S59 
ZOOLOGY. 
is a tolerably good measure, generally speaking, of the edu- 
abiiity of an animal. At the same time, we must not push 
this conclusion too far, nor expect to find that a dolphin, 
because its brain is as large as a man’s, can retain as many 
conceptions in its memory, or associate as readily those con¬ 
ceptions with external impressions; or, that a man with a 
large head has more of these faculties than one with a smaller 
one. The changes that are wrought on the brain are too subtle 
for our view; but no fact is more clear than that the cerebral 
functions never go on, unless there is a due supply of aerated 
blood. This supply varies in quality in different animals, 
and may vary in this respect in minute particulars among in¬ 
dividuals; it varies in quantity, not only in different indivi¬ 
duals, but at different periods; and the connexion between this 
variation, and the excess or diminution of the cerebral func¬ 
tions, has been observed by every one. 
Though there is no subject more interesting in Zoology 
than the present, our limits, already exceeded, forbid us to 
enter further into its consideration. The same reason com¬ 
pels us to abbreviate very much of what we had to say on 
the senses. 
The senses of taste and smell are probably enjoyed by 
nearly all animals, though they do not reside in the same parts. 
In such as have no tongues, the digestion of particular foods 
may be so pleasant as to render the stomach a true gustatory 
organ; and where the nasal cavities are absent, palpi, or 
even a respiratory cavity, may receive various modifications 
from the existence of different aromee in the air. 
With respect to the organ of sight, though the whole 
bodies of some zoophytes seem capable of perceiving light, 
it is only in a proper eye that the form of objects can be per¬ 
ceived : and this organ exists in the sepia and helix alone, 
and in the class vermes. 
In insects, two kinds of eyes, very dissimilar in their struc¬ 
ture, are found. One sort is small and simple (stemmata); 
the others, which are large, seem to consist of an aggregation 
of smaller eyes; for their general convexity is divided into 
an immense number of small hexagonal convex surfaces, 
which may be considered as so many distinct corneae. The 
first kind is formed in different numbers in most of the 
aptera, as also in the larvae of many winged insects. When 
these undergo the last or complete metamorphosis, aud re¬ 
ceive their wings, they gain at the same time the large com¬ 
pound eyes. Several genera of winged insects, and aptera 
(as the largest species of monoculi) have stemmata besides 
their compound eyes. 
The internal structure has hitherto been investigated only 
in the large polyedrous eyes. The back of the cornea 
(which is the part, divided in front into the hexagonal sur¬ 
faces, called in French, facettes ) is covered with a dark 
pigment. Behind this are numerous white bodies, of a 
hexagonal prismatic shape, and equal in number to that 
of the facettes of the cornea. A second coloured membrane 
covers these, and appears to receive the expansion of the optic 
nerve. 
Blumenbach thinks it probable, that the polyedrous eyes 
are adapted for distant objects, and the simple ones for such 
as are more near. 
We have engraved two figures, in order to shew the eyes in 
insects. 
Fig. 42 is a representation of the parts of the eye, as they 
are attached to the brain in the scarabaaus: a a , the pyramidal 
fibres; the cornea which surmounted them being removed : 
b, the fibrous membrane which receives the optic nerve, c. 
This membrane is white on the exterior surface, but dark 
where it is in contact with the nerve. This dark appearance 
is represented at d. 
Fig. 43 represents the eye of the drone opened from below: 
x, the medulla spinalis; y, the first ganglion that the medulla 
forms after emerging from the cranium; s, the cortical; t, 
the medullary substances of the brain; z, the facettes of the 
cornea, and the pyramidal bodies; q, the cortical substance 
of the eye which receives the optic nerve; x, the inner coats; 
o, the eye; u, the part where the pyramidal bodies are largest. 
The peculiarities in the eye'of fishes, which belong either 
to the whole class, or to most of the genera and species, con¬ 
sist in the division of their choroid coat and retina into se¬ 
veral manifestly distinct laminae; and in the existence of two 
small organs within the eye, which belong exclusively to this 
class. 
1. A body, generally resembling in shape a horse-shoe, 
lies between the internal and middle layers of the choroid ; 
some have thought it muscular, and others glandular. 2. 
The tunica Ruyschiana gives origin to a vascular membrane, 
resembling in its form a bell (campanula of Haller). This 
goes towards the lens, and has, therefore, some resemblance 
to the marsupium of birds. 
No true ciliary body is found, at least in the bony fishes. 
The crystalline lens of most fishes is very large in com¬ 
parison with the size of the eyeball, and nearly or entirely 
spherical. The vitreous humour on the contrary is small, 
and the aqueous in many cases is hardly discernible. 
The eyes are very large in birds. . 
The distinction between certain parts of the eye, where 
the membranes have been supposed to be continuous, appears 
more plainly in some birds, than in any other animals. The 
boundaries of the choroid coat and iris are very clearly 
defined in the horned owl (strix bubo); and those of the 
margin of the retina, and the posterior border of the ciliary 
body, very distinct in the toucan (ramphastos tucanus). 
The ciliary processes of birds are not very prominent; they 
consist rather of striae than of loose folds. They are always 
closely connected to the crystalline capsule. There is no 
tapetum in this class. 
A great peculiarity in the eye of birds consists in the mar¬ 
supium (pecten plicatum; in French, la bourse, le peigne,) 
the use of which has not hitherto been very clearly ascer¬ 
tained. It arises in the back of the eye, proceeding appa¬ 
rently through a slit in the retina; it passes obliquely into the 
vitreous humour, and terminates in that part, reaching in 
some species to the capsule of the lens. The figure of its 
circumference is a truncated quadrangle. Numerous blood¬ 
vessels run in the folds of membrane which compose it; and 
the black pigment by which it is covered, suggests an idea 
that it is chiefly destined for the absorption of the rays of 
light, when they are too strong or dazzling. 
The third eyelid, or membrana nictitans, is a thin, semi¬ 
transparent fold of the conjunctiva; which, in the state of 
rest, lies in the inner corner of the eye, with its loose edge 
nearly vertical, but can be drawn out so as to cover the whole 
front of the globe. By this, according to Cuvier, the eagle 
is enabled to look at the sun. 
The eye of mammalia in general differs little from that of 
man, except in their form and size, or in the separation of 
individual parts. 
The sclerotica of the cetacea is distinguished by the great 
thickness of its posterior part; when the eye-ball equals an 
orange in size, the back of this membrane is an inch thick; 
so that, although the globe be spherical, the space containing 
the vitreous humour is of a different form. The extent of the 
cornea seems to be greatest in the porcupine (hy strix cristata), 
where the cornea extends over half the globe. 
The choroid coat consists more plainly in the cetacea than 
in any other mammalia, of two distinct laminae, of which 
the internal (membrana Ruyschiana) is covered with a dull 
tapetum. 
The tapetum occupies the temporal side of the bottom of 
the eye-ball; i. e. it is placed exteriorly to the entrance of the 
optic nerve. It exists in the carnivorous and ruminating 
animals; in the solipeda, pachydermata, and cetacea. In 
the dog, wolf, and badger, it is of a pure white, bordered by 
blue. 
The iris varies in thickness in the different genera. In no 
instance can we discover true muscular fibres; the examina¬ 
tion of the part in the elephant and whale having afforded 
in this respect the same result, as the tender and almost trans¬ 
parent iris of the white rabbit. 
The size of the crystalline lens varies in proportion to that 
of the vitreous humour; and sometimes very considerably. 
Blumenbach found the largest lens in this point of view in 
the 
