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NERVOUS SYSTEM. (Comparative Anatomy.) 
ganglion, situated in the median line; in the 
perch, gurnard, cod, pike, roach, chub, carp, 
and dace, of a pair of ganglia, and this is the 
most usual arrangement. In the skate, one of 
the Plagiostome fishes, where the brain is alto- 
gether more highly developed, there is one large 
ganglion or cerebral mass; it is solid, but in 
some of the sharks it contains a cavity. From 
these eminences, whatever be their number, the 
_ olfactory nerves (processes or lobes) arise (figs. 
350 and 351, k, k), which, running together in 
an osseous canal for some little distance, 
diverge, and form large tubercles on the cribri- 
form plate of the ethmoid bone (fig. 352, 1); 
from these tubercles nerves arise, which are 
distributed to the pituitary membrane of the 
1 nose. 
2dly. The optic lobes, or second cerebral 
mass (figs. 351 and 352, c,b), which Collins,* 
- Monro,* Camper,* Ebel,* Treviranus,* and 
Cuvier,* considered as analogous to the cere- 
ral hemispheres of the mammalia, but which, 
with Serres,+ Desmoulins,t Arsaky,+ Carus,+ 
and Tiedemann,+ I consider as analogous to 
the tubercula quadrigemina, are generally of 
large size in fishes, and contain internal tuber- 
cles and cavities, which communicate with the 
rth ventricle. These masses may be said to 
arrive in this class at their maximum of deve- 
ment, and we may recollect to have traced 
out their first radiments in the cerebral ganglion 
of the Gasteropodous Mollusca, and to have 
noticed their successive complication of deve- 
ent in the varied classes of articulated 
animals. In the lamprey these optic lobes are 
er and more developed than any other parts 
of the brain; and this is what we should be 
led to expect from the low organization and 
vermiform nature of these Cyclostomous fishes : 
they contained in their interior a cavity. In 
the eel, perch, cod (fig. 351, c), gurnard, 
mackarel, pike, roach, chub (fig. 352, 6), carp, 
and dace, true osseous fishes, the optic lobes 
are well developed, and, excepting in the eel, 
much larger than the olfactory tubercles before 
them ; they are hollow, and contained tubercles, 
which vary in number, size, and position. Inthe 
eel there are two of these tubercles in each hollow 
lobe, equal in size, and situated posteriorly ; in the 
cod there are also two, the outermost being the 
largest, and smooth, the inner one being smaller, 
and constricted in the middle (fig. 351, e); in 
the mackarel there are two, the anterior one 
being exceedingly small, the posterior much 
larger, slightly convoluted, somewhat resemb- 
ing the Greek letter =; in the pike there are 
two, and the floor of the cavity had a striated 
appearance; in the roach there is only one 
large tubercle; and in the carp there are two, 
the anterior being rather long, and passing 
backwards in a curved manner. From these 
lobes the optic nerves (fig. 351, /) arise, and 
cross each other, without, however, any other 
connexion than mere cellular tissue. The third, 
fourth, and sixth pairs have also their origins 
from these ganglia. 
[The optic lobes have a direct relation in 
* Op. cit. + Op. cit. 
619 
point of volume with that of the eyes, and in 
the pleuronecta, in which the eyes are 6 une- 
qual size, Gottsche states that the optic lobes 
are unequal. | 
The tubercles situated on the inferior surface 
of the brain, and immediately beneath the 
optic lobes just described, are generally of 
small size, and seldom contain a cavity; be- 
tween them are the infundibulum and pituit 
gland, generally of very large proportional size, 
Respecting their analogies and names, very 
much difference of opinion exists. Haller 
termed them the inferior protuberances of the 
olfactory nerves ;* Cuvier considered them as 
the true optic lobes ;+ Dr. Grant calls them 
the cerebral hemispheres, and supposes they 
are the representatives of those parts in the 
higher animals ;{ Serres considers them appen- 
dages to the optic nerves, and analogous to the 
tuber cinereum ;§ Vicq d’Azyr,|| Arsaky,§] and 
Carus, consider them analogous to the corpora 
mamumnillaria of higher animals :** Tiedemannt+ 
does not decide upon this point, but judges 
(from the situation and form of the tubercles) 
that the latter hypothesis is the more probable 
one. 
3dly. The cerebellum, or third cerebral mass 
(fig. 351, f; fig. 352, c), is but imperfectly 
developed in fishes ; it is generally of a round 
form, and covers in the cavity formed by the 
divergence of the two cords of the spinal mar- 
row and an enlargement of its canal, the fourth 
ventricle. In the lamprey there are scarcely 
any traces of a cerebellum, a thin transverse 
band of medullary matter being all that stands 
for it; the fourth ventricle is here, therefore, 
quite open and exposed. In the eel it is large, 
and of a rounded form ; in the perch its sum- 
mit is directed backwards; in the mackarel, 
forwards; in the cod (fig. 351, f) and pike it 
consists of a tongue-shaped lobe ; in the gur- 
nard, roach, chub (fig. 352, c), and dace, it is 
round, and of moderate size; in the carp it is 
also of a rounded form, but immediately be- 
hind and below it is situated another ganglion 
of smaller size, on each of which is a larger 
ganglion, principally destined for the origin of 
the branchial nerves, thus rendering the struc- 
ture of the cerebellum very complicated, and 
its size very voluminous. In the Plagiostome 
fishes the cerebellum is much more highly de- 
veloped. In the skate it is of large relative 
size, furnished with two lateral appendages, 
the commencement of lateral hemispheres, on 
the external surface of which transverse and 
longitudinal strie were developed. 
On reviewing these statements of the nervous 
system of the fishes, we observe two things 
that more particularly mark its low organization 
—the equality and the horizontal position of the 
brain and spinal marrow. In fact, as regards 
* Opera minora, vol. ii. 
+ Anatomie (omparée. 
¢ Lectures on Comparative Anatomy. 
Anatomie Comparée du Cerveau. 
i Opera minora. 
{ De piscium cerebro. 
**® Anatomie Comparée. 
tt Anatomy of the Feetal Brain. 
