854 
ZOOLOGY. 
rior ganglion, which forms the under surface of the brain, and 
which receives the optic nerves -, b bb, the seven ganglia that 
give off nerves in the thorax and abdomen; ccc, nerves, 
which arise from the threads that connect these ganglia; d d, 
their doubleor separated threads; e, the conical portion of the 
spinal chord, which is enclosed in a contracted piece of shell; 
f, the thoracic portion of the spinal chord, which furnishes 
nerves to the muscles of the legs and wings; g, the part of the 
spinal chord enclosed in that strangulating scale that separates 
the thorax from the abdomen; h, the abdominal portion of the 
chord, which gives origin to the nerves that supply the muscles 
of the abdominal rings and the sting; i i, two nerves passing to 
the jaws; k k, two nerves going to the proboscis [Swammer¬ 
dam conjectures these may be for taste]; 11, two nerves that 
supply the muscles of the proboscis; mm, the optic nerves; 
n n, two nerves which pass from the thoracic portion of the 
spinal chord through the scaly strangulation, with the me¬ 
dulla itself, and which are distributed to the muscles on the 
dorsum of the abdomen. The ganglia are firmer and more 
striated in this animal’s spinal chord than the chord itself, 
and are also more amply supplied with tracheae, according to 
the observations of Swammerdam on bees, Malpighi says 
the same of the silkworm. 
Fig. 41 represents the spinal chord of an ephemera, formed 
of a series of ganglia, answering to the annular rings of the 
body, united to each other by two columns of spinal marrow, 
and sending off nerves; the foremost ganglion forming the 
brain and receiving the optic nerves. 
At figs. 42 and 44, we have representations of the brain in 
the scarabaeus monoceros: a, the brain, composed of two he¬ 
mispheres, its front part sending off four small nerves; b, two 
large nerves sent off posteriorly, which run round the oeso¬ 
phagus, unite, and form a ganglion; from this a continued 
spinal chord descends, dotted with fourteen ganglia. Nerves 
that pass to the whole body of the insects issue symmetrically 
from this medulla. 
Fig. 42. ii, the brain, formed of two lobes, and studded 
with tracheae; k, the optic nerves, of large size altogether, 
are somewhat contracted where they pass off from the brain at 
11 , but dilate considerably at m rn; they retract again at n n, 
hut form, ultimately, a large swelling at o, where they become 
invested by the internal parts of the eye. 
Fig. 59. A male cray-fish (astacus fluviatilis), opened from 
above; the viscera are removed, the tail is cutaway, with the 
exception of the chain of ganglia. The canal in which the 
chain is lodged above the legs, and formed by the horny 
laminae, to which the legs are attached (a kind of vertebral 
canal, though placed on the abdominal surface), is, for the 
most part, broken away, so as to expose the whole extent of 
the central part of the nervous system: a, the oesophagus, cut 
through immediately behind the triangular mouth; b, the 
cerebral ganglion ; c, the nervous collar of the neck, with a 
pair of nerves'arising from each side; the remaining eleven 
ganglia are designated by figures; d, the bony pedicle and 
muscles of the mandibulae; e, the eyes; f, the horny laminae 
forming the canal for the chain of abdominal ganglia; g, a 
part of this canal not broken open; h, the lower extremities 
of the seminal ducts. 
Of the nervous system in fishes.—With respect to the two 
divisions of the great nervous central mass in this class, 
the superiority of bulk is constantly on the side of the spinal 
marrow, which stretches, with few exceptions, through the 
whole extent of the vertebral column, and consequently, 
which is not the case in superior animals, through the caudal 
veitebrae; whence, from the great number of vertebrae, it at- 
taiusa remarkable length. According to Arsaky, the spinal 
marrow is peculiarly short, in proportion to the vertebral 
canal, in some cartilaginous fishes, as the tetrodon mola and 
lophius piscatorius, the nerves belonging to it forming a kind 
of cauda equina, as in man. In the lampreys (petromyzon 
rnarinus, fluviatilis, and branchial is), the spinal marrow 
differs from that of all other vertebral animals. In them the 
inferior of the two grooves of the spinal marrow expands so 
much immediately below the brain, that the former assumes 
a complete riband-like appearance, and the canal usually 
contained within it totally disappears. The spinal marrow 
of fishes terminates in a single thread, and usually at the last 
caudal vertebra. Its nerves arise by superior and inferior 
roots, of which the latter are detached somewhat more pos¬ 
teriorly (caudally) than the former. As in man, it is only 
on the inferior roots that small ganglia are found. The supe¬ 
rior are connected with the inferior roots externally to the ver¬ 
tebral canal; which, particularly in the osseous fishes, is but 
imperfectly closed by the slender arches supporting the 
spinous processes. In those situations where large nerves 
arise from the spinal marrow, distinct swellings, or enlarge¬ 
ments of the latter, are distinctly visible; an observation 
which applies to the remarkably curtailed spinal marrow in 
the sun fish and frog fish, as well as to the upper portion of 
the spinal marrow in a species of flying fish, (trigla) in which 
the pectoral fins are developed in an unusual degree, and 
each of the pairs of nerves belonging to it, corresponds to a 
pair of ganglia on the upper side of the spinal marrow. The 
brain also, in fishes, is little else than a similar series of pairs 
of ganglia on the upper side of the medullary cord; and as 
by this arrangement the individual portions of the brain are 
behind, and not below each olher, the form of the whole re¬ 
cedes much from the spherical shape of the perfectly deve¬ 
loped brain of man. The size of the brain also, is very in¬ 
considerable; and that with relation as well to the spinal 
marrow as to the whole body. 
As to the diversified conformation of the brain in fishes, 
we shall be best able to trace it by means of the variations 
which the separate masses composing it undergo in different 
genera. The cerebral mass from whence the olfactory nerves 
proceed, Carus distinguishes as the first cerebral mass. In 
the osseous fishes it is very inferior in point of structure, and 
frequently also in size, to the other portions of the brain. In 
the eel genus it presents three or four pair of ganglia, which 
successively diminish in size as they advance forwards, and of 
which the posterior and largest pair are connected, here as in 
other instances, by a small commissure. The olfactory nerves 
are usually slender, except in the conger-eel, (murcena 
conger ) in which each divides into two tolerably thick 
branches. In other osseous fishes, this portion of the brain 
is composed of one or more pairs of ganglia. The latter 
consists almost entirely of grey substance, and have no cavity 
within them. In the cartilaginous fishes, we observe changes 
in the first cerebral mass, by which means it approximates to 
the form of the hemispheres of the mammalia. The middle 
cerebral mass is distinguished in the osseous fishes by the 
perfection of its internal structure, and by the greater quantity 
of fibrous matter contained within it: it consists on the dorsal 
surface of the brain of a pair of ganglia, which are frequently 
nearly consolidated into one. Within this is contained a 
spacious cavity, into which again some other ganglia are 
found projecting. These parts collectively should be viewed 
as true optic tubercles, i. e. as identical with the corpora 
quadrigemina in man. From the covering of the cavity of 
these optic tubercles, arises on each side the broad, ribbon- 
like, optic nerve, which is generally evidently disposed in 
numerous folds. With a few, as in the soles and cod, the 
right optic nerve passes to the left side, and the left over it 
to the right, without however forming a perfect decussation, 
although they are connected by a commissure at their origin. 
From the middle cerebral mass also arise the accessory cords 
of the optic nerve, viz. the third pair from the great internal 
ganglion of the cavity of the optic tubercle; the fourth pair 
from the medullary lamella (valve of the brain) connecting 
the optic tubercle with the third cerebral mass; and the sixth 
pair from the medulla oblongata, immediately below the 
fourth, as the third is below the second pair. The identity of 
this middle cerebral mass with the corpora quadrigemina, is 
fully proved by a reference to the progress of formation of 
the same parts in the foetus of man and other mammalia. 
The inferior surface of the middle cerebral mass presents 
some elevations of gray substance, corresponding precisely to 
the gray substance about the infundibulum in the human 
brain. The conformation of the middle cerebral mass is re¬ 
markably distinct in the thoracici and abdominales. In the 
eel 
