1. =Cranial Nerves. Nervi cerebrales.=--I. N. OLFACTORIUS.--The
fasciculi of the olfactory nerves arise from the olfactory bulb (Fig.
144, _a_) and pass through the foramina of the cribriform plate, upon
which the bulb lies, to be distributed to the olfactory mucous membrane
of the nose.
II. N. OPTICUS.--The second nerve or optic (Fig. 138, _II_) arises from
the optic chiasma (Fig. 138, _c_), passes through the optic foramen, and
extends in an S-shaped curve to the eyeball. Its course is craniodorsad.
It pierces the sclerotic and choroid coats of the eye and spreads out
into the retina.
III. N. OCULOMOTORIUS.--The third or oculomotor nerve arises (Fig. 138,
_III_) from the pedunculus cerebri and passes into the orbit through the
orbital fissure. It passes between the lateral and superior recti, sends
a large branch to the superior rectus, supplies the medial rectus and
the retractor oculi, passes laterad of the optic nerve and supplies the
inferior rectus, and sends a long branch to the inferior oblique (Fig.
166, _f_). At the point where the branch is given off to the inferior
oblique muscle is the triangular reddish =ciliary= (or ophthalmic)
=ganglion=, about two millimeters in diameter. It is directly attached
to the inferior oblique branch of the third nerve and receives a
delicate sensory filament from the ophthalmic nerve. From the ciliary
ganglion proceed two =short ciliary nerves= which pass along each side
of the optic nerve to the eyeball. These nerves penetrate the sclerotic
at the sides of the eyeball to be distributed to the ball.
IV. N. TROCHLEARIS.--The fourth nerve, the trochlearis (or patheticus),
arises from the lateral border of the velum medullare anterius, as
already described (Fig. 141, _n_). It passes craniad, emerges through
the orbital fissure in company with the oculomotor, abducens, and
ophthalmic, passes dorsad of the superior rectus muscle, and reaches
(Fig. 154, _l_) the caudal border of the belly of the superior oblique
muscle near its middle. It supplies only the superior oblique.
V. N. TRIGEMINUS.--The fifth nerve, the trigeminus (or trifacial) arises
(Fig. 138, _V_) by two roots, a large sensory and a small motor root
(4), as already described (p. 347). One of these, the motor (4), is
smaller and more ventral; the other, the sensory root, is larger and
dorsal. The dorsal root soon enlarges to form a large ganglion, the
=semilunar= (or Gasserian) =ganglion= (Fig. 138, _k_), from which three
branches diverge. One branch is joined by the ventral root (4), which
passes over the ventral surface of the semilunar ganglion; and the nerve
thus formed is the =mandibular= division (1) of the fifth nerve. Of the
other two branches from the ganglion, the middle and longest is the
=maxillary= nerve (2), and the smallest is the =ophthalmic= (3). The
mandibular nerve is thus mixed, motor and sensory, while the others are
sensory.
1. =N. ophthalmicus.=--The ophthalmic or first division of the fifth
nerve arises from the semilunar (or Gasserian) ganglion. It passes out
of the cranial cavity and into the orbit by way of the orbital fissure,
in company with the third, fourth, and sixth nerves and with the
extension of the carotid (arterial) plexus.
It passes between the superior and medial recti along with the third
nerve, crosses dorsad of the optic nerve, and divides into
=infratrochlear= and =ethmoidal= branches. In the orbital fissure it
gives off the =frontal= nerve, and while crossing the optic it gives off
the =long ciliary= nerve.
_a._ =N. frontalis.=--The frontal nerve passes along the lateral border
of the superior oblique muscle and then laterad of the pulley to near
the middle of the supraorbital crest of the frontal bone. Here it passes
out of the orbit and is distributed to the integument of the upper
eyelid and the adjacent region at the side of the nose.
_b._ =N. infratrochlearis.=--The infratrochlear nerve passes between the
superior rectus and the superior oblique in the first part of its
course. It then passes ventrad of the superior oblique and ventrad of
the pulley to be distributed to the integument of the upper eyelid near
the inner angle.
_c._ =N. ethmoidalis.=--The ethmoidal nerve passes along with the
ethmoidal artery through the ethmoidal foramen (or foramina) in the
orbital plate of the frontal bone. It is finally distributed to the
mucosa of the nose and to the cartilage and integument of the snout.
_d._ =N. ciliaris longus.=--The long ciliary nerve arises from the
ophthalmic and passes along the optic nerve to be distributed to the
eyeball. It divides into several branches before penetrating the
sclerotic.
One or two small communicating branches to the =ciliary ganglion= are
given off at about the same point as the long ciliary nerve. (For a
description of this ganglion see the account of the oculomotor nerve,
page 369.)
2. =N. maxillaris.=--The maxillary nerve, the second division of N.
trigeminus, rises from the semilunar (Gasserian) ganglion and leaves the
skull by the foramen rotundum. It is the sensory nerve of the palate,
upper teeth and upper lip, and of part of the forehead and cheek.
On leaving the foramen the maxillary nerve divides into three branches,
the two =infraorbital= nerves (Fig. 154, _f_) and the =sphenopalatine=
(Fig. 154, _g_). Two smaller branches are likewise given off either
within or just outside of the foramen, the =lachrymal= nerve (Fig. 154,
_j_) and the =zygomatic= (subcutaneus malæ) (Fig. 154, _i_).
_a._ =N. lachrymalis= (Fig. 154, _j_; Fig. 155, _m_).--The lachrymal
nerve passes along the periorbita to the lachrymal gland (Fig. 154, 11),
to which it gives branches. It then continues caudad of the zygomatic
process of the temporal to the integument; here it turns caudad and is
distributed to the integument in the region between the eye and the
external ear (Fig. 155, _m_). It anastomoses with the zygomatic branch
of the seventh nerve (Fig. 155, _h_).
_b._ =N. zygomaticus= (subcutaneus malæ) (Fig. 154, _i_).--This arises
with the preceding and follows it for some distance. It passes through a
foramen in the frontal process of the malar bone and is distributed to
the lower eyelid and adjacent integument.
_c._ =Nn. infraorbitales= (Fig. 154, _f_; Fig. 155, _l_).--The
infraorbital nerves are two of nearly equal size. They pass through the
orbit ventrad of the eyeball to the infraorbital canal. On their course
each divides once or twice and each divides again in the infraorbital
foramen, so that about eight branches emerge from the infraorbital
foramen and diverge to the integument and whiskers of the upper lip and
to the side and wing of the nose (Fig. 155, _l_). In the infraorbital
canal, and before reaching it, branches are given to the molar teeth,
and a branch continues in the bone to the canine, incisor, and premolar
teeth.
_d._ =N. sphenopalatinus= (Fig. 154, _g_).--The sphenopalatine nerve
turns mediad from the infraorbitals, directing its course toward the
sphenopalatine foramen. Before reaching this it gives off the =greater
palatine= nerve (=N. palatinus major=), which enters the posterior
palatine canal and passes to the hard palate. The sphenopalatine then
usually divides into two branches which pass along side by side to enter
the =sphenopalatine ganglion=. This is a large elongated triangular
ganglion lying on the dorsal surface of the external pterygoid muscle,
just laterad of the sphenopalatine foramen.
The following nerves are connected with the sphenopalatine ganglion:
_a._ =N. palatinus minor.=--This leaves the craniolateral angle of the
ganglion and passes to the soft palate.
_b._ =N. nasalis posterior.=--The posterior nasal nerve enters the nasal
cavity by the sphenopalatine foramen and is distributed to the mucosa of
the ventral and middle parts of the nasal cavity.
_c._ =N. canalis pterygoidii= (Vidian Nerve).--This is a large nerve
which leaves the caudal angle of the sphenopalatine ganglion and passes
caudad. It enters the orbital fissure, lying in a groove on its ventral
wall (the cranial end of this groove is sometimes converted into a
canal). The groove ends caudally in a foramen which pierces the sphenoid
bone between the wing and the body and lies just mediad of the foramen
rotundum. The groove and foramen constitute the pterygoid canal (from
which the nerve is named). After emerging from the pterygoid canal onto
the ventral surface of the basisphenoid the nerve enters the tympanic
bulla along with the Eustachian tube, lying on the medial side of the
latter. Just after entering, on reaching the internal carotid artery, it
divides into two. One of these, =N. petrosus superficialis major= (p.
375), passes into the hiatus facialis of the petrous bone and joins the
facial nerve. The other, =N. petrosus profundus=, accompanies the
internal carotid artery caudad, turning therefore out of the bulla and
passing caudad along its medial side; it finally joins the superior
cervical ganglion of the sympathetic system.
3. =N. mandibularis.=--The third division of the fifth, the mandibular
nerve, takes origin by a strong root from the semilunar (or Gasserian)
ganglion, and receives also after separating from the ganglion the
smaller ventral root (portio minor) of the fifth nerve. The root from
the ganglion is sensory; the smaller root is motor, so that the
mandibular nerve is both sensory and motor. The nerve passes through the
foramen ovale, and sends off at once the following branches:
_a._ =N. auriculotemporalis= (Figs. 154 and 155, _n_).--This passes
dorsad between the cartilaginous auditory meatus and the zygomatic
process of the temporal bone (Fig. 154, _n_), emerges at the caudal
border of the masseter muscle (Fig. 155, _n_), and divides into two
chief branches. One, the =auricular= branch, passes along the cranial
side of the external ear and is distributed to its integument. The
other, the =temporal= branch, passes along the zygomatic arch, gives
branches to the temporal muscle and to the skin, follows the zygomatic
arch almost to the angle of the mouth, and anastomoses with the superior
buccal branch of the facial (Fig. 155, _k_).
_b._ =N. temporalis profundus.=--One or more large branches passing
mediad of the zygomatic arch to the temporal muscle.
_c._ =N. massetericus.=--The masseteric nerve passes dorsocraniad to the
masseter muscle.
_d._ =N. pterygoideus.=--One or more pterygoid branches pass to the
pterygoid muscles. A small twig from the nerve to the internal pterygoid
passes into the tympanic cavity and supplies the tensor tympani muscle.
_e._ =N. buccinatorius= (Fig. 154, _e_).--This passes craniad along the
dorsolateral surface of the pterygoid muscles (Fig. 154, 8) to the angle
of the mouth. Here it divides into branches to the masseter muscle, the
mucosa of the mouth, and to the lips.
[Illustration: FIG. 154.--DISSECTION SHOWING A NUMBER OF THE CRANIAL
NERVES.
The mandible and zygomatic arch have been cut and removed; the
temporal and internal pterygoid muscles are also removed. _a_, N.
hypoglossus; _b_, N. lingualis; _c_, N. chorda tympani; _d_, N.
alveolaris inferior (cut); _e_, N. buccinatorius; _f_, Nn.
infraorbitales; _g_, N. sphenopalatinus; _h_, branch of N.
oculomotorius; _i_, N. zygomaticus; _j_, N. lachrymalis; _k_, N.
abducens; _l_, N. trochlearis; _m_, N. frontalis; _n_, N.
auriculotemporalis. 1, cut end of zygomatic process of temporal; 2,
tympanic bulla; 3, M. digastricus; 4, M. constrictor pharyngis medius;
5, M. hyoglossus; 6, M. genioglossus; 7, M. geniohyoideus; 8, M.
pterygoideus externus (cut); 9, cut cranial end of zygomatic arch; 10,
mandible (cut); 11, lachrymal gland.]
After giving off the above branches the mandibular nerve passes three or
four millimeters laterad along the root of the zygomatic process of the
temporal bone, and divides into two large branches, the =inferior
alveolar= (or dental) nerve (_d_) and the =lingual= nerve (_b_).
_f._ =N. alveolaris inferior= (Fig. 154, _d_).--This passes toward the
mandibular foramen, but before reaching it gives off a branch to the
mylohyoid and digastric muscles. It then enters the foramen, passes
through the mandibular canal, and gives numerous =inferior dental=
branches to the teeth. At the mental foramen (or foramina) it passes out
of the canal and divides into several =mental= nerves, to the chin,
lower lip, and mucosa of the lower jaw.
_g._ =N. lingualis= (Fig. 154, _b_).--The lingual nerve passes between
the internal and external pterygoid muscles, and two or three
millimeters from its origin it receives the =chorda tympani= nerve
(_c_), a communicating branch from the facial (see page 376). It passes
onto the ventral surface of the external pterygoid, thence onto the side
of the tongue, beneath the mylohyoid. On the side of the tongue it
divides into branches which enter the tongue and are distributed to its
mucous membrane. One branch passes to the mucosa of the pharynx. Just
before entering the tongue a branch is given off to the sublingual and
submaxillary glands.
VI. N. ABDUCENS.--The sixth nerve, the abducens, arises from the
medulla, as already described (Fig. 138, _VI_, and page 347). It passes
into the orbit through the orbital fissure, then extends obliquely
cranioventrad along the medial surface of the lateral rectus muscle
(Fig. 154, _k_). At about the middle of the length of the muscle it
divides into two or three branches which enter at once into the lateral
rectus and supply it.
VII. N. FACIALIS (Fig. 155).--The seventh or facial nerve arises from
the trapezium at the caudal margin of the pons, craniad of the origin of
the eighth nerve, as described in the account of the brain (Fig. 138,
_VII_). It passes into the internal auditory meatus, traverses the
facial canal through the petrous bone, and emerges at the stylomastoid
foramen.
Within the facial canal the nerve bears an enlargement, the =ganglion
geniculi=. It gives off within the canal a branch to the stapedius
muscle, and the =superficial petrosal= and =chorda tympani= nerves.
The =superficial petrosal= (N. petrosus superficialis major) nerve
passes into the hiatus facialis, through the canal which forms its
continuation, and joins the sphenopalatine ganglion (p. 373).
[Illustration: FIG. 155.--SUPERFICIAL NERVES OF THE FACE.
_a_, N. facialis; _b_, branch to digastric; _c_, N. auricularis
posterior; _d_, branch to inside of ear; _e_, ventral ramus; _f_,
dorsal ramus; _g_, temporal nerve; _h_, zygomatic nerve; _i_, nerve to
stylohyoid muscle; _j_, inferior buccal nerve; _k_, superior buccal
nerve; _l_, infraorbital branches of fifth nerve; _m_, lachrymal
branches of fifth nerve; _n_, auriculotemporal branches of fifth
nerve. 1, M. digastricus; 2, M. stylohyoideus; 3, M. masseter.]
The =chorda tympani= is given off two or three millimeters before the
emergence of the facial at the stylomastoid foramen. It passes into the
tympanic cavity, extends across it between the malleus and incus,
passing close against the tensor tympani muscle, and leaves it (by a
small cleft, the canal of Huguier) between the bulla and the squamous
portion of the temporal. It then passes craniad (Fig. 154, _c_), ventrad
of the root of the zygomatic process of the temporal, and joins the
lingual nerve (Fig. 154, _b_) (branch of the third division of the fifth
nerve).
On emerging from the stylomastoid foramen the facial nerve (Fig. 155,
_a_) gives off at once a small branch (_b_) to the digastric muscle (1),
and a larger branch, =N. auricularis posterior= (_c_), which passes
dorsocaudad about the base of the ear, supplying some of its muscles.
Another small branch (_d_) pierces the cartilaginous external ear and is
distributed to its inner surface. The facial nerve then curves craniad
about the proximal part of the cartilaginous external ear, and divides
five or six millimeters from its emergence into two main branches, which
may be designated as the =dorsal= (_f_) and =ventral= (_e_) rami. A
third small branch may also rise from the point of union of the two; it
is usually, however, a twig from the dorsal ramus, and passes to M.
submentalis.
The dorsal ramus (_f_) sends two or three small branches to the cheek
(including the one just mentioned), and divides into the =temporal=
(_g_) and the =zygomatic= (_h_) branches. The former (_g_) passes along
the cranial margin of the external ear, supplying the superficial
muscles, and extends into the temporal region, where it lies deeper than
the terminal branches of the lachrymal nerve (_m_) (from the fifth
cranial). The =zygomatic= branch (_h_) passes across the malar bone to
the caudal angle of the eye, sends branches into both eyelids,
anastomoses with twigs from the lachrymal branch (_m_) of the fifth
nerve, and passes along the medial side of the eye to the lateral
surface of the nose, where it ramifies.
The ventral ramus (_e_) gives off a minute branch (_i_) to the
stylohyoid muscle (2), then proceeds toward the angle of the mouth and
divides into =superior= (_k_) and =inferior= (_j_) buccal branches, the
former to the muscles of the upper lip and the contiguous regions, the
latter to those of the lower lip and chin.
The seventh nerve thus supplies most of the muscles of the head except
those of mastication, and of these it supplies the digastric.
VIII. N. ACUSTICUS.--The eighth or auditory nerve takes origin (Fig.
138, _VIII_) from the floor of the fourth ventricle, as before described
(page 347). It passes into the internal auditory meatus and is
distributed to the internal ear within the petrous bone.
IX. N. GLOSSOPHARYNGEUS (Fig. 156, _a_).--The ninth or glossopharyngeal
nerve takes origin (Fig. 138, _IX_) from the side of the medulla, as
described in the account of the brain. Close to its origin its trunk
bears a small enlargement, the =ganglion superius=. It passes along with
the tenth and eleventh nerves through the jugular foramen. At its exit
it has a ganglionic enlargement, the =ganglion petrosum=; this is
connected by fine fibres with the ganglion nodosum (Fig. 156, _d_) of
the vagus. The glossopharyngeal (Fig. 156, _a_) then passes craniad over
the surface of the tympanic bulla (15) and mediad of the digastric
muscle. It continues mediad of the carotid artery, and as it approaches
the cranial cornu of the hyoid divides into two portions, one of which
passes to the muscles and mucosa of the pharynx, and the other to the
tongue, where it is the special nerve of taste.
X. N. VAGUS.--The vagus nerve arises from the side of the medulla in the
manner described under the Brain (page 346 and Fig. 138, _X_). It passes
through the jugular foramen along with the glossopharyngeal and
accessory nerves.
=Cervical Portion of the Vagus= (Fig. 156, _d_, _d′_).--In the foramen,
or just before entering it, it presents a ganglionic enlargement, the
=ganglion jugulare= (or “ganglion of the root”), and a short distance
beyond the foramen it forms a second ganglion, the =ganglion nodosum=
(_d_) (or “ganglion of the trunk”), which lies dorsocaudad of the
superior cervical sympathetic ganglion (_e_). The ganglia of the vagus
and sympathetic are closely bound together by connective tissue, and
that portion of the vagus craniad of the ganglion nodosum is
interconnected by a network of nervous fibres with the sympathetic
(_e_), hypoglossal (_b_), and accessory (_c_) nerves. From the ganglion
nodosum the vagus (_d′_) passes caudad, closely bound up with the
sympathetic; the two lie at the side of the common carotid artery. Just
before entering the thorax the two separate, the vagus being the larger
and lying ventrad of the sympathetic. (In rare cases the two are
distinctly separated throughout their length.)
[Illustration: FIG. 156.--GLOSSOPHARYNGEAL, HYPOGLOSSAL, VAGUS,
SYMPATHETIC, AND FIRST CERVICAL NERVES IN THE NECK.
_a_, N. glossopharyngeus; _b_, N. hypoglossus; _c_, N. accessorius;
_d_, ganglion nodosum of vagus nerve; _d′_, N. vagus; _e_, ganglion
cervicale superius of sympathetic; _f_, ramus descendens of N.
hypoglossus (united with first cervical); _g_, branch to M. thyrohyoid
from ramus descendens of N. hypoglossus; _h_, N. laryngeus superior of
N. vagus; _i_, united vagus and sympathetic nerves; _j_, first
cervical nerve; _k_, second cervical nerve; _l_, N. pharyngeus from
vagus. 1, M. masseter; 2, outline of external ear; 3, M. splenius; 4,
M. levator scapulæ ventralis; 5, M. longus capitis; 6, trachea; 7, M.
sternothyreoideus; 8, M. sternohyoideus; 9, M. thyreohyoideus; 10, M.
constrictor pharyngis inferior; 11, M. constrictor pharyngis medius;
12, cut end of M. stylohyoideus; 13, M. hyoglossus; 14, M.
stylopharyngeus; 15, bulla tympani; 16, M. jugulohyoideus; 17, M.
styloglossus; 18, M. mylohyoideus.]
Branches of the vagus in the cervical region (Fig. 156).
_a._ =N. auricularis.=--This leaves the ganglion jugulare, passes into
the petrous bone to the facial canal, leaves the skull with the facial
nerve by the stylomastoid foramen, and is distributed to the external
ear.
_b._ =N. pharyngeus= (_l_).--This leaves the vagus craniad of the
ganglion nodosum, passes ventrad, sends a small communicating branch
to N. laryngeus superior (_h_), and is distributed to the pharyngeal
muscles and cranial part of the œsophagus.
_c._ =N. laryngeus superior= (_h_).--This arises from the ganglion
nodosum, passes ventrad, crossing the medial surface of the carotid
artery, and reaches the larynx at about the level of the caudal end of
M. thyreohyoideus. It passes into the larynx and supplies its mucosa.
=Thoracic Portion of the Vagus= (Fig. 157).--As the vagus (_b_) and
sympathetic (_c_) separate to enter the thorax, the vagus (_b_) lies
more ventrad. The right vagus now lies along the lateral surface of the
trachea, the left one (Fig. 157) along that of the œsophagus. At about
the level of the first rib or further craniad there arise, apparently
from the vagus (but really from the sympathetic), two slender nerves
which pass caudad parallel and close to the vagus, lying slightly dorsad
of it. These are the =cardiac nerves= (_i_); they arise from the
sympathetic farther craniad in the neck region, accompany the vagus, and
seem to branch from it. In cases where vagus and sympathetic are
separate throughout their courses, these nerves arise clearly from the
sympathetic. The cardiac nerves pass to the heart and aid in forming a
network of nerves known as the =cardiac plexus= (_k_).
The vagus usually receives also, at about the level of the first rib,
one or two communicating branches from the middle cervical ganglion
(_d_) of the sympathetic. It then passes to the roots of the lungs,
crossing the lateral surface of the aortic arch on the left side, and
the medial surface of the azygos vein on the right side. As it crosses
the aortic arch the left vagus gives off =N. laryngeus inferior= (_j_).
This curves around the caudal side of the aortic arch to the lateral
surface of the trachea (_o_), then extends on the lateral and ventral
surface of the trachea craniad into the neck region. In the neck it
passes to the dorsolateral side of the trachea and reaches the larynx.
It passes into the larynx between the cricoid and thyroid cartilages,
and supplies the muscles of the larynx. It anastomoses with the superior
laryngeal.
[Illustration: FIG. 157.--SYMPATHETIC, VAGUS, AND PHRENIC NERVES IN
THE NECK AND THORAX.
1-13, the thirteen ribs (cut); _V_-_VIII_, the fifth to eighth
cervical nerves (cut); _I′_, first thoracic nerve (cut). _a_, combined
trunk of vagus and sympathetic; _b_, vagus; _c_, sympathetic; _d_,
middle cervical ganglion of sympathetic; _e_, inferior cervical
ganglion; _e′_, communicating branch to sixth, seventh, and eighth
cervical nerves; _f_, phrenic nerve; _g_, loop of sympathetic about
subclavian artery, between the middle and inferior cervical ganglia;
_h_, cardiac branches from the inferior cervical ganglion; _i_,
cardiac branch passing from sympathetic along with vagus to heart;
_j_, inferior laryngeal nerve; _k_, pulmonary and cardiac plexus; _l_,
ventral œsophageal branch of vagus; _l′_, similar branch from the
opposite side; _m_, dorsal œsophageal branch of vagus; _n_, great
splanchnic nerve; _o_, trachea; _p_, œsophagus; _q_, aorta; _r_,
heart; _s_, root of lung.]
The _right_ inferior laryngeal nerve is given off from the right vagus
much further craniad than the left; it curves around the right
subclavian artery just caudad of the origin of the internal mammary
artery, then passes to the trachea, and has a course and distribution
like that of the left side.
On reaching the root of the lungs the vagus divides into numerous
branches which form the =pulmonary plexus= (_k_) over the roots of the
lungs, and extends onto the basis of the heart as the =cardiac plexus=.
From these plexuses numerous branches pass to the heart, lungs,
pulmonary artery, pericardium, etc. The cardiac branches from the
sympathetic, mentioned above, are also connected with the cardiac
plexus.
Caudad of the pulmonary plexus the vagus is continued on each side as
two trunks, a =dorsal= (_m_) and a =ventral= (_l_) which pass along the
œsophagus (_p_). The ventral branches of right and left vagi (_l_ and
_l′_) unite a short distance caudad of the root of the lung, and the
single trunk extends into the abdominal cavity on the ventral surface of
the œsophagus. The dorsal branches of right and left vagi unite farther
back, near the diaphragm, and the single trunk thus formed (_m_) enters
the abdominal cavity on the dorsal surface of the œsophagus. Both
divisions give fine nerves to the œsophagus.
=Abdominal Portion of the Vagus= (Fig. 164, page 407).--After passing
through the diaphragm the ventral division (Fig. 164, _l_) reaches the
lesser curvature of the stomach (1), over which it ramifies, some
branches being traceable almost to the pylorus. The network of branches
thus formed is the =anterior gastric plexus=. Some twigs from the
ventral division pass transversely across the cranial end of the stomach
and join the plexus formed by the dorsal division.
The dorsal division (_m_) of the vagus reaches the greater curvature of
the stomach, where it ramifies, forming the =posterior gastric plexus=.
Branches from this anastomose with the anterior gastric plexus and with
the cœliac plexus of the sympathetic (_e_).
XI. N. ACCESSORIUS.--The accessory (or spinal accessory) nerve arises
(Fig. 138, _XI_) by numerous rootlets from the lateral surface of the
medulla and of the spinal cord as far caudad as the fifth to seventh
cervical nerve. These spinal rootlets join to form a nerve which enters
the cranium through the foramen magnum. After receiving the rootlets
from the medulla, it leaves the cranial cavity along with the vagus and
glossopharyngeal by the jugular foramen. Just outside the foramen (Fig.
156, _c_) it becomes involved in a plexus of fine branches which
interconnect it with the vagus, sympathetic, and hypoglossal. It then
turns laterad and caudad (Fig. 158, 1) and pierces the cleidomastoid
muscle (_d_), to which it gives small branches. It then divides: one
branch enters the sternomastoid muscle; the other passes caudad along
the dorsal border of the levator scapulæ ventralis, sends branches to
the clavotrapezius, and may be traced to the acromiotrapezius and
spinotrapezius, which it supplies.
XII. N. HYPOGLOSSUS (Fig. 156, _b_).--The twelfth or hypoglossal nerve
arises from the ventral side of the medulla (Fig. 138, _XII_), as
previously described. It emerges from the skull through the hypoglossal
canal. At first it passes ventrad, then gradually turns craniad,
following thus a curved course and passing successively laterad of the
vagus and sympathetic nerves (Fig. 156, _d_ and _e_), the common carotid
artery, and the cranial cornu of the hyoid bone. A short distance from
its origin the nerve gives off a =ramus descendens= (Fig. 156, _f_),
which receives a communicating branch from the first cervical nerve
(_j_), and divides into two branches, one (_g_) to the thyrohyoid muscle
(9), the other to the sternohyoid (8) and sternothyroid muscles (7). The
main nerve passes beneath the mylohyoid muscle into the tongue, at first
following the lingual artery, then laterad of it, then crossing it
again. It sends branches to all the muscles of the tongue, and may be
traced to its tip.
The hypoglossal nerve is involved in the plexus just outside of the
jugular foramen, with which the vagus, sympathetic, and accessory nerves
are also connected.
2. =Spinal Nerves.=--The origin and general features of the spinal
nerves have been described in connection with the account of the spinal
cord (page 337). A description of their peripheral distribution will now
be given.
A. CERVICAL NERVES.--There are eight pairs of cervical nerves. The first
leaves the vertebral canal through the atlantal foramen. The second
passes out between the arches of the atlas and axis, not through a
special intervertebral foramen, so that its ganglion lies among the
muscles of the back of the neck. The others emerge through the
intervertebral foramina, the eighth one from between the last cervical
and first thoracic vertebræ.
[Illustration: FIG. 158.--SUPERFICIAL NERVES OF THE NECK.
M. clavotrapezius has been partly removed. 1, N. accessorius; 2-4,
ventral rami of second to fourth cervical nerves; 5, N. auricularis
magnus; 6, N. cutaneus colli. _a_, M. clavotrapezius (cut); _b_, M.
splenius; _c_, M. levator scapulæ ventralis; _d_, M. cleidomastoideus;
_e_, M. sternomastoideus; _f_, parotid gland.]
=Dorsal Rami= (Rami posteriores).--The dorsal ramus of the first nerve
(N. suboccipitalis) supplies the short dorsal muscles which move the
head and connect the atlas and skull. In the second nerve the dorsal
ramus is much larger, forming =N. occipitalis major=. It sends small
branches to the muscles about its origin, then turns craniad on the
surface of the obliquus superior muscle, passes through the biventer
cervicis and splenius, joins a small branch from the third nerve, and
reaches the dorsal surface of the back of the head. It passes craniad,
lying beneath the levator auris longus, emerges from between the two
divisions of this muscle, and supplies the skin and cutaneous muscles
between the two external ears. The dorsal rami of the other cervical
nerves supply muscles and integument on the back of the neck.
=Ventral Rami= (=rami anteriores=).--These pass ventrad between the
transverse processes of the vertebræ, except in the case of the =first=
nerve (Fig. 156, _j_). This passes ventrad from the atlantal foramen
along the groove for the vertebral artery, through the notch in the wing
of the atlas, and across the lateral surface of the longus capitis
muscle (5). Here it sends a branch caudad to join the second cervical
(_k_), then crosses the vagus (_d′_) and sympathetic and the carotid
artery, giving off communicating branches to the vagus and sympathetic,
and uniting with a branch of the descending ramus of the hypoglossal
(_f_). At the lateral surface of the larynx it turns caudad, following
the lateral border of the sternothyroid muscle (7), and is distributed
to the sternohyoid (8) and sternothyroid (7).
The ventral ramus of the =second= nerve (Fig. 158, 2) receives a branch
from that of the first nerve, passes laterad between the levator scapulæ
ventralis (_c_) and the cleidomastoid (_d_), receives a communicating
branch from the third cervical (3), sends a branch to N. accessorius (1)
and numerous small nerves into the sternomastoid (_e_) and cleidomastoid
(_d_), then turns craniad and divides into =N. auricularis magnus= (5)
and =N. cutaneus colli= (6).
=N. auricularis magnus= (5), the great auricular nerve, passes
dorsocraniad across the lateral surface of the sternomastoid (_e_) to
the lateral and caudal surface of the external ear and parotid gland
(_f_), where it ramifies. The =cutaneus colli= (6) is the smaller,
ventral, division of the second nerve; it may receive also an accession
from the third. It passes to the integument over the ventral part of M.
masseter and ventrad of that muscle.
The =third= nerve (Fig. 158, 3) communicates with the second and
supplies the levator scapulæ ventralis (_c_), cleidomastoid (_d_),
sternomastoid (_e_), longus capitis, and other muscles of this region
and aids in forming the cutaneus colli (6). The =fourth= (4) and =fifth=
are distributed to the muscles and integument of the sides of the neck.
A branch of the fourth supplies the integument in the hollow of the
shoulder, and one from the fifth follows the vena cephalica and supplies
the integument over the shoulder. The fifth by sending a branch to aid
in forming the phrenic nerve (Fig. 157, _f_) may be considered to enter
partly into the brachial plexus.
Owing to the intercommunicating branches between the ventral roots of
the first five cervical nerves, these are sometimes considered as
forming a loose plexus which receives the name =cervical plexus=.
The sixth, seventh, and eighth cervical nerves (with a part of the
fifth) become interconnected with each other and with the first thoracic
to form the =brachial plexus=.
=The Brachial Plexus= (Fig. 159).--The brachial plexus is formed by the
ventral rami of the fifth, sixth, seventh, and eighth cervical nerves
and the first thoracic. Of the fifth cervical only a small part enters
into the plexus, forming part of the phrenic nerve. The formation of the
plexus is due to the union of the different nerves by means of strong
connecting branches or =ansæ=. The plexus lies in the axilla, along with
the axillary artery and vein; all its component nerves pass laterad in
front of the first rib. Its branches supply the arm and shoulder.
The precise arrangement of the different strands is somewhat variable.
The plexus is commonly made up in approximately the following manner
(Fig. 159). From the =fifth= cervical nerve (_V_) a small branch joins a
similar one from the sixth to form the phrenic nerve (_a_); the
remainder of the fifth does not enter into the plexus. From the =sixth=
cervical (_VI_) arise parts of the phrenic nerve (_a_), the
suprascapular (_b_), the cranial one of the three subscapular nerves
(_c_), the axillary (_d_), and the musculocutaneous (_f_). The sixth
also gives off close to its origin a nerve (_b′_) which passes to the
inner surface of the levator scapulæ and ramifies over the surface,
supplying this muscle and extending to the rhomboideus, which it also
innervates. The =seventh= cervical (_VII_) is the largest nerve entering
into the plexus; it furnishes parts of one or both of the anterior
thoracic nerves (_k_ and _n_), of the posterior thoracic (_m_), the
three subscapular nerves (_c_, _e_, and _i_), the axillary (_d_),
musculocutaneous (_f_), radial (_h_), and median (_g_) nerves. The
=eighth= cervical (_VIII_) supplies parts of one of the anterior
thoracic nerves (_k_), the caudal one of the subscapular nerves (_i_),
the radial (_h_), median (_g_), and ulnar (_j_) nerves. The =first
thoracic= (1) curves craniad, so as to leave the thorax on the cranial
side of the first rib; it supplies the median cutaneous nerve (_l_) and
parts of the radial (_h_), ulnar (_j_), and median (_g_) nerves;
sometimes also a part of one (_k_) of the anterior thoracic nerves.
[Illustration: FIG. 159.--DIAGRAM OF THE RIGHT BRACHIAL PLEXUS.
_V_, _VI_, _VII_, _VIII_, the fifth to eighth cervical nerves. 1, the
first thoracic nerve. _a_, phrenic nerve; _b_, suprascapular; _b′_,
nerve to serratus anterior and levator scapulæ muscles; _c_, first or
cranial subscapular nerve; _d_, axillary nerve; _e_, second
subscapular; _f_, musculocutaneous; _g_, median; _h_, radial; _i_,
third subscapular; _j_, ulnar; _k_, second anterior thoracic; _l_,
medial cutaneous; _m_, posterior thoracic; _n_, first anterior
thoracic.]
The following are the nerves which arise from the brachial plexus:
A. =Nerves of the Shoulder and Breast= (Fig. 160).
1. =Nn. thoracici anteriores.=--There are two anterior thoracic nerves.
One (Fig. 160, _h_; Fig. 159, _n_) arises from the seventh cervical and
passes to the pectoral muscles (9) along with the anterior thoracic
artery. The other (Fig. 159, _k_; Fig. 160, _q_ and _r_) arises usually
by two roots, one from the eighth cervical, one from the first thoracic;
it passes along with the long thoracic artery to be distributed to the
pectoralis muscles (Fig. 160, 8) and sometimes also to the latissimus
dorsi (7).
2. =N. thoracicus posterior.=--The posterior thoracic (or long thoracic)
nerve (external respiratory) (Fig. 159, _m_) arises from the seventh
cervical nerve near its beginning. It passes within the scalenus muscle
and extends caudad on the outer surface of the serratus anterior muscle,
which it supplies.
3. =N. suprascapularis= (Figs. 159 and 160, _b_).--The suprascapular
nerve arises from the sixth (and sometimes the seventh) cervical. It
passes laterad and gives off a branch which passes over the
shoulder-joint and penetrates the clavodeltoid muscle to be distributed
to the integument on the ventral surface of the upper arm. It then
follows the transversa scapulæ artery into the supraspinatus fossa and
accompanies the artery to be distributed to the supraspinatus and
infraspinatus muscles.
4. =Nn. subscapulares.=--The subscapular nerves are three. The cranial
one (Figs. 159 and 160, _c_) arises from the sixth and seventh cervical
and supplies the subscapular muscle. The middle one (Fig. 159, _e_)
arises from the seventh; it supplies principally the teres major. The
caudal one (Fig. 159, _i_) arises from the seventh and eighth cervical
and supplies the latissimus dorsi. (The points of origin from the plexus
vary.)
5. =N. axillaris= (or =circumflexus=) (Fig. 159, _d_).--The axillary
nerve arises from the sixth and seventh cervical nerves. It passes
toward the shoulder-joint, then follows the posterior circumflex artery
ventrad of the long head of the triceps to be distributed to the
spinodeltoid and acromiodeltoid. A branch of it continues to the
clavobrachial, which it supplies.
B. =The Phrenic Nerve= (Fig. 157, _f_, page 381).
6. =N. phrenicus.=--The phrenic nerve (internal respiratory) is formed
by the junction of two slender branches, one from the fifth and one from
the sixth cervical nerves (Fig. 159, _a_). (It is said to receive
sometimes a branch from the fourth.) It passes caudad into the thorax on
the ventral surface of the subclavian artery and then extends to the
diaphragm lying at the side of the inferior and superior venæ cavæ. It
is the motor nerve of the diaphragm.
[Illustration: FIG. 160.--BLOOD-VESSELS AND THE MORE VENTRAL NERVES OF
THE AXILLA, VENTRAL VIEW.
The pectoral and clavobrachial muscles have been cut and laid aside,
their ends being shown; only a part of the nerves are exhibited. 1, M.
clavobrachialis; 2, cut end of M. pectoralis major; 3, cut end of M.
pectoralis minor; 4, M. biceps; 5, M. teres major; 6, M.
epitrochlearis, partly cut and turned back; 7, M. latissimus dorsi,
partly cut; 8, M. pectoralis minor; 9, M. pectoralis major; 10, short
portion of caput mediale of M. triceps brachii. _a_, branches of the
thyrocervical axis to clavobrachial and clavotrapezius muscles; _b_,
suprascapular nerve; _c_, first subscapular nerve; _d_,
musculocutaneous nerve; _e_, median nerve; _f_, V. axillaris; _g_, A.
axillaris; _h_, first anterior thoracic nerve, accompanied by the
anterior thoracic artery and vein; _i_, V. subscapularis; _j_, large
muscular branch (to subscapular muscle) of the subscapular vein and
brachial artery; _l_, A. subscapularis; _l′_, A. circumflexa humeri
anterior; _m_, radial nerve; _n_, ulnar nerve; _o_, medial cutaneous
nerve; _p_, A. thoracica longa; _q_, _r_, second anterior thoracic
nerve; _s_, V. thoracica longa; _t_, A. and V. thoracicodorsalis; _u_,
A. profunda brachii; _v_, branch of brachial artery accompanying
medial cutaneous nerve; _w_, branch of A. collateralis radialis
superior; _x_, A. collateralis radialis superior; _y_, V. mediana
cubiti; _z_, A. collateralis ulnaris.]