614 
hermit crab. One ganglion, indeed, is present 
in the Pagurus, but both its situation and office 
alike militate against the hypothesis of its spe- 
cial subserviency to sensation: it is developed 
upon the end of the smooth abdominal chords, 
and seems to have been called into existence 
solely to regulate the actions of the muscles of 
the claspers by which the hermit keeps firm 
hold of the columella of its borrowed dwel- 
ling.” *} 
in reviewing these statements of the»ner- 
vous system of the entomoid Articulata, we 
observe that the superior ganglion of the pri- 
Mary nervous ring, or the cerebral ganglion, 
ey through several degrees of complication 
rom the Crustacea, where it presents only 
slight traces of a division laterally up to the 
Insecta, as in the bee for instance, where it 
preponderates greatly in size over the gan- 
glions, and where the sensorial nerves arising 
from it present distinct ganglionic enlarge- 
ments. e anterior or cephalic primary ner- 
vous ring itself we see to be gradually de- 
creasing in size from the Crustacea, where it is 
large and lengthened, to the highest Insecta, 
the Lepidoptera, where it is much smaller, 
and almost coalescing the superior and inferior 
ganglions developed on it into one ganglion. 
We observe that the number of primary ner- 
vous rings, with their ganglia, gradually be- 
comes more constricted from the Crustacea 
through the Myriapoda (where they are de- 
veloped in an undetermined length) and the 
Arachnida, where they are much fewer, to the 
Insecta, where, in their larva state, they ap- 
proach the Annelides, but in their perfect state 
we find them developed in a regular series, 
and more concentrated in the regions of the 
head, thorax, and abdomen. These anato- 
tical details, together with the complicated 
nature of the longitudinal commissures, a dis- 
tinct system of nerves supposed to be for re- 
Spiration, and a par vagum, demonstrate a 
close analogy between the ganglionic cord of 
the Insecta and the spinal cord of the Verte- 
brata, and may be considered as reasonable 
grounds for ranking this interesting tribe of 
animals the highest of the Articulata. 
Further details respecting the anatomy of the 
nervous system of Insects will be found in the 
article Insecta. 
Vertesrata.—We now to the last 
and highest group, the Vertebrata, where the 
powery nervous rings of the preceding classes 
ave become ganglions, and their commissures 
have become primary nervous rings. In each 
segment of their bodies there is but one gan- 
glion developed, but that one large, and situ- 
ated on the dorsal aspect, and each one in the 
different segments is united to the other by 
commissures, thus forming a large median ner- 
vous mass, the primary characteristic of a true 
cerebral system. This will be, of course, sub- 
ject to infinite modifications aud degrees of 
organization. In the lower Vertebrata the gan- 
glia and their commissures will be nearly 
* Owen’s Lectures, p. 171-72. 
NERVOUS SYSTEM. (Comparative Anatomy.) 
ually developed; in the higher ones the 
pone 1s Soe eetion will predominate; and — 
as these animals are vyte we pA 
dominance of ganglion, its great develo a 
takes place ior that -: of their body. which 
is itself the most highly developed, the head, — 
and the ganglionary mass itself is called the — 
brain. On the ye the developement of — 
the commissures, or of the longitudinal fibres, 
takes place in the opposing point to the he Z 
viz. the trunk, and from that results what is 
called familiarly the spinal marrow. Again, 
as it is the very characteristic of the nervous 
matter to accumulate and develop itself on t 
dorsal aspect in preference, it can easily be 
conceived that as the ganglionic nervous matter, — 
or brain, increases in developement, so will it 
influence the direction of the spinal marrow, 
and, indeed, also of the whole body. For 
instance, in the lower nervous formations the” 
brain and spinal marrow are perfectly hort 
zontal. ‘As the former proceeds in develope- 
ment, an angle, at first very acute, is formed, 
which gradually decreases until, in the human 
brain, the most perfect of all, it becomes acom 
plete right angle. Another important poit 
is, the number of ganglia and commissures tha 
may be developed. We have already observe 
that each segment of the body of the Ve 
brata contains a ganglion and a primary né 
vous ring: the number, therefore, of the 
latter is unfixed and variable, and depends e 
tirely on the number of the segments of 
body, or, in other words, the length of # 
spinal cord depends on the length of the an 
mal. But, with regard to the masses of gal 
glionic nervous matter situated in that segm 
of the body which is the most highly dev 
loped—the head—they ought to be develop 
in a manner fixed and determined ; and st 
indeed, is the case: a division into three 
observable in the brains of all the Vertebrat 
an anterior portion or cerebrum, @ poste 
portion or cerebellum, a median portion, 
tubercula quadrigemina. Thus the numbe 
ganglia forming the brain, the most hig 
organized part of the nervous mass, is def 
and invariable, while the number of yan 
forming the spinal cord, the least highl} 
ganized part of the nervous mass, is inde’ 
and variable. The three portions of the¢ 
bral mass, the anterior, median, and post 
will be designated by the names of first, se 
and third cerebral masses ; and we shal 
deavour to point out the analogies whieh 
of these portions bears in the brains of t 
ferent animals, as we ascend the scale, resp 
which anatomists have various opinions. — 
These observations being premised, we 
to the consideration of the vertebrated | 
individually, in the manner proposed, ~ 
mencing with the lowest, the fishes. = 
1. Pisces.—In these animals the ner 
system presents an immense variety of f 
and degrees of development. Even in- 
Cyclostomata, a division into brain and 8 
marrow (in the general acceptation of the ter” 
is evident: in the former, a division into ti 
‘ 
