THE NERVOUS SYSTEM
=The nerves of the Fish.=--The nervous system in the fish, as in the
higher vertebrates, consists of brain and spinal cord with sensory,
or afferent, and motor, or efferent, nerves. As in other vertebrates,
the nerve substance is divided into gray matter and white matter, or
nerve-cells and nerve-fibres. In the fish, however, the whole nervous
system is relatively small, and the gray matter less developed than in
the higher forms. According to Günther the brain in the pike (_Esox_)
forms but 1/1305 part of the weight of the body; in the burbot (_Lota_)
about 1/720 part.
The cranium in fishes is relatively small, but the brain does not
nearly fill its cavity, the space between the dura mater, which lines
the skull-cavity, and the arachnoid membrane, which envelops the brain,
being filled with a soft fluid containing a quantity of fat.
=The Brain of the Fish.=--It is most convenient to examine the
fish-brain, first in its higher stages of development, as seen in the
sunfish, striped bass, or perch. As seen from above the brain of a
typical fish seems to consist of five lobes, four of them in pairs, the
fifth posterior to these and placed on the median line. The posterior
lobe is the _cerebellum_, or _metencephalon_, and it rests on the
_medulla oblongata_, the posterior portion of the brain, which is
directly continuous with the spinal cord.
In front of the cerebellum lies the largest pair of lobes, each of
them hollow, the optic nerves being attached to the lower surface.
These are known as the _optic lobes_, or _mesencephalon_. In front of
these lie the two lobes of the cerebrum, also called the hemispheres,
or _prosencephalon_. These lobes are usually smaller than the optic
lobes and solid. In some fishes they are crossed by a furrow, but are
never corrugated as in the brain of the higher animals. In front of
the cerebrum lie the two small olfactory lobes, which receive the large
olfactory nerve from the nostrils. From its lower surface is suspended
the hypophysis or pituitary gland.
[Illustration: FIG. 78.--Brain of a Shark (_Squatina squatina L._).
(After Dean.)
I. First cranial nerve (olfactory).
P. Prosencephalon (cerebrum).
E. Epiphysis.
T. Thalamencephalon.
II. Second cranial nerve.
IV. Fourth cranial nerve.
V. Fifth cranial nerve.
VII. Seventh cranial nerve.
V4. Fourth ventricle.
M. Mesencephalon (optic lobes).
MT. Metencephalon (medulla).
EP. Epencephalon (cerebellum).]
[Illustration: FIG. 79.--Brain of _Chimæra monstrosa_. (After Wilder
per Dean.)]
[Illustration: FIG. 80.--Brain of _Protopterus annectens_. (After
Burckhardt per Dean.)]
In most of the bony fishes the structure of the brain does not differ
materially from that seen in the perch. In the sturgeon, however,
the parts are more widely separated. In the Dipnoans the cerebral
hemispheres are united, while the optic lobe and cerebellum are very
small. In the sharks and rays the large cerebral hemispheres are
usually coalescent into one, and the olfactory nerves dilate into
large ganglia below the nostrils. The optic lobes are smaller than
the hemispheres and also coalescent. The cerebellum is very large,
and the surface of the medulla oblongata is more or less modified or
specialized. The brain of the shark is relatively more highly developed
than that of the bony fishes, although in most other regards the latter
are more distinctly specialized.
=The Pineal Organ.=--Besides the structures noted in other fishes the
epiphysis, or pineal organ, is largely developed in sharks, and traces
of it are found in most or all of the higher vertebrates. In some of
the lizards this epiphysis is largely developed, bearing at its tip a
rudimentary eye. This leaves no doubt that in these forms it has an
optic function. For this reason the structure wherever found has been
regarded as a rudimentary eye, and the "pineal eye" has been called the
"unpaired median eye of chordate" animals.
[Illustration: FIG. 81.--Brain of a Perch, _Perca flavescens_. (After
Dean.)
R. Olfactory lobe.
P. Cerebrum (prosencephalon).
E. Epiphysis.
M. Optic lobes (mesencephalon).
EP. Cerebellum (epencephalon).
ML. Medulla oblongata (metencephalon).
I. First cranial nerve.
II. Second cranial nerve.
IV. Fourth cranial nerve.
V. Fifth cranial nerve.
VII. Seventh cranial nerve.
VIII. Eighth cranial nerve.
IX. Ninth cranial nerve.
X. Tenth cranial nerve.]
[Illustration: FIG. 82.--_Petromyzon marinus unicolor_ (Dekay). Head of
Lake Lamprey, showing pineal body. (After Gage.)]
It has been supposed that this eye, once possessed by all vertebrate
forms, has been gradually lost with the better development of the
paired eyes, being best preserved in reptiles as "an outcome of the
life-habit which concealed the animal in sand or mud, and allowed the
forehead surface alone to protrude, the median eye thus preserving its
ancestral value in enabling the animal to look directly upward and
backward." This theory receives no support from the structures seen in
the fishes.
In none of the fishes is the epiphysis more than a nervous enlargement,
and neither in fishes nor in amphibia is there the slightest suggestion
of its connection with vision. It seems probable, as suggested by
Hertwig and maintained by Dean that the original function of the pineal
body was a nervous one and that its connection with or development into
a median eye in lizards was a modification of a secondary character.
On consideration of the evidence, Dr. Dean concludes that "the pineal
structures of the true fishes do not tend to confirm the theory that
the epiphysis of the ancestral vertebrates was connected with a median
unpaired eye. It would appear, on the other hand, that both in their
recent and fossil forms the epiphysis was connected in its median
opening with the innervation of the sensory canals of the head. This
view seems essentially confirmed by ontogeny. The fact that three
successive pairs of epiphyseal outgrowths have been noted in the roof
of the thalamencephalon[6] appears distinctly adverse to the theory of
a median eye."[7]
=The Brain of Primitive Fishes.=--The brain of the hagfish differs
widely from that of the higher fishes, and the homologies of the
different parts are still uncertain. The different ganglia are all
solid and are placed in pairs. It is thought that the cerebellum is
wanting in these fishes, or represented by a narrow commissure (_corpus
restiforme_) across the front of the medulla. In the lamprey the brain
is more like that of the ordinary fish.
In the lancelet there is no trace of brain, the band-like spinal cord
tapering toward either end.
=The Spinal Cord.=--The spinal cord extends from the brain to the tail,
passing through the neural arches of the different vertebræ when these
are developed. In the higher fishes it is cylindrical and inelastic.
In a few fishes (headfish, trunkfish) in which the posterior part of
the body is shortened or degenerate, the spinal cord is much shortened,
and replaced behind by a structure called cauda equina. In the headfish
it has shrunk into "a short and conical appendage to the brain." In the
Cyclostomes and chimæra the spinal cord is elastic and more or less
flattened or band-like, at least posteriorly.
=The Nerves.=--The nerves of the fish correspond in general in place
and function with those of the higher animals. They are, however,
fewer in number, both large nerve-trunks and smaller nerves being less
developed than in higher forms.
The _olfactory nerves_, or first pair, extend through the ethmoid bone
to the nasal cavity, which is typically a blind sac with two roundish
openings, but is subject to many variations. The _optic nerves_, or
second pair, extend from the eye to the base of the optic lobes. In
Cyclostomes these nerves run from each eye to the lobe of its own side.
In the bony fishes, or Teleostei, each runs from the eye to the lobe of
the opposite side. In the sharks, rays, chimæras, and Ganoids the two
optic nerves are joined in a chiasma as in the higher vertebrates.
Other nerves arising in the brain are the third pair, or _nervus
oculorum motorius_, and the fourth pair, _nervus trochlearis_, both
of which supply the muscles of the eye. The fifth pair, _nervus
trigeminus_, and the seventh pair, _nervus facialis_, arise from the
medulla oblongata and are very close together. Their various branches,
sensory and motor, ramify among the muscles and sensory areas of the
head. The sixth pair, _nervus abducens_, passes also to muscles of the
eye, and in sharks to the nictitating membrane or third eyelid.
The eighth pair, _nervus acousticus_, leads to the ear. The ninth
pair, _glosso-pharyngeal_, passes to the tongue and pharynx, and forms
a ganglion connected with the sympathetic system. The tenth pair,
_nervus vagus_, or pneumogastric nerve, arises from strong roots in
the corpus restiforme and the lower part of the medulla oblongata. Its
nerves, motor and sensory, reach the muscles of the gill-cavity, heart,
stomach, and air-bladder, as well as the muscular system and the skin.
In fishes covered with bony plates the skin may be nearly or quite
without sensory nerves. The eleventh pair, _nervus accessorius_, and
twelfth pair, _nervus hypoglossus_, are wanting in fishes.
The spinal nerves are subject to some special modifications, but in
the main correspond to similar structures in higher vertebrates. The
anterior root of each nerve is without ganglionic enlargement and
contains only motor elements. The posterior or dorsal root is sensory
only and widens into a ganglionic swelling near the base.
A sympathetic system corresponding to that in the higher vertebrates is
found in all the Teleostei, or bony fishes, and in the body of sharks
and rays in which it is not extended to the head.
FOOTNOTES:
[6] The thalamencephalon or the interbrain is a name given to the
region of the optic thalami, between the bases of the optic lobes and
cerebrum.
[7] Fishes Recent and Fossil, p. 55.