Only the head and anterior part of the trunk will be required for the
dissection of the nervous system. Cut across the body back of the
pectoral fins; the posterior part of the body will not be required
further unless it is desired to study the muscles and skeleton.
The manner of dissecting the brain depends somewhat upon the specimens
at the disposal of the student. If a large head is to be used especially
for the dissection of the cranial nerves, only the brain, eye and ear
need be studied in the present specimen. But in most cases it will be
found best for the student to dissect the first dogfish as thoroughly as
possible, working out the cranial nerves as well as the brain, and
reserving the second head for a thorough review of the entire nervous
system. Chapter III of Herrick and Crosby’s “Laboratory Outline of
Neurology” should be used in connection with such a review.
If a line be drawn over the dorsal surface of the head connecting the
two spiracles, two small pores will be found near the middle. These are
the external apertures of the ducti lymphatici. Cut carefully through
the skin in a small circle around the pores, and remove the skin from
the remainder of the dorsal surface of the skull without disturbing the
small section containing the pores. The latter part should now be lifted
gently; beneath it will be seen two delicate tubes passing from the
pores to apertures in a depression of the skull below them. These tubes
are the _ducti endolymphatici_, through which a passage exists between
the internal ear and the exterior. As they cannot be preserved in the
subsequent dissection, the pores by which they pass through the skull to
the internal ear should be found now, and a memorandum-sketch made of
the ducts themselves.
DORSAL SURFACE OF THE BRAIN. The roof of the skull should be removed
from over the brain. Use a sharp scalpel and take very thin slices of
cartilage. Do not cut beyond the brain at the sides. No attempt should
be made at this time to expose more than the dorsal surface of the
brain.
Above the anterior end of the brain there is a small median foramen
through the skull, the _epiphysial foramen_. A strand of tissue, the
_epiphysis_, leading from this to the surface of the brain, should be
carefully observed and retained. The cartilage should also be cut away
from above the portion of the spinal cord next the skull. Gently wash
away any coagulated lymph.
The brain and spinal cord are invested by two membranes (_meninges_).
The tough _dura mater_ lines the cavity in which they lie, clinging
closely to the cartilage; in fact it forms the perichondrium of the
internal surface of the cranium. The _pia mater_ envelops closely the
brain and cord, and contains numerous blood vessels. Between the two is
the _arachnoid space_, traversed by occasional fine threads of
connective tissue and filled with lymph.
As the spinal cord passes forward into the skull it enlarges and merges
with the posterior portion of the brain, the _medulla oblongata_
(_myelencephalon_). The roof of the medulla is extremely thin, and is
broken if the cartilage has not been removed with extreme care, exposing
a cavity within, the _fourth ventricle_.
In front of the medulla, and overlapping its anterior extremity, is a
large oval organ, the _cerebellum_ (_metencephalon_). Ventral to the
cerebellum, each side of the medulla is expanded in an ear-shaped lobe,
the _corpus restiformis_. Anteriorly, the cerebellum overlaps a pair of
rounded lobes, the _optic lobes_, which together form the dorsal portion
of the _midbrain_ (_mesencephalon_).
In front of the optic lobes are two slightly larger lobes united in
their posterior portions but separated anteriorly, the _cerebral lobes_
or _hemispheres_. Together they constitute the _prosencephalon_. (The
prosencephalon is not divided in Eugaleus.) Between the mesencephalon
and the prosencephalon is a depressed region belonging to the
brain-stem, the _diencephalon_ (_thalamencephalon_), from which the
epiphysis arises. The roof of the diencephalon also is very thin and is
frequently broken during the exposure of the brain. The cavity seen
within the diencephalon is the _third ventricle_.
Stalked bodies arising from the antero-lateral angles of the cerebral
hemispheres are the _olfactory lobes_. The portion of the brain
including the cerebral hemispheres and the olfactory lobes constitutes
the _telencephalon_.
DISSECTION OF THE INTERNAL EAR. The structures composing this organ
lie in the projecting cartilage at the side of the medulla (auditory
capsule). Remove the cartilage of the auditory capsule in thin slices
and bit by bit, following the ductus endolymphaticus to the
membraneous labyrinth. Dissect away the surrounding cartilage leaving
the membraneous canals in place, until the entire labyrinth is
exposed. The _membraneous labyrinth_ consists of a large central sac
(_utriculo-saccular chamber_) into which the endolymphatic duct opens,
and three membraneous tubes (_semicircular canals_) external to the
chamber but communicating with it in various ways. Two, one anterior
and the other posterior to the sacculus, lie in a nearly vertical
plane (_anterior and posterior semicircular canals_); one is external
to the sacculus and lies in a nearly horizontal plane (_horizontal or
external semicircular canal_). At the ventral ends of the vertical
canals are nearly spherical enlargements called _ampullae_. The
ampulla of the horizontal canal is at its anterior end. The dorsal
ends of the vertical canals open near each other into the upper part
of the utriculo-sacculus. The ventral extremity of the anterior
vertical canal and the anterior extremity of the horizontal canal open
beside each other into an anterior projection of the sacculus. The
ventral extremity of the posterior vertical canal opens into the
posterior and lower part of the sacculus. The posterior extremity of
the horizontal canal opens into the posterior side of the sacculus.
During life the utriculo-sacculus and the semicircular canals are
filled with a lymphatic fluid, and the sacculus contains a large
calcareous ear-stone (_otolith_), which is usually dissolved by the
formalin used in preserving the dogfish.
Whitish patches of thickened sensory epithelium may be seen in the
ampullae (_cristate acusticae_) and in the utriculo-saccular chamber
(_maculae acusticae_). Branches of the eighth nerve can be followed to
all these areas.
A projection of the ventral wall of the utriculo-sacculus is the
_lagena_, the rudiment from which the cochlea of higher animals
developed. It also contains a macula acustica.
EXTERNAL FEATURES OF THE EYE. Observe the transparent cornea covering
the external surface of the eye; the dark ring of the iris; the central
opening in the iris, the pupil; the conjunctival sac surrounding the
external half of the eyeball. Cut away sufficient of the upper wall of
the cartilaginous orbit to expose the eyeball and its muscles. Note the
considerable amount of soft connective tissue around the eye and explore
the orbital sinus (p. 13). Take notice of the following nerves, in order
to ensure their preservation until the time comes to trace them more
completely. A large nerve crossing the medial side of the orbit, the
superficial ophthalmic; a nerve leaving the cranium opposite the optic
lobe, passing under the superficial ophthalmic to the anterior muscle of
the eyeball, the trochlear; several long ciliary nerves passing to the
eyeball; several other nerves visible in the deep angle of the orbit.
Six muscles move the eye. Four of these arise close together at the deep
postero-medial angle of the orbit. Diverging, they are inserted upon
four sides of the eyeball, and from the position of their insertions are
named the _superior_, _posterior_, _inferior_, and _anterior recti_. Two
muscles arise from the antero-medial angle of the orbit, the _superior
and inferior oblique muscles_.
Between the recti muscles can be seen a mushroom-shaped stalk of
cartilage, the _ophthalmic peduncle_; the eyeball rests against its
expanded end. (There is no peduncle in Eugaleus.)
THE CRANIAL NERVES. The cranial nerves are twelve pairs of nerves
arising from the brain, and thus distinguished from the spinal nerves
which arise from the sides of the spinal cord. They are distributed
chiefly to the head and neck, though branches of the vagus nerve go to
the viscera and to the sense organs of the lateral line. Since the
nerves are all paired, the distribution of both nerves of a pair being
alike, the descriptions will mention but one nerve of a pair. As the
cranial nerves are traced dissect away the sides of the cranium down to
the foramina penetrated by the nerves, and follow each nerve from its
origin on the brain to the parts innervated by it. Features of the
dissection which are not found in tracing the nerve of one side should
be sought on the other side.
The _olfactory nerve_. The anterior surface of the olfactory lobe fills
a large foramen in the anterior wall of the cranium and is pressed
closely against the posterior surface of the nasal sac. Numerous small
nerves, collectively forming the olfactory nerve, arise from the
anterior face of the lobe, penetrate the membraneous wall of the
olfactory organ, and are distributed to its highly folded surface.
The _terminal nerve, Nervus terminalis_, is a slender nerve running
along the medial surface of the stalk of the olfactory lobe. Follow it
backward to its origin on the anterior surface of the cerebral
hemisphere, deep in the median fissure (in Eugaleus on the ventral
surface). Trace it forward over the dorsal surface of the olfactory lobe
to where it enters the nasal sac. The terminal nerve is a true cranial
nerve which has escaped notice until recent years. It is associated with
the olfactory nerve in vertebrates generally from fishes to men. The
fibres of the terminal nerve remain distinct from those of the olfactory
nerve, both in the olfactory organ and in the brain. Its function is
unknown. There is still a division of opinion among authorities as to
whether the terminal nerve should be considered to be a distinct cranial
nerve, or a portion of the olfactory nerve.
The _optic nerve_ can be seen at the bottom of the orbit between the eye
and the skull, nearly under the superior oblique muscle. It arises from
the ventral side of the diencephalon, passes outward, penetrates the
orbit at its infero-medial angle, and continues directly outward to the
eyeball.
The _trochlear nerve_, or _patheticus_, penetrates the wall of the orbit
opposite the optic lobe. Follow it back to its origin from the dorsal
surface of the brain in the depression between the optic lobes and the
cerebellum. Then follow it from the skull to the superior oblique
muscle, which it innervates.
The _oculo-motor nerve_ arises from the ventral surface of the midbrain,
passes outward, and penetrates the orbit on a level with and just
anterior to the origins of the recti muscles. It divides immediately
into three parts; two pass to the anterior and superior recti
respectively, while the third passes downward along the posterior
surface of the eyeball to the inferior rectus and inferior oblique
muscles. In tracing this nerve the palatine process of the upper jaw
will be seen projecting from below into the orbit.
The _trigeminal_, _facial_, and _auditory nerves_ spring from the side
of the medulla below the corpus restiformis. The roots, and some of the
branches, of the trigeminal and facial nerves are so mingled as to be
indistinguishable except by special neurological technique. The common
root of the trigeminal and facial nerves shows a partial division into a
dorsal and a ventral portion; the dorsal portion belongs to the facial
nerve, while the ventral root is mixed. The root of the auditory nerve
lies close behind the trigeminal-facial root, but can be distinguished
fairly well. Both the trigeminal and facial nerves divide into several
trunks, namely: