They contain elements of both the portio major and the portio minor.
These two branches come from the lower part of the outer region of the
ganglion, and pass together through a cavity which is located between
the os petrosum and alæ and basis sphenoid, and then branch.
The ramus secundus, or _superior maxillary division_, of the trifacial,
is the second branch and passes into the orbit below the optic nerve and
the eyeball. It is called the recurrent infra-orbitale. It gives an
ascending branch to the gland of Harder, one to the conjunctiva, one to
the membrana nictitans, and one to the eyelids. It also gives a branch
to the skin below the eye and to the angle of the mouth. This latter
branch is called the _recurrent subcutaneous_. These two branches
communicate with the recurrent nasal ciliaris of the first trigeminus
branch. The second branch passes below the nasal opening, then passes on
and forms the alveolar nerve on the side between the gums. It sends
several recurrent posterior branches to the elevations on the back part
of the mouth. It extends forward to the point of the beak.
The _inferior maxillary division_ of the fifth is larger than the other
two. It is directed downward and outward then upward to the temporal
region. At the temporal region it divides into five parts. It gives a
branch to the temporalis muscle, one to the pterygoid muscle, and one to
the mylo-hyoideus. The main branch gives twigs to the parotid gland
region and enters into the dental canal of the lower jaw. Numerous
filaments break through the lower jaw bone, and then spread out on the
skin and rim of that bone. The largest branch, called the recurrent
maxillaris externa, comes out near the coronoid process. The rest of the
trunk extends to, and comes out of, several foramina at the anterior
point of the jaw bone.
These filaments are contained in grooves or cavities of the lower jaw
and terminate in touch buds.
The trifacialis fuses with the other cranial nerves and with the
sympathetic nervous system. Some of these fusions are as follows: The
recurrent ophthalmica fuses with the orbito-nasalis ganglion. Indirect
fusion of the recurrent maxillary division of this nerve takes place
near the Gasserian ganglion through the sympathetic nerves, the
temporo-lacrimalis, the facial nerves, the large cervical ganglion, and
indirectly with the glosso-pharyngeal and the vagus. This fusion has
been called the superior recurrent branch of the trigeminus, or
trifacialis.
There is a direct fusion of the superior recurrent maxillary division of
the fifth just before entering the upper jaw. This fusion is with the
spheno-palatine ganglion and the sympathetic carotidis cephalica, and
also with the large cervical ganglion.
=The Abducens= (Fig. 62, No. _C_, 13).—This nerve originates in the
somatic and motoric column along with the other nerves of the eye
muscles. Its nucleus lies in the circle of the pars commissuralis of the
cerebellum. The nerve then passes ventrally, as does the motoris oculi
and leaves the brain along the median line. This is a comparatively
large nerve and passes somewhat laterally and ventrally from the foramen
opticum through a canal in the sphenoid. It then enters the orbital
cavity. Some muscular twigs are given off to the quadratus and to the
pyramidalis muscle. This nerve also innervates the external rectus
muscle of the eye. The abducens anastomoses with the ramus ciliaris and
the ophthalmic nerve and gives fine nerve twigs to the ramus ciliaris
externus of the ciliary ganglion.
=The Facialis and the Acusticus.=—The facial nerve originates with the
auditory (Fig. 62, No. _C_, 14 and 15) in a very vaguely known manner,
from the cerebellum. It divides into three parts.
The first of these probably comes from the complex ganglion with the
posterior roots of the auditory. This part belongs to the somatic
sensory group of nerves. From this same group originates the auditory,
which spreads out into the cochlea and takes the impressions of sound.
This nerve is short and thick, and, at the point where it loses its
medullary covering on entering the cochlea, there is developed a
ganglion. This ganglion is similar to the spinal ganglia.
The second part is provided with one root which originates mesially and
ventrally, from the deeper ganglion cells. Some of the fibers from this
root constitute the vestibular branches. They accompany the auditory and
supply the anterior part of the ear labyrinth and the semicircular
canals. The larger part of the fibers of this trunk make up the
intermediate part of the facial. The geniculate ganglion is formed at
their fusion. The sympathetic spheno-palatine nerve emerges from this
ganglion, coming out of the aqueduct of Fallopius.
The third part, called the portio dura, is the main facialis. It is
located opposite the auditoria intermedia. Its roots may be traced to
the complex ganglion, from which they take a direction ventrally from
the median portion.
The facialis, after emerging from the aqueduct of Fallopius, takes a
curved course and partly fuses with the sympathetic temporo-lacrimalis
on the upper posterior wall of the ear drum. It is here accompanied by
the carotid and the cephalic arteries. It leaves the ear drum through an
opening in the quadrate bone, giving off a large branch to the
digastricus muscle and a small one to the stapedius of the columella
auris. The facialis trunk is quite large. It then passes downward along
the quadrate bone, where it receives a large branch from the
glosso-pharyngeus. This nerve also gives branches to the mylo-hyoideus
and the stylo-hyoideus muscle, crosses laterally the glosso-pharyngeus,
and finally fuses with branches of the subcutaneous and with the first,
the second, the third, and the fourth cervical nerves. After this fusion
the nerves innervate the skin of the anterior of the neck and the
constrictor colli muscle.
The facialis anastomoses as follows: with a fine branch of the
sympathetic temporo-lacrimalis, indirectly with the ramus trigeminus,
with the large cervical nerve ganglion and with the spheno-palatine
ganglion and nerve.
=The Vagus Group.=—The ninth, the tenth, and the eleventh cranial nerves
are by some anatomists called the vagus group, first so called by
Willis. They are made up of both sensory and motor nerves.
=The Glosso-pharyngeus.=—The roots of the glosso-pharyngeus (Fig. 62,
No. _C_, 16; Fig. 76, No. 13) emerge, along with those of the vagus,
from the medulla oblongata, and enter, as a short trunk, the foramen
jugulare et caroticum. Between these two nerves there is usually found a
thin portion of bone. The trunk of the glosso-pharyngeus forms a
ganglion in the foramen jugulare et caroticum where it receives
connecting branches from the near-by ganglion radicis and vagus nerve.
The glosso-pharyngeus passes out of the foramen jugulare et caroticum
above the large superior sympathetic nerve ganglion, with which it
communicates. The glosso-pharyngeus passes diagonally over the ramus
temporo-lacrimalis of the sympathetic system, and then receives a very
strong branch from the vagus (Fig. 76, No. 12). It sends a short branch
to the recurrent lacrimalis of the sympathetic system. At this point
there is formed a reddish-yellow ganglion, the petrosal ganglion, which
is similar to the petrosal ganglion of mammals. Frequently it is found
close below the large superior cervical nerve ganglion. The petrosal
ganglion is frequently connected by special fibers with the cervical
nerve ganglion, the large superior cervical nerve ganglion and the
ganglion radicis vagi. The glosso-pharyngeus is divided into the
following branches.
First, the recurrent pharyngeus which gives branches to the upper part
of the throat and which is tortuous in its course. It receives a branch
from the superior cervical nerve ganglion and gives off branches to the
salivary glands and papillæ of the posterior tongue region.
Second, the recurrent lingualis, which passes with the lingual artery
over the hyoid bone to the base of the tongue and to the papillæ.
Another branch passes below the tongue bone and supplies the tongue and
the pharynx. The glosso-pharyngeal is a mixed nerve.
[Illustration:
FIG. 76.—Photograph of spinal cord of a hen. 1, Cerebrum. 2, The
olfactory nerve terminating at 3, the turbinated bones. 4, The
superior maxillary division of the fifth nerve dividing. 5, The eye
with the upper part laid open. 6, The cervical segment. 7, The
lumbo-sacral segment. 8, The lungs. 9, The kidneys lying above the
peritoneum. 10, The terminal filament of the spinal cord. 11, The
hypoglossal nerve. 12, The vagus nerve. 13, The glosso-pharyngeal
nerve. 14, The branch from the vagus.
]
=The Vagus.=—The vagus, or pneumogastricus (Fig. 18, No. 5; Fig. 62, No.
_C_, 17; Fig. 76, No. 12), a mixed nerve, forms a ganglion in the
foramen through which it passes. The foramen lies between the os
petrosum and os occipitale, close to the foramen jugulare, above and to
the inside of it.
The ganglionic root of the vagus communicates with the superior cervical
nerve ganglion. The vagus, after coming out of the foramen, takes up
branches of the spinal accessory, passes to the superior nerve ganglion,
crosses the carotid artery, and then, accompanied by the internal
jugular, it connects by a branch with the hypoglossal. It then
communicates with the petrosal ganglion, and receives a long branch from
the superior cervical ganglion. It then extends along the neck with the
jugular vein and often fuses with the sympathetic ganglionic plexuses.
As it passes, it is interwoven with the glosso-pharyngeus, but each
nerve element retains its own individuality. Extending down the neck, on
entering the thoracic cavity, it lies between the plexus brachialis and
the carotid artery. Then it passes below the subclavian artery, and
between the bronchial tubes, the aorta pulmonalis, and the subclavian
vein. Then ventrally it rests upon the glands, and the right and the
left vagi fuse or unite. From here they radiate down to the stomach in
fan-shape, and, continuing, they fuse with the sympathetic system.
The other branches of the vagus are:
The first branch is the recurrent laryngeus, which supplies the lower
end of the bronchial tubes, and the esophagus, then enters above the
bronchus near the origin of the subclavian artery as the recurrent
cardiacus.
The second are the recurrent pulmonale which pass into the lungs, each
fusing with its fellow from the opposite side, and giving off branches
inferior to the vena cava including a branch to the heart.
The third are the recurrent hepatici which pass through the diaphragm
and are distributed to the liver.
=The Accessorius Spinalis.=—This is a very small motor nerve (Fig. 62,
No. _C_, 18). It comes out between the dorsal and the ventral root of
the third cervical nerve. It lies close to the neck and extends
anteriorly, receiving roots from the first and the second cervical
nerve. It passes through the occipital foramen into the brain cavity and
then enters the ganglion radicis vagi. It passes out of the cranial
cavity through the foramen jugulare. It then partly fuses with the vagus
and partly, as a fine branch, with the subcutaneous colli.
=The Hypoglossus.=—The hypoglossus is a motor nerve (Fig. 62, No. _C_,
19). It originates anterior to the eleventh pair of cranial nerves, and
from the same ganglion as the abducens and the motor oculi. It leaves
the medulla oblongata from its ventral surface, and passes with a
posterior and an anterior branch out of the cranial cavity through two
separate foramina in the os occipitale laterale. The posterior branch,
much smaller than the anterior, passes between the cranial bones and the
cervical sympathetic nerve; it then passes at right angles through the
rectus capitis anticus, and, while closely following the carotid artery,
it fuses with the anterior branch. The larger, anterior branch, much
stronger than the posterior, sends a short strong branch to the
complexus muscle. The rest of this branch, which probably has
sympathetic elements, is disposed as is a spinal nerve. It crosses the
sympathetic nerve, and at this point forms a typical cervical
sympathetic ganglion. It also forms a short loop with the recurrent
ventralis of the first cervical plexus. From this loop, after it has
given off several branches to the muscles of the neck, it gives off one
or two strong branches, which fuse with each other and with the
posterior thin branch, thus forming the trunk of the hypoglossus. It
receives many elements from the first cervical nerves.
The hypoglossus sometimes communicates with the vagus, crosses over the
latter, and divides itself into two main branches, as follows:
First, the _recurrent laryngo-lingualis_, which passes between the
cornua of the os hyoideum and the larynx to the anterior part, and
furnishes the principal tongue muscles. It extends along the inferior
surface of the tongue and fuses with the one of the other side and
extends to the free tip of the tongue. This nerve probably receives
sensory elements from the second root and from the confluent of the
first cervical nerve. This form of anastomosis is well marked in birds
with thick tongues, as ducks.
Second, the _recurrent laryngeus_ furnishes the muscles of the superior
larynx and of the tongue skeleton. It extends downward and also
furnishes the muscles of the trachea. It follows the course of the
jugular vein and at the entrance of the chest supplies the furcula. The
nerve passes into the thorax, downward along the side of the bronchial
tubes, and supplies innervation to all the muscles of the inferior
larynx.
THE SPINAL CORD
=Structure of the Cord.=—The spinal cord is called the myelon or medulla
spinalis. It is a comparatively large, white, irregularly cylindrical
cord, flattened from above downward. It extends from the foramen magnum
at the base of the medulla to the caudal portion of the spinal canal
where it terminates in a fine filament. In order to allow considerable
motion of the spinal column without danger of injury to the spinal cord,
the cord is loosely suspended in the canal. The meninges, or coverings,
are continued from the brain. In addition to these there are to be found
arteries, veins, and nerves entering and others making their exit from
the canal and from the cord. At the cervico-dorsal juncture, where the
brachial plexus is given off, the cord is enlarged. There is another
enlargement at the point where the lumbo-sacral plexus is given off. At
this point, superiorly, there is a longitudinal cavity called the _sinus
rhomboidalis_. This sinus contains a gelatinous substance. There is a
pair of nerves given off at each intervertebral foramen. Thus there are
as many pairs of spinal nerves as there are vertebral segments. The
inferior root is the motor root and carries the impulses from the cord
to the periphery. The superior root is the sensory branch and carries
the impulses from the periphery to the cord, and thence the impulse is
carried to the centers in the brain. The sensory and the motor roots are
of about equal size. The inferior have more numerous filaments. The
ganglion on the superior or sensory root is relatively large. In the
sacral region the sensory and the motor branches pass through their own
bony canal.
White _rami communicantes_ are given off at each intervertebral foramen
to the sympathetic system and gray rami are received from the
sympathetic ganglia. Thus there is established a direct communication
between the sympathetic and the spinal system. There is a ganglion on
the superior root just outside the spinal cord.
The ganglionic portion, or _gray matter_, is arranged in the center of
the cord in two comma-shaped parts forming an X. The white matter, or
the material forming the fibers, is arranged around the central
ganglionic portion.
The spinal cord may be divided into two lateral symmetrical halves.
There are two longitudinal fissures, one on the upper and the other on
the lower half. The upper called the _superior median fissure_ is narrow
but deep. The one on the inferior side, called the _inferior median_, is
usually more pronounced. The superior parts of the gray matter are
called the superior cornua, or horns; the inferior parts the inferior
cornua, or horns. The center of the cord is pierced by a _central canal_
which communicates with the fourth ventricle at the calamus scriptorius.
STRUCTURE OF THE NERVE TRUNKS AND GANGLIA
The protoplasmic processes called _dendrites_ have a similar structure
to the cell body. The dendrites branch dichotomously, become rapidly
smaller, and usually end at no great distance from the cell body.
The _axone_ or _axis cylinder process_, differs from the cell body and
from the dendrites. It does not contain chromophilic granules. It
consists entirely of neurofibrils and perifibrillar substance. It
emerges from the cell at an enlargement known as the axone hill. This
hill is free from chromophilic bodies.
Nerves are divided into two kinds, medullated and non-medullated.
_Medullated nerves_ are divided into two kinds: medullated nerves with a
neurolemma, and medullated nerves without a neurolemma.
Medullated nerves with a neurolemma consists of an axone, a medullary
sheath, and a neurolemma. A delicate membrane called an axolemma, or
periaxial sheath envelops the axone. The medullary sheath called myelin,
is semifluid, somewhat resembling fat. The outer covering is the
neurolemma, or sheath of Schwann. It is a delicate, structureless
membrane which incloses the myelin. There is under this sheath an
occasional oval nucleus. At intervals there are constrictions, or nodes,
called the constrictions of Ranvier. The part between the nodes is
called the internode.
The medullated axones without a neurolemma are the medullated nerve
fibers of the central nervous system.
The non-medullated axones, or _non-medullated nerve fibers_, are divided
into non-medullated axones with and those without, a neurolemma.
The non-medullated axone without a neurolemma is merely a naked axone.
They are confined to the gray matter and to the beginnings and endings
of sheath axones, all the latter being uncovered for a short distance
after leaving the nerve cell body and also just before reaching their
terminations.
The long axones serve to make connections with the peripheral, or
distant, nerve cell, muscle cell, or gland cell; while the shorter
axones of certain neurones divide into terminal branches in the
immediate vicinity of its cell body, presumably to come into relation
with other nerve cells in the same or adjacent groups.
[Illustration:
FIG. 77.—Histological structure of tissues.
1. Non-striated or involuntary muscle cell. _a._ 1, Nucleus; 2,
Protoplasm. _b._ Transverse section showing nuclei in the center of
the cells that are cut through the nuclear zone. _c._ A
cross-section of the connective tissue which binds together the
muscle cells. _d._ A section showing the so-called intercellular
bridges.
2. A cross-section of the rectus abdominalis. 1, The endomysium which
binds the cells into primary fasciculi or bundles. 2, The perimysium
which surround the bundles of fasciculi. 3, The epimysium which
surrounds the muscle. These binding structures are white fibrous
connective tissue.
3. A longitudinal section at the juncture of a muscle and tendon. _a_,
The juncture between the muscle and tendon. Note the many nuclei in
both muscle and tendon. This is voluntary or striated muscle which
make up the dermal, dermo-osseous and skeletal muscles.
4. A longitudinal section of heart muscle. _a_, Connective-tissue
cell. _b_, Nucleus of a muscle cell. _c_, Cement line between the
muscle discs. _d_, The cell or segment.
5. A bipolar ganglionic nerve cell. _a_, The nucleus. _b_, The
nucleolus. _c_, The fibrillar structure. _d_, The medullary sheath.
6. A diagram showing the scheme of the peripheral nerve trunk. _a_,
The neuraxis of the peripheral sensory neurone. _b_, The spinal
ganglion of the superior or sensory root. _c_, The dendrite or
peripheral nerve fiber of the sensory nerve. _d_, The nerve trunk.
_f_, The sympathetic nerve ganglion connected with the spinal cord
through the white and the gray ramus communicans. _e_, The neuraxis
of the sympathetic neurone. _g_, Neuraxis or trunk of the motor
neurone or nerve cell. _h_, The anterior horn of the gray matter of
the spinal cord.
7. A diagram of a peripheral sensory neurone. _a_, The neuraxis which
ends in the spinal cord or brain. _b_, The T-shaped division of
Ranvier. _c_, The dendrite or sensory nerve fiber in the nerve
trunk. _d_, The nucleus and nucleolus of the cell _e._, _f._ the
axis cylinder process of the cell. _g_, The telodendrions or
terminal branches of the dendrite or axis cylinder.
8. A schematic diagram of the sensory motor reflex. _a_, The
telodendria. _b_, The dendrite. _c_, Nerve cell of the motor
neurone. _d_, The motor neurone. _f_, The muscle fiber. _g_, The
neuraxis of both sensory and motor neurones, the upper being the
sensory. _h_, The nerve cell in the sensory ganglion. _i_, The
sensory neurone or axis cylinder (nerve fiber). _j_, The skin with
peripheral telodendrion of sensory neurone.
]
_Neurones_ are devoted to the maintenance of functions. Reproductive
neurones are so arranged as to receive afferent nerve impulses from
other tissues; emissive neurones give off efferent nerve impulses. The
former are sensory neurones; the latter are motor neurones if connected
with muscles, and excito-glandular if connected with gland cells. The
basis, then, of the nerve system is a series of neurones, with
projecting and association processes, coördinated for the purpose of
performing specific actions manifested either by motion, by trophic
changes, or by the apperception of stimuli of a chemic, mechanic
(tactile and auditory), thermal, or photic nature.
The whole of the nerve structure is composed of the nerve tissue and
supporting connective tissue. The neurones constitute the nerve tissue,
while the supportive tissue is composed of neuroglia and of white
fibrous tissue derived either from the investing membrane or from the
sheaths of its numerous vascular channels.
The neurones, or nerve cells, exhibit marked variations as to external
characters, dimensions, and form. The neurone presents a swollen cell
mass and a nucleus; it is known as the ganglion cell. From this cell
body are given off a number of processes of two distinct kinds: first,
protoplasmic processes, which are commonly branched, called dendrites;
second, a single, thinner, and paler process; the axis cylinder process.
The bodies of cells vary in size from 4 to 100 microns or more in
diameter. The largest cells occur in the inferior horn of the spinal
cord, in the spinal ganglia, in the large pyramidal cell layer of the
cerebral cortex, in the Purkinjean cell layer of the cerebellum, and in
Clark’s column of the spinal cord. Very small cells occur in the
olfactory bulbs, in the granular layer of the cerebral and cerebellar
cortex, and in the gliosum cornuale of the cord.
Nerve cells are classified according to the number of processes arising
from the cell body, and neurones are referred to as unipolar, bipolar,
and multipolar.
_Unipolar cells_ are met with frequently in early stages of embryonic
development, but are rare in adults, occurring only in the retina, in
the olfactory bulb, and within the baskets of the Purkinjean cells of
the cerebellum. The cells of the cerebro-spinal ganglia, except the
cochlear and vestibular, are apparently unipolar, but they are
developmentally and functionally of bipolar nature.
_Bipolar cells_ are found almost exclusively in the peripheral sensory
system, as in the olfactory membrane, in the retina, in the cochlear and
vestibular ganglia, and in the cerebro-spinal ganglia of the embryo.