transparency 
of this fluid is subject; so that fishes necessa- 
rily require a greater developement of visual 
apparatus than would suffice /and animals for 
an equal amount of vision. 
Birds.—In birds the sense of sight exists in 
great perfectio, and the optic nerves exhibit 
corresponding developement. 
Mammalia.—In Mammalia the faculty of 
vision ceases to preponderate, and accordingly 
the proportions of the optic nerves in this class 
are no longer excessive. 
Many facts in comparative anatomy war- 
rant the conclusion that the senses of smell 
and vision are at times supplemental to each 
other; for example, the mole either possesses 
no optic nerve, or if any such exist it is so 
diminutive as to be most difficult of recogni- 
tion, but the olfactory lobes of the brain and 
the whole olfactory apparatus of the animal 
exist in great perfection, and its subterranean 
habits enable it to turn this latter function to 
‘account, whilst a highly finished organ of 
vision would have been an useless appendage. 
In certain fish which frequent the mud or slimy 
waters (as for instance the eels), the visual 
apparatus is poorly developed, and the optic 
