ZOOLOGY. 
858 
take their course over it. We have further to notice here, 
also, the very great breadth of the medulla oblongata, which 
gradually diminishes in the superior species, and particularly 
in the porpoise. In the fourth ventricle, which is formed in 
the same manner as in man, the medullary strise connecting 
the ganglia of the auditory nerves in man are still deficient. 
The corpora olivaria likewise, are either wanting in most of 
the mammalia, or at least do not contain the same arborescent 
ramification of grey and white substance as in man : in the 
porpoise, however, these parts are very much developed. 
It had been generally admitted that the olfactory nerves 
were wanting in the porpoise, and probably in the other ce¬ 
tacea, until Jacobson and Blainville asserted that they had 
discovered them. Rudolphi, however, contradicts the state¬ 
ment from his own observations, and those of Dr. Otto; who 
examined the brains of several specimens in the fresh state, 
and sent others to Rudolphi, still enveloped in the arach¬ 
noid, so that, had the nerves existed, they could not have 
escaped observation. The same remark applies to the 
whale, (balaena mystecetus ) and narwhale, (monodon mo- 
noccros ) examined by Rudolphi. 
The optic nerves generally have, in mammalia, altogether 
the same course as in man, their size only varying materially. 
In mice, rats, bats, and hedgehogs, for instance, they are 
very slender; on the contrary, in the squirrel, rabbit, and 
hare, very large. 
The remaining cerebral and spinal nerves in this class, are 
distributed in every essential respect as in man: we may, 
however, notice the very striking size of the fifth cerebral 
pair in most mammalia. 
In mammalia, as in man, the cervical portion of the sym¬ 
pathetic nerve, no longer lies immediately upon the vertebral 
column, and the number of its ganglia in that region no 
longer corresponds to the number of vertebrae. So few 
other important peculiarities are to be found in the remaining 
divisions of the ganglionic system, that a distinct investiga¬ 
tion of it appears superfluous. 
With respect to the functions of the nervous system, we 
remark that nerves are longitudinally divided into thin fibrils, 
and that this arrangement enables such of them as are sentient 
to take cognizance of the particular forms, as well as the 
mere presence of bodies. Had they been homogeneous, they 
must have possessed two functions, a power of acknowledg¬ 
ing the form of objects, and that of acknowledging simply 
their presence. 
The size of a motive nerve appears to depend less on the 
bulk of the part it supplies than upon the variety of move¬ 
ments that can perform. The size of a sentient nerve less on 
the intensity of the impressions it conveys but on their 
number. Sentient nerves are, cater is ‘paribus , larger than 
motive. Since many plants and animals move that have no 
nerves, these elementary parts may be considered as only 
necessary to concentrate the movements of distant organs, or, 
in other words, to excite remote muscles, the intermediate 
parts remaining quiescent Thus a medusa has no nerves, 
though, when a stimulant is applied to it, it contracts: for 
the medusa does not contract a particular line or range of 
its strise, but all of them simultaneously. For example, sup¬ 
pose the line a - b were stimulated at the end marked a, 
a contraction would take place, were the line pure muscle, 
through its whole extent, strongest at a, and gradually getting 
weaker towards b, an effect which it is not altogether in¬ 
conceivable to suppose may be effected by a power of 
attraction. But if the line in question were traversed by a 
nerve, the ends a and b would begin to contract, whilst the 
intermediate parts were quiescent; and the consequence 
would be, that the line would become curled, and would 
enclose any substance placed between its arms. Now this 
is what occurs in the animals where the first trace of the 
nervous system is found. In the asterias and iiolo- 
thurize a nervous collar surrounds the entrance to the sto¬ 
mach, and sends a nerve to each ray. These animals lie 
outstretched in the water; if any matter be placed on their 
feelers, the nerves carry the impression to the nervous circle: 
this excites the extremities of all the rays; they contract, and 
the matter in question is thrown into the stomach. This 
nervous circle has some ganglions in it; the functions of 
which are probably two in number, to increase the nervous 
power at the part where it must evidently be required in the 
greatest degree; and, secondly, to convert the sentient impulse 
it derives from one nerve into the motive influence it imparts 
to another. There is no evidence, nor is it probable that 
any consciousness exists here; the operation of the nerves is 
doubtless under the governance of an imperative law. The 
next step in the gradation of function is, that by means of a 
peculiar apparatus nerves become capable of receiving im¬ 
pressions from light, or perceiving vibrations in the media that 
surround them, or even of distinguishing the chemical quali¬ 
ties of those media. Hence seeing, hearing, &c., are super- 
added to the mere sense of touch. But there is no evidence 
that these may not, to a certain degree, exist without any 
very clear consciousness on the part of the animal possessing 
them. A form deciphered on the eye of a cuttle fish may 
excite a motion as determinate and automatic in the nerves 
and ganglia of that animal as the immediate contact of a 
knife could be capable of exciting in the separated muscle of 
a frog. 
That pleasure and pain are felt very low in the crea¬ 
tion is evident, both from experiment and from the consi¬ 
deration that animals of the fullest life require a sense of 
appetite, to stimulate them to supply themselves with food. 
Perhaps the next instincts we observe in animals, is that 
of approaching things which are pleasant, and receding from 
such as are disagreeable. To lay hold on that of which the 
presence is associated with the remembrance of a pleasant 
taste, is perhaps the next. Here we have association, and 
consequently reminiscence developed, as very early powers, 
though very confused ones, the movements of the animals 
being determined by a few objects. These associations are 
of course in the feeble origin, the result of original determinate 
laws, but become strengthened and fully formed in the con¬ 
sequence of repetition; and on these habits become deve¬ 
loped, of which nothing more can be said, than that they are 
produced by the combined operation of internal powers and 
external circumstances. Upon the whole, we remark that 
in the animal kingdom, the prime symptoms of cerebral 
powers begin at a very low scale, at a point which is unde¬ 
termined; that they acquire, insensibly but rapidly, a high 
degree of developement, and furnish no marked point of 
difference, except at that single step which separates the 
mind of man from the rest of the creation. With respect to 
the parts of the nervous system, in which particular instincts 
reside, it appears fair to infer the ganglionic enlargement, 
which is formed at the union of any particular sets of nerves, 
to be the spot in which the instinct, or habit, or association is 
developed, that refers to the impressions those nerves are cal¬ 
culated to convey : that, for example the first cerebral lobe is 
the seat of the remembrance of smells in a fish: and the 
second, the seat of sights. What renders this investigation 
complex, is the circumstance that the different parts of the 
brain being connected intimately together, the affections of 
one part influence in a great degree the other—a reciprocating 
action which a mere glance of the nervous movements must 
convince us is absolutely requisite. In the higher animals, 
this union becomes so perfect that the central part of the 
nervous system forms, as it were, but four parts; the cerebrum 
and cerebellum united by the tuber annulare and the spinal 
marrow ; and these even are intimately connected. Here it 
is probable that the cerebrum is at once the seat of the asso¬ 
ciations that are exercised on the impressions of sight, smell¬ 
ing, and touch. 
With respect to the size of a central lobe of the nervous 
system, that does not appear to be in proportion to the size 
of the nerves that terminate in it, or the perfection of the 
sense in the parts those nerves supply; neither does it depend 
on the intensity or force of the instinct. But it appears to be 
intimately connected with the number of habits, or associa¬ 
tions, that it is required to receive. Hence the size of the brain 
is 
