superficial superficial
ophthalmic ophthalmic
deep ophthalmic buccal
maxillary otic
mandibular hyomandibular
The _superficial ophthalmic_ trunks of the two unite in a single nerve
which passes along the inner wall of the orbit above the muscles of the
eye to a foramen in the antero-medial angle of the orbit, through which
it passes to the dorsal surface of the snout. The superficial ophthalmic
nerve of Squalus is composed almost entirely of fibres of the facial
nerve. The superficial ophthalmic trunk of the trigeminal gives rise to
several small nerves leaving the common trunk near its origin and
passing to the skin above the eye. The superficial ophthalmic trunk of
the facial, nearly the whole of the common nerve, branches profusely to
supply the sensory organs of the dorsal and lateral surfaces of the
snout.
The superficial ophthalmic of Eugaleus rises from the dorsal part of the
trigemino-facial root and leaves the cranium by a separate foramen above
and anterior to the roots of the recti muscles.
Directly under the origin of the superficial ophthalmic will be found a
comparatively slender nerve, which passes between the superior and
posterior rectus muscles, and forward along the medial surface of the
eyeball; it penetrates the anterior wall of the orbit by a separate
foramen, and emerges under the superficial ophthalmic. It is distributed
to the skin of the dorsal and lateral surfaces of the snout. This is the
_deep ophthalmic_ (_ophthalmicus profundus_) of the trigeminal nerve. A
slender branch (_posterior ciliary nerve_) passes from the deep
ophthalmic near its origin to the posterior surface of the eyeball.
Farther forward the same trunk gives off an _anterior ciliary nerve_ to
the anterior part of the eyeball.
A large nerve which crosses the floor of the orbit, beneath the eyeball,
consists of the _maxillary_ trunk of the trigeminal and the _buccal_
trunk of the facial nerve. These remain associated, even into the small
branches. Near the anterior margin of the orbit the maxillary-buccal
trunk divides into three parts; the smallest and outer one passes to the
surface lateral and anterior to the eye. The other two dip downward and
pass in front of the jaw to the ventral surface of the snout. Reflect
the skin of the ventral surface of the snout, and by dissection expose
these nerves as they emerge from the orbit. The larger branch runs
forward close to the median line of the snout, giving off numerous
twigs; the other, which appears to be a pure trigeminal branch, is
distributed near the angle of the mouth. The fibres of the maxillary
trunk supply the skin, while those of the buccal go to the canal organs
and ampullae of Lorenzini.
The _mandibular_ trunk of the trigeminal nerve arises beneath and behind
the maxillary. It passes outward in front of the levator maxillae
superioris muscle, sending a few twigs into this muscle, and turns
downward over the palato-quadrate cartilage. It divides here, one part
entering the adductor mandibularis muscle, the other passing downward
along the edge of the mandible, innervating the skin of the lower jaw
and the first ventral superficial constrictor muscle.
The mandibular and maxillary-buccal trunks of Eugaleus are united until
they approach the edge of the orbit, and the palatine branch is much
larger; otherwise the trigemino-facial branches are much as in Squalus.
The _hyomandibular_ trunk of the facial nerve can be found just beneath
the skin behind and close to the spiracle. From here it can be followed
back to the brain. It arises from the ventral part of the
trigemino-facial root, emerging from the cranium through the
hyomandibular canal. It divides into a number of branches just beyond
the spiracle:
1. The _external mandibular_ branch consists of two portions, a small
anterior nerve extending antero-ventrally to the canals above and below
the angle of the mouth, and a larger nerve which passes laterally and
suddenly breaks up into a brush of twigs which innervate the hyoidean
group of ampullae.
2. The _internal mandibular_ branch arises at about the same level as
the external mandibular, but under it, passes inward around the edge of
the hyoid cartilage, under the adductor mandibularis muscle, and then
forward along the mandibular cartilage.
3. The _hyoid_ branch separates from the hyomandibular trunk at about
the same level as the preceding nerves, and then passes, deep in the
tissues, around the angle of the jaw to the ventral side where it is
distributed to the superficial constrictor muscles. Several nerves pass
from the hyomandibular trunk and the hyoid branch to the dorsal
superficial constrictors.
4. The _palatine_ branch springs from the base of the hyomandibular
trunk inside the hyomandibular canal. It passes outward and forward,
dividing into numerous branches which innervate the mucous membranes of
the mouth. It can be traced completely later.
One or more small nerves proceeding to the pseudobranch and anterior
wall of the spiracle arise near the point of origin of the palatine
branch.
The _otic nerve_, passing from the root of the facial nerve to the
postorbital canal, is not likely to be found in this dissection.
Observe the enlargement near the base of the hyomandibular trunk, and
within the cartilaginous canal, the _geniculate ganglion_. The
_gasserian ganglion_, a component of the trigeminal nerve, lies in the
ventral portion of the trigeminal-facial root, and can now be located.
The _auditory nerve_ arises close behind the ventral division of the
trigemino-facial root. The root of the auditory nerve encloses a large
_auditory ganglion_. A _vestibular nerve_ arises from the anterior end
of the auditory ganglion and passes into the ear capsule, innervating
the upper part of the utriculo-sacculus and the ampullae of the anterior
and horizontal canals. From the posterior part of the ganglion nerves
pass to the ventral part of the sacculus and the ampulla of the
posterior canal. Trace these branches as thoroughly as possible.
The _abducens nerve_ emerges from the cranium under and close to the
origin of the posterior rectus muscle, into which muscle it enters. To
expose this nerve the trigeminal, facial and auditory nerves must be
lifted and cut as they pass through the wall of the cranium. It can be
traced obliquely backward and inward, through a long canal, to its
origin near the midline of the ventral surface of the medulla.
The _glossopharyngeal nerve_ passes through the base of the ear capsule
from the side of the medulla to the upper end of the first branchial
pouch. A ganglionic enlargement is found near where it emerges from the
cartilage. Outside the cranium the glossopharyngeal divides into a
_pretrematic branch_, passing down in front of the first gill pouch, and
a _posttrematic branch_ running behind the pouch. The pretrematic branch
quickly sends off a _pharyngeal nerve_ which runs antero-ventrally to
the roof of the pharynx. The pretrematic and posttrematic branches can
be followed along the gill arch to the ventral side of the pharynx. A
fourth branch of the glossopharyngeal, the _supratemporal_, springs from
the dorsal side of the ganglion; passing through the ear capsule it runs
to the dorsal surface of the head, where it is distributed to the sense
organs of a short section of the lateral line canal. This small nerve
can be demonstrated by carefully separating the muscles and
perichondrium from the posterior surface of the auditory capsule.
The _vagus nerve_ (or _pneumogastric_) arises by an extensive series of
roots from the side of the medulla. An easily distinguished ribbon-like
portion of the root, the _lateral line root_, runs forward as far as the
root of the glossopharyngeal. Note the canal by which the vagus leaves
the cranium, and trace the nerve along the inner side of the anterior
cardinal vein.
The principal branches of the vagus are:
1. The _supratemporal_ branch, a small nerve running dorsal through the
posterior part of the ear capsule to the lateral line canal and other
sense organs of the head. It will be found near the supratemporal branch
of the glossopharyngeal.
2. The _lateral line branch_, a large nerve which separates from the
trunk of the vagus just outside the cranium and runs backward through
the muscles, parallel to the vertebral column on a level with the
lateral line. It sends off numerous small twigs to the sense organs of
the lateral line canal.
3. Four _branchial_ nerves, which can be seen through the floor of the
anterior cardinal vein, leave the outer side of the vagus trunk. Each
divides into a pretrematic and posttrematic branch; a pharyngeal branch,
the last of which is the largest, arises from each posttrematic.
4. Beyond the branchial nerves the remainder of the vagus passes
backward as the _intestinal_ or _visceral_ trunk, to the end of the
pharynx, where it divides into a number of branches which are
distributed chiefly to the wall of the stomach. Near the point of this
last division the vagus is crossed by the hypobranchial nerve, which
should be noted and preserved.
The _occipital nerve_ penetrates the lateral wall of the cranium close
behind the root of the vagus and enters the canal of the vagus, along
which it passes. On emerging, it sends small branches to the nearby
muscle, while the principal portion runs on to join the hypobranchial
nerve. The occipital nerve will be found to arise from the ventral
surface of the medulla, below and behind the root of the vagus, by two
or more distinctly separated roots, which may represent distinct nerves.
SPINAL NERVES. The spinal nerves are those nerves which arise from the
sides of the spinal cord. They differ from the cranial nerves not only
in their origin outside the cranium, but also in that each spinal nerve
arises by two roots which spring from the spinal cord near the dorsal
and ventral surfaces. Each root passes through a foramen in the
cartilaginous wall of the neural canal, the ventral a little anterior to
the dorsal, after which they unite to form the spinal nerve. Between the
junction of the roots and its foramen the dorsal root contains a mass of
ganglion cells, which cause an enlargement known as the _dorsal root
ganglion_. The typical course of a spinal nerve is around the body to
the ventral surface, giving off branches to the muscles and skin of its
segment. A short distance from the vertebral column the spinal nerves
lie just outside the peritoneum, through which many of them can be seen
and followed to about the level of the lateral vein. At this point they
pass outward into the muscles of the body wall. To dissect any of the
spinal nerves make a longitudinal incision along the lateral line and
separate the dorsal muscle mass from the lateral muscles for some
distance. The dorsal muscles can then be pressed toward the vertebral
column and dissected away from the peritoneum. The spinal nerves, lying
against the peritoneum, will be exposed and can be followed easily,
first to their roots, next ventrally.
The _hypobranchial nerve_, to which attention was called at the point
where it crosses the vagus, is formed by the union of the principal
branches of the occipital and first two spinal nerves. The third spinal
nerve receives a branch from the second, and itself accompanies the
hypobranchial nerve closely without actually becoming a part of it. The
union of nerves thus formed is known as a plexus. After crossing the
vagus the hypobranchial nerve forks, one division passing medial to, the
other lateral to the anterior cardinal vein; both run ventrally,
following the last gill arch, and reunite on the lateral wall of the
pericardium, forming a trunk which runs forward. At the anterior end of
the pericardium this divides into a dorsal and a ventral branch which
innervate the surrounding muscles. The hypobranchial nerve innervates
the skin of the region immediately in front of the pectoral girdle, and
the coraco-arcualis communis, coraco-mandibularis, coracohyoideus, and
coraco-branchialis muscles.
The third, fourth, fifth and sixth spinal nerves pass backward and
ventrad till they reach the level of the articulation of the pectoral
fin with the girdle. Here they join to form a simple _brachial plexus_,
from which arise branches proceeding to the musculature of the dorsal
and ventral faces of the fin. The seventh to eleventh spinal nerves pass
downward to the level of the fin, and then branch, one portion entering
the muscles of the ventral body wall, while the other passes into the
depressor muscles of the fin.
The pelvic fin is innervated by eight or nine spinal nerves which pass
backward and downward along the medial edge of the septum between the
myomeres, entering the dorsal side of the fin along its axis. No plexus
is formed.
OLFACTORY ORGAN (NASAL SAC). Dissect away the skin and other tissues
around the nostril so as to expose completely the olfactory organ; this
will be found to be a dark-colored, nearly spherical mass, of half the
diameter of the eye, firmly attached at its base. By cutting away the
cartilage dorsal to the nasal sac its base will be exposed, and the
olfactory bulb will be shown to be closely adherent to a considerable
part of the postero-dorsal surface of the organ. Numerous short nerves
can be demonstrated to pass from the olfactory bulb into the olfactory
organ; all these nerves together are considered as the first cranial or
olfactory nerve. Remove the olfactory organ from the head; divide it by
a median longitudinal cut; observe the arrangement and structure of its
double series of internal folds (lamellae), and the complete median
septum.
VENTRAL SURFACE OF THE BRAIN. Cut the cord in two some distance back of
the brain. Cut all cranial nerves just inside the cranium and carefully
lift the brain out. Parts of the ventral portion of the brain lie in a
recess beneath the mesencephalon and must be disengaged very gently.
Identify and examine the ventral parts of the brain. Note the
considerable lateral compression of the mesencephalon. The optic nerves
cross beneath the diencephalon, forming the _optic chiasma_. From the
sides of the chiasma slightly elevated _optic tracts_, formed by the
fibres of the optic nerves, can be traced into the optic lobes.
Back of the optic chiasma the projecting ventral portion of the
diencephalon forms the _hypothalamus_. The posterior lobe of this
structure is the _hypophysis_ or _pituitary body_.
The oculo-motor nerves emerge over the posterior end of the
hypothalamus.
The ventral portion of the mesencephalon is formed by the _cerebral
peduncles_ (_crura cerebri_), columns of fibres passing between the
myelencephalon and telencephalon.
The abducens nerves arise on the ventral surface of the myelencephalon
near the midline and just back of a line connecting the roots of the
auditory nerves.
The _internal carotid arteries_ reach the brain at the sides of the
hypothalamus. Branches are sent upward and forward over the surface of
the brain. Anastomoses between the vessels of the opposite sides are
formed anterior to the optic chiasma. The main branches of the carotids
pass backward along the sides of the hypothalamus and unite behind this
organ. The median artery thus formed runs along the ventral surface of
the myelencephalon and the spinal cord. Numerous transverse vessels are
given off to the myelencephalon.
Identify the roots of the remaining cranial nerves.
CAVITIES OF THE BRAIN. Divide the brain into exactly equal halves by a
vertical longitudinal cut.
Each lobe of the prosencephalon contains a large cavity. These are the
_prosocoels_. They are commonly known either as the lateral ventricles,
or the left cavity as the first ventricle and the right as the second
ventricle. The prosocoels are continued into the olfactory lobes, these
portions being known as rhinocoels.
The _thalamocoel_ is the cavity within the diencephalon, often called
the third ventricle. The prosocoels communicate with the thalamocoel by
lateral openings, the _foramina of Monro_. The roof of the thalamocoel
is very thin and is non-nervous; it is frequently torn during the early
dissection. Where the lobes of the prosencephalon meet the dorsal wall
of the diencephalon this thin roof is pushed into the prosocoels,
carrying with it the pia mater and its blood vessels, and thus forms
vascular ingrowths known as the _choroid plexi_. The thalamocoel
continues above into the epiphysis and below into the hypothalamus.
The _myelocoel_ is the large cavity of the myelencephalon. It also is
frequently apparently open to the exterior at the posterior end by the
accidental breaking of the thin, non-nervous dorsal wall of this region.
The myelocoel is also known as the fourth ventricle.
The thalamocoel and myelocoel are connected by a narrow passage through
the mesencephalon, the _aqueduct of Sylvius_ (_iter, mesocoel_).
The optocoels are large cavities within the optic lobes which open into
the aqueduct of Sylvius.
A large metacoel in the metencephalon opens into the myelocoel. The
myelocoel is also continued into the corpora restiforma; posteriorly it
joins the _central canal_ which extends down the center of the spinal
cord.
DISSECTION OF THE EYE. Remove one of the eyes from its orbit, and divide
it into inner and outer halves by an equatorial cut around the eyeball
(not directly through it, as this tears the lens from its fastenings).
Place the halves under water and observe:
In the inner half:
The _posterior chamber_, the cavity of the eyeball which has been
opened. During life it is filled by a gelatinous substance, the
_vitreous humor_.
The _retina_, a delicate yellowish-white membrane lining the interior of
the eye, loosely attached to the outer coats except at the point of
entrance of the optic nerve.
The _choroid coat_, a thin, black membrane outside the retina. It can be
pulled away from the outer coat quite easily except near the optic
nerve.
The _sclerotic coat_, the outer coat of the eye. This is composed of
connective tissue having an almost cartilaginous consistency, is only
slightly pigmented, and is somewhat translucent. The muscles of the eye
are inserted upon the sclerotic.
In the outer half:
The _ora serrata_, an irregular line along which the retina ends.
The _iris_, a fold of the choroid extending inward like a shelf, and
perforated centrally to form the pupil. Around the iris the choroid is
folded radially into the _ciliary processes_.
The _lens_, a spherical body, transparent and elastic during life, but
opaque and hard in preserved specimens. It projects into the pupil and
is suspended from the ciliary processes by a delicate membrane, the
_suspensory ligament_.
The _anterior chamber_, in front of the iris and lens, filled with a
watery fluid, the _aqueous humor_.
The transparent _cornea_, forming the outer side of the eyeball,
continuous with the sclerotic.
Take out the other eye and cut it in two by a section through the pupil
and optic nerve. Review the relation of the parts.