The =trigeminal nerve= (N. trigeminus) is the largest of the cranial
series. It is connected with the lateral part of the pons by a large
=sensory root= and a smaller =motor root= (Fig. 499).
The =sensory root= (Portio major) extends forward through a notch on the
lower part of the petrosal crest and widens out to join the semilunar
ganglion.
The =semilunar= (or Gasserian) =ganglion= (Ganglion semilunare) is a
crescent-shaped mass of nerve-fibers and cells which overlies the
antero-external part of the foramen lacerum basis cranii, and is partly
embedded in the dense fibrous tissue which occupies the foramen except
where vessels and nerves pass through. Its long axis, which is about an
inch (2.5 cm.) in length, is directed forward and inward and its convex
anterior face gives rise to the ophthalmic, the maxillary, and the
sensory part of the mandibular division of the nerve. The surface of the
ganglion is irregularly striated. It is connected by filaments with the
adjacent carotid plexus of the sympathetic, and sends delicate twigs to
the dura mater.
The fibers of the sensory root arise from the ganglion as axones of
the ganglion cells, and the fibers of the nerves which extend
peripherally from the ganglion are dendrites of the cells. The sensory
root-fibers enter the tegmentum of the pons and divide into anterior
and posterior branches, which terminate about the cells of the sensory
nucleus of termination of the trigeminus. This nucleus extends from
the pons to the sixth cervical segment of the spinal cord (Dexler).
The posterior branches of the fibers are collected into a compact
bundle, the spinal tract or root of the trigeminus, which lies lateral
to the substantia gelatinosa in the medulla. The central connections
of the sensory part of the trigeminus are very extensive. The most
important paths are: (1) Axones of cells of the sensory nucleus and
the substantia gelatinosa pass chiefly as arcuate fibers across the
raphé to the thalamus, whence impulses are transmitted by
thalamo-cortical fibers to the cerebral cortex. In ungulates a
distinct tract extends from the anterior part of the sensory nucleus
to the thalamus of the same side (Wallenberg). It is probable that
collaterals of the arcuate fibers go to the motor nuclei of the fifth,
seventh, ninth, and tenth cranial nerves. (2) Axones of cells of the
sensory nucleus enter the posterior cerebellar peduncle of the same
side and reach the cerebellar cortex. (3) Collaterals are distributed
to the nuclei of origin of the hypoglossal and of the motor part of
the trigeminal and facial nerves.
The =motor root= (Portio minor) extends forward beneath the sensory root
and the semilunar ganglion and is incorporated with the mandibular
division of the nerve. Its fibers arise chiefly from the so-called
masticatory nucleus, which is situated in the pons near the inner face
of the sensory nucleus; a few of these fibers come from the nucleus of
the opposite side and cross in the raphé. Other fibers, which constitute
the mesencephalic root, arise from cells in the outer part of the
central gray matter of the mid-brain.
It is evident from the foregoing statements that the trigeminus has
essentially the same arrangement as a typical spinal nerve. It divides
into three branches.
I. The =ophthalmic nerve= (N. ophthalmicus) (Figs. 438, 439, 440) is
purely sensory and is the smallest of the three branches of the
trigeminus. It arises from the inner part of the front of the semilunar
ganglion, passes forward along the outer side of the cavernous sinus,
and is blended with the maxillary nerve for some distance. It enters the
foramen lacerum orbitale with the third and sixth nerves and divides
into three branches.
1. The =lacrimal nerve= (N. lacrimalis) runs forward on the rectus
superior and the levator palpebræ superioris and ramifies chiefly in
the lacrimal gland and the upper eyelid. A branch (Ramus
zygomatico-temporalis) exchanges twigs with the zygomatic branch of
the maxillary nerve, perforates the periorbita, and emerges from the
orbital fossa behind the supraorbital process; it forms a plexus with
branches of the auriculo-palpebral and frontal nerves, and ramifies in
the skin of the temporal region.
2. The =frontal nerve= (N. frontalis), also termed the supraorbital,
runs forward almost parallel with the superior oblique muscle, at first
within, then outside of, the periorbita. It passes through the
supraorbital foramen with the artery of like name and ramifies in the
skin of the forehead and upper eyelid, forming a plexus with the
lacrimal and auriculo-palpebral nerves. It divides into three branches.
3. The =naso-ciliary nerve= (N. naso-ciliaris), also termed the
palpebro-nasal, runs forward along the inner side of the optic nerve
between parts of the retractor muscle and divides into two branches. Of
these the =ethmoidal nerve= (N. ethmoidalis) is the continuation of the
parent trunk. It accompanies the ethmoidal artery through the foramen of
like name into the cranial cavity and crosses the lower part of the
ethmoidal fossa. Leaving the cranium through an opening in the
cribriform plate close to the crista galli, it enters the nasal cavity
and ramifies in the mucous membrane of the septum nasi and the superior
turbinal. The =infratrochlear nerve= (N. infratrochlearis) runs forward
to the inner canthus and ramifies in the skin in this region; it
detaches twigs to the conjunctiva and caruncula lacrimalis, and a long
branch which supplies the third eyelid and the lacrimal ducts and sac.
The naso-ciliary nerve furnishes the =sensory= or =long root= (Radix
longa) of the ciliary ganglion.
The =ciliary ganglion= is placed on the inferior branch of the
oculomotor nerve. It is usually not larger than a millet-seed, and is
best found by following the nerve to the inferior oblique muscle back to
its origin. The ganglion receives (_a_) =sensory fibers= from the
naso-ciliary nerve; (_b_) =motor fibers= from the oculomotor nerve; and
(_c_) =sympathetic fibers= from the sphenopalatine plexus. It detaches
filaments which unite with twigs from the ophthalmic and maxillary
nerves and from the sphenopalatine ganglion to form the ciliary plexus.
From the latter emanate five to eight delicate =short ciliary nerves=
(Nn. ciliares breves), which pursue a somewhat flexuous course along the
optic nerve, pierce the sclera near the entrance of that nerve, and run
forward between the sclera and chorioidea to the circumference of the
iris. Here the branches of adjacent nerves anastomose to form a circular
plexus (Plexus gangliosus ciliaris), from which filaments go to the
ciliary body, iris, and cornea.
The circular fibers of the iris and the ciliary muscle are innervated
by fibers derived from the oculomotor nerve, the radial fibers of the
iris by the sympathetic.
II. The =maxillary nerve= (N. maxillaris), also termed the superior
maxillary, is purely sensory and is much larger than the ophthalmic. It
extends forward from the semilunar ganglion in the middle cranial fossa
in the large groove on the root of the temporal wing of the sphenoid. It
is related internally to the cavernous sinus and superiorly to the
ophthalmic nerve, with which it is blended for some distance. It emerges
through the foramen rotundum, passes forward in the pterygo-palatine
fossa above the internal maxillary artery and embedded in fat, and is
continued in the infraorbital canal as the infraorbital nerve (Fig.
439). Its branches are as follows:
1. The =zygomatic nerve= (N. zygomaticus s. subcutaneus malæ), also
termed the orbital branch, arises before the maxillary nerve reaches the
pterygo-palatine fossa (Figs. 438, 439, 514). It pierces the periorbita
and divides into two or three delicate branches which pass along the
surface of the external straight muscle to the external canthus and
ramify chiefly in the lower lid and the adjacent skin. Anastomoses are
formed with branches of the lacrimal nerve.
2. The =sphenopalatine nerve= (N. sphenopalatinus) is given off in the
pterygo-palatine fossa from the lower border of the maxillary nerve
(Fig. 439). It is broad and flat and forms a plexus in which several
small =sphenopalatine ganglia= are interposed. It divides into posterior
nasal, and greater and lesser palatine nerves. (1) The =posterior nasal
nerve= (N. nasalis aboralis) passes through the sphenopalatine foramen,
in which it bears one or more minute ganglia, enters the nasal cavity,
and divides into internal and external branches (Figs. 440, 464). The
internal branch (N. septi narium) runs forward in the submucous tissue
of the lower part of the septum nasi, gives twigs to the mucous membrane
here and to the vomero-nasal organ (of Jacobson), passes through the
palatine cleft and ramifies in the anterior part of the hard palate. The
external branch (Ramus lateralis) ramifies in the mucous membrane of the
inferior turbinal and the middle and inferior meatus nasi. (2) The
=greater= or =anterior palatine nerve= (N. palatinus major s. anterior)
(Figs. 438, 439) is the largest of the three branches. It runs forward
in the palatine canal and groove and ramifies in the hard palate and
gums. It also supplies twigs to the soft palate, and gives off branches
which pass through the accessory palatine foramina to supply the mucous
membrane of the inferior meatus.
The branches of the two nerves anastomose in the hard palate and form
a plexus about the branches of the palatine arteries.
[Illustration: