ZOOLOGY. 
855 
eel genus, on the contrary, the optic tubercles are smaller and 
less developed internally. This is still more the case in the 
rays and sharks. 
. The most uniform and most essential part of the third 
cerebral mass , is a single ganglion, principally composed of 
gray substance, and which may be considered as the proto¬ 
type of the cerebellum. It always lies immediately behind 
the optic tubercles, is usually of a rounded shape, and con¬ 
tains a cavity which is continuous with the general cerebral 
cavity formed by the divergence of the upper columns of the 
spinal marrow, and the expansion of the canal contained 
within it. Its formation is thus simple in the eel for instance. 
In other fishes there are lateral appendages to this ganglion ; 
small in the pike, larger in the haddock. Occasionally also 
there is a second azygous ganglion below it, as in the carp 
and cobitis fossilis. Lastly, there are occasionally two 
other ganglia behind it, which then are principally connected 
with the origin of the branchial nerve 3 as in the carp, cobitis 
fossilis, and herring. 
The circumstances connected with the third cerebral mass 
in cartilaginous fishes, are peculiarly remarkable; for here 
the ganglion of %vhich it consists, and which as regards its 
character, corresponds to the cerebellum, more evidently 
presents the structure that it possesses in man. Thus, in rays 
and sharks, we find it is a simple medullary lamella, cover¬ 
ing the fourth cavity of the brain ; and in several of the 
latter, not only of considerable extent, but also disposed in 
transverse folds ; in which respect it coincides more particu¬ 
larly with the cerebellum of birds, hereafter to be described. 
The medulla oblongata is divided superiorly, as in man, so 
as to form the fourth ventricle; interiorly it is flattened, and 
of considerable breadth. 
The nerves that come from the brain in fishes, are the 
olfactory, which frequently form a ganglion previous to their 
termination. The optic, which are usually developed in 
proportion to the optic tubercles and the eyes; the third and 
fourth and sixth pairs, the origins of which we have described. 
(The fifth pair, and the par vagum, are distinctly recognis¬ 
able as pairs of nerves passing out between two vertebrae, 
like those of the spine; the former passing between the an¬ 
terior and middle, the latter between the middle and poste¬ 
rior, cranial vertebrae.) The branchial nerve is usually of 
considerable size in fishes, and is ordinarily distributed in 
three particular directions. The anterior, and thicker bran¬ 
ches, proceed to the respiratory organs, placed immediately 
below the head ; a twig divided into two fibrils being distri¬ 
buted to each lamina of the gills. The middle branches are 
destined chiefly for the neighbouring muscles. The third 
posterior branch, lastly, proceeds directly outwards, and then 
runs along the sides of the body, immediately below the 
skin, forming a lateral line, visible externally ; an arrange¬ 
ment of which the distribution of the accessory nerve in 
man, appears to form a repetition. The auditory nerves, 
though in immediate opposition with nerves of the fifth pair 
at their origin, are distinct from them, according to Des¬ 
moulins, in all species of fishes, except rays. A similar state¬ 
ment is made by Rudolphi, as regards the sturgeon ; and by 
Weber as to lampreys, and certain osseous and cartilaginous 
fishes. In many fishes there isan accessory auditory nerve cor¬ 
responding to the facial, arising either directly from the brain, 
or from the auditory and maxillary nerves, and distributed 
partly to the internal ear, and partly to the muscles of the 
branchial apparatus, os hyoides, &c. The nerve which sup¬ 
plies the place of the glosso-pharyngeal in fishes, is a division 
of the vagus nerve, given off from the first of the branchial 
nerves. On the other hand, that which corresponds to the 
hypoglossal, is a branch of the maxillary or fifth pair of 
nerves, and, according to Weber, has also, in the cyprini, an 
additional origin from the medulla oblongata. 
As to the spinal nerves, from the absence of proper extre¬ 
mities, are distributed in a very simple manner, between the 
ribs and the long spinous processes. In lampreys, they are 
extraordinarily small, which appears to be connected with 
the deficiency of ribs and fins; whilst on the contrary, in 
other cases, where the fins are highly developed, the bulk of 
the nerves is increased in a corresponding degree. This is 
particularly evident in rays; in which the bones of the 
shoulder and pelvis, as well as the pectoral and abdominal 
fins, are highly developed. Whilst in osseous fishes only 
the two first spinal nerves on each side unite for the supply 
of the pectoral fins, in the rays the first twenty-four pairs are 
united together, within a cartilaginous canal, into a broad 
band, a kind of axillary plexus, for the supply of the same 
parts. 
In all of the four superior classes of animals, there is a 
nervous chord on each side of the front of the vertebral 
column, the office of which appears to be to connect toge¬ 
ther the branches from the spinal marrow, destined for the 
organs of vegetative life, and to combine them into a general 
system, forming the medium for the mutual influence of the 
animal and vegetative spheres. Hence, it is necessarily 
connected with the nerves of the spine, as well as with the 
intervertebral nerves of the head : the fifth is the only pair 
of cerebral nerves with which it is connected in them. In 
the cyclopterus lumpus the internal filaments of the third 
ganglion, form an arch under the first vertebra, from which 
proceeds a fasciculus, passing along the oesophagus to the sto¬ 
mach, liver, and commencement of the intestine. A similar 
fasciculus arises from the eleventh ganglion, and is distributed 
to the ovary. The remaining ganglia are much smaller, and 
accompany the aorta and venae cavce, in the sub-vertebral 
canal. One of the most remarkable circumstances however, 
is, that, according to Desmoulins, the nerves of the sympa¬ 
thetic system are much less intimately connected with the 
arteries in fishes, than in the superior classes of animals. 
As an example of the parts in question, in osseous fishes, 
we have represented at fig. 45, the brain and spinal marrow 
of the cyprinus alburnus, viewed from above; a, the ganglia 
of the olfactory nerves, or first cerebral mass; b, optic tuber¬ 
cles ; c, cerebellum ; d, the medulla oblongata. As far as a-, 
the spinal chord is lodged within the dorsal vertebra, and 
after x, in the caudal; u, the olfactory nerves; 5, the fifth 
pair. 
Fig. 49 shews the basis of the brain in another osseous 
fish, the cyprinus barbus; c, is the.medulla spinalis; h, 
medulla oblongata; a b, the optic lobe; d u, the cerebral 
hemispheres; e, the olfactory lobe; 2 , the optic nerve; 3 , 
the third pair ; 45, the fifth nerves, formed of separate fibrils; 
7, the facial nerve; 8 , the auditory nerve; 9, the glosso¬ 
pharyngeal; 10 , the pneumo-gastric; 11 , the hypoglossal; 
12 , the accessory; a a, the cerebellum. 
Fig. 50 represents the upper surface of the same brain: ], 
the cerebellum; 2, the optic lobes; 3, the cerebral hemi¬ 
spheres ; i i, the olfactory lobe ; m, its pedicle; b b, the hypo¬ 
glossal nerve; b, the glosso-pharyngeal; c, the pneumo-gastric 
nerve; d, auditory; E, facial; a a, fourth pair; g, h, j, 
nervus trigeminus ; K, third pair ; L , optic nerve ; n o, ante¬ 
rior branch of trigemini. 
Fig. 48 represents the nervous system in a cartilaginous 
fish, viz. the upper surface of the brain; the raia rubus ; 
1 , medulla spinalis; 1 , bis corpus restiforme; 2 , colonus 
scriptorius; 3 and 4, middle lobe of the cerebellum ; 4, bist, 
and 9, lateral lobes ; 5, fifth and seventh nerves united ; 6, 
the fourth nerve; 7, optic lobe; 8 ,third nerve; 13, 14. the 
cerebral hemispheres; 10 , 11 , the optic nerve ; 16, pedicles 
of the olfactory ganglion ; 17, the ganglion itself. 
In the class amphibia the two main divisions of the great 
central nervous mass are still pretty nearly similar; for, al¬ 
though the brain is somewhat more developed than in fishes, 
the spinal marrow preponderates considerably in point of 
size. In salamanders, serpents, and lizards, as in fishes, the 
spinal marrow extends through the whole spine, even to the 
caudal vertebrae, and is, consequently, of considerable lenglh. 
In the frog, its fibres terminate at the sacrum. The form of 
the spinal marrow does not vary essentially from that ob¬ 
served in the preceding class. In the frog, the fourth ven¬ 
tricle extends pretty low into the spinal marrow. Here, also, 
as in fishes, the principal divisions of the brain are placed 
one behind another, and its form coincides most completely 
with that of the brain of cartilaginous fishes, particularly 
rays 
