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NERVOUS SYSTEM. (Comparative Anatomy.) 
is at once seen; in the latter, the ganglia are 
numerous and undetermined. We will notice 
these parts separately. 
The spinal cord* (fig. 351, g,) is remarkable 
for its great relative size in this class of animals: 
it is continued (with but very few exceptions) 
the whole length of the vertebral column, even 
into the caudal vertebra, and it has on its an- 
terior and posterior aspects a longitudinal fis- 
sure (fig. 351, h), the latter being the deepest ; 
internally it is hollowed out by a canal (i) 
which traverses it in its whole extent, and 
which, at the upper part, immediately posterior 
to or underneath the cerebellum, forms a con- 
siderable dilatation or enlargement—the fourth 
 yentricle (fig. 352, e). The posterior fissure 
extends to this canal. In a river lamprey 
( Petromyzon fluvialis ), weighing 570 grains, 
brain weighs only four-tenths of a grain, 
while the spinal cord weighs three grains, the 
proportions being as 100: 750. We thus ob- 
serve how much the latter preponderates in 
| size, being seven and a half times heavier than 
the brain. 
It is inclosed in a semicartilaginous 
_ case, and I satisfactorily traced it into the ex- 
treme point of the caudal extremity of the 
animal : it presents a thin flattened appearance, 
_ so much so that no trace of a central canal is 
perceptible ; but immediately posterior to the 
Eo. the rudimentary corpora restiformia of the 
two lateral longitudinal columns diverge to 
_ form a large excavation, which is covered over 
a net-work of delicate vessels, a sort of 
us choroides ; this is the fourth ventricle. 
_ Amongst the true osseous fishes I have found 
acanal traversing the spinal marrow with this 
dilatation or ventricle at its superior portion, 
in the eel (Anguilla), perch ( Perca fluvialis), 
gurard ( Trigla gurnardus ), cod ( Gadus mo- 
vhua), mackarel (Scomber vulgaris), pike 
( Esox lucius ), roach ( Leuciscus rutilus ), dace 
Leuciscus vulgaris ), chub ( Leuciscus ?), 
carp( Cyprinus carpio ), and skate ( Raia ?) 
Tn the gurnard there are six pair of ganglia 
developed on the superior surface, immediately 
posterior to the cerebellum, at the origins of 
the nerves distributed to the large pectoral fins ; 
this remarkable conformation only exists in this 
genus. In all the other species the spinal cord 
is of nearly equal diameter throughout, except- 
ing towards its termination ; and in the dace I 
traced it running to the extremity of the tail, 
and ending in a point: in the moon fish ( Te- 
trodon mola) it is remarkably short, and termi- 
Nates in a true cauda equina. 
_ [Asimilar exception to the usual length of 
the spinal cord in fishes, is found in the Lophius 
_ piscatorius, in which that organ ceases as high 
as the eighth vertebra, and in one instance 
by Leuret as high as the second. 
rest of the canal is occupied by cauda 
equina. } 
The superior portion of the spinal cord, which 
takes the name of medulla oblongata, is large and 
* In the description of the spinal cord the terms 
anterior and posterior are used in the same signi- 
fication as in the human subject ; anterior to signify 
the surface next the bodies of the vertebrz, posterior 
that next the spinous processes. 
615 
broad in most fishes: on it are perceptible the 
corpora pyramidalia and restiformia; the t.ivaria 
are not yet develo The former, situated on 
either side of the anterior longitudinal groove, 
are flattened and broad, and are distinctly seen 
continuous with the crura cerebri, the pons 
Varolii being wanting. The corpora restiformia, 
or cerebellic fasciculi, are situated posteriorly ; 
they separate (as before observed) at their upper 
part to form the fourth ventricle, and pass after- 
wards into the cerebellum. According to 
Leuret there is no decussation of the fibres of 
the spinal cord in fishes. 
[A singular little fish which haslately attracted 
the attention of naturalists, and for the reception 
of which Mr. Yarrell has instituted the genus 
Amphioxus, exhibits the apparent anomaly of 
an absence of all outward distinction between 
the brain and spinal cord. It is the Amphioxus 
Lanceolatus, of which a very perfect specimen 
has lately been presented to the Museum of 
King’s College, London, by Professor Edward 
Forbes. 
An elaborate examination of the anatomy 
of this little creature has been published by 
Mr. John Goodsir in the Transactions of the 
Royal Society of Edinburgh, from which we 
extract the following account of its neuro- 
skeleton and of its nervous system. 
“ Neuro-skeleton.—The osseous system, pro- 
perly so called, consists of a “ chorda dorsalis” 
tapering at both ends, without the vestige of a 
cranium, and of a dorsal and ventral series of 
cells, the germs of superior and inferior inter- 
spinous bones and fin rays. The “ chorda 
dorsalis” consists of sixty to seventy vertebrae, 
the divisions between which are indicated by 
slight bulgings, and lines passing obliquely from 
above downwards on the sides of the column. 
In this way a separation into individual ver- 
tebre is rather indicated than proved to exist; 
for although the column has certainly a ten- 
dency to divide at the points above-mentioned, 
yet that division is rather artificial than natural. 
There’ is no difficulty in ascertaining above 
sixty divisions, those at each end above the 
number stated run so much into one another 
that no correct result can be obtained. 
_ The chorda dorsalis is formed externally 
of a fibrous sheath, and internally of an im- 
mense number of lamin, each of the size and 
shape of a section of the column at the place 
where it is situated. When any portion of the 
column is removed, these plates may be pushed 
out from the tubular sheath, like a pile of coins. 
They have no great adhesion to one another, 
are of the consistence of parchment, and appear 
like flattened bladders, as if formed of two 
tough fibrous membranes pressed together. 
“ As the fibres of the sheath are principally 
circular, provision is made for longitudinal 
strains on the column by the addition of a su- 
perior and inferior vertebral ligament, as strong 
cords stretching along its dorsal and ventral 
aspects. The superior ligament lies imme- 
diately under the spinal cord, and may be re- 
cognized as a very tough filament, when the 
column is torn asunder, or some of the ver- 
tebree removed. The inferior ligament may be 
