The =sympathetic nervous system= (Systema nervorum sympathicum) is that
part of the nervous system which serves (1) to transmit stimuli to the
heart-muscle, unstriped muscle, and glands; (2) to conduct impulses from
the viscera to the cerebrospinal system.
Many of the fibers are derived from the cerebrospinal system and are
rearranged and distributed in the sympathetic system. Numerous
sympathetic fibers are contributed to the cerebrospinal nerves,
through which they are distributed to the unstriped muscular tissue
and glands, as vasomotor, pilomotor, and secretory nerves. The
sympathetic, like the cerebrospinal system, consists of neurones, each
of which comprises the cell-body, an axone, and numerous branched
dendrites. The cell-bodies are aggregated into =ganglia=, some of
which are large and more or less constant in position and form, while
others are microscopic; and are scattered in an irregular manner
through the peripheral part of the system. It is probable that simple
visceral reflexes may be mediated by sympathetic neurones alone.
In descriptive anatomy the sympathetic system is regarded as consisting
of (1) a chain of =ganglia= extending along either side of the vertebral
column and connected by association fibers to form the =sympathetic
trunk=; (2) =central branches= to and from the cerebrospinal nerves; (3)
=peripheral branches=, which form =plexuses= with each other and the
cerebrospinal nerves; (4) the =peripheral ganglia= which are interposed
in the plexuses.
The =sympathetic trunk= (Truncus sympathicus) extends on either side
from the base of the cranium to the tail. In it are interposed, at
intervals of varying regularity, the =ganglia of the sympathetic trunk=
(Ganglia trunci sympathici). These are connected with the cerebrospinal
nerves by central branches, the =rami communicantes=.
Two kinds of rami communicantes occur. Of these, one type consists
largely of medullated fibers derived from the spinal nerves and
ganglia; they have therefore a white appearance, and are termed =white
rami=. They contain both efferent and afferent fibers. The =efferent
splanchnic fibers= are derived from the ventral roots of the spinal
nerves and terminate in great part about the cells of the nearest
sympathetic ganglion; others end in more distant or in peripheral
ganglia. The =afferent splanchnic fibers= are chiefly peripheral
processes of the cells of the spinal ganglia, but some are sympathetic
fibers which enter the spinal nerve-trunk and terminate about cells of
the spinal ganglion. The =gray rami= consist mainly of non-medullated
fibers derived from the sympathetic ganglia directly or through the
trunk, which proceed centrally to the spinal nerves and are
distributed along the somatic divisions of the latter to unstriped
muscle and glands as =vasomotor=, =pilomotor=, and =secretory fibers=.
Some go to the membranes of the spinal cord, and a few terminate about
cells of the spinal ganglia as sensory sympathetic fibers.
Similar but more complex and irregular communications which exist
between the sympathetic system and the cranial nerves—with the exception
of the first and second—have been referred to in the accounts of the
latter.
It is convenient for descriptive purposes to divide the sympathetic
system into cephalic, cervical, thoracic, abdominal, and pelvic parts.
1. The =cephalic part= (Pars cephalica systematis sympathici) comprises
the =otic=, =sphenopalatine=, and =ciliary ganglia=, which may be
regarded as homologues of the ganglia of the trunk of other regions. It
also includes three plexuses formed by branches derived from the
superior cervical ganglion. The ganglion gives off two or three
filaments which subdivide to form the =internal carotid plexus= around
the artery of like name. The =cavernous plexus= surrounds the artery
within the cavernous sinus and communicates with that of the opposite
side. The =external carotid plexus= is formed around the homonymous
artery, and filaments from it go to the vessel and its branches and to
the salivary glands.
2. The =cervical part= (Pars cervicalis s. sympathici) includes two
ganglia and the trunk which connects them.
The =superior= or =anterior cervical ganglion= (G. cervicale craniale)
lies on the guttural pouch below the occipito-atlantal articulation
(Fig. 437). It is reddish-gray in color, fusiform, and about an inch
(ca. 2 to 3 cm.) in length. It is connected by rami communicantes with
the last four cranial and first cervical nerves, sends branches to form
the plexuses mentioned above, and contributes to the pharyngeal plexus.
The =cervical trunk= of the sympathetic connects the superior and
inferior cervical ganglia. On leaving the former it is associated in a
common sheath with the vagus along the dorsal face of the common carotid
artery. At the root of the neck it separates from the vagus and joins
the inferior ganglion.
The =inferior= or =posterior cervical ganglion= (G. cervicale caudale)
is situated under the first rib and the insertion of the scalenus (Figs.
428, 429). On the right side it lies upon the longus colli and the
trachea, on the left side upon the same muscle and the œsophagus. It is
flattened, very irregular and variable in outline, and is blended more
or less with the first thoracic ganglion. It is connected by large gray
rami with the ventral branches of the last two cervical nerves, and by
small filaments with the recurrent nerve. It receives a nerve—termed by
Schmaltz the nervus transversarius—which accompanies the vertebral
artery; through it rami come from the second to the sixth cervical
nerves inclusive.
A =middle cervical ganglion= may be present, and occurs oftenest on the
right side. It lies on the carotid artery an inch or two in front of the
first rib and is intercalated in the vago-sympathetic trunk.
The =cardiac nerves= (Nn. cardiaci) are formed by branches from the
inferior cervical and first thoracic ganglia, together with twigs from
the sympathetic trunks and the vagi. They form a plexus (P. cardiacus)
on the ventral face of the trachea with each other and with branches of
the vagus and recurrent nerves. They are subject to considerable
variation, but their general arrangement may be stated as follows:
(_a_) On the right side there are usually two cardiac nerves. Of these
one passes back with the vagus in the angle between the right brachial
artery and the common carotid trunk, pierces the pericardium, crosses
the aortic arch, and divides into branches which are mingled with those
of the left nerves. The second crosses obliquely over the right face of
the trachea and joins the vagus, where the latter gives off the right
recurrent nerve; a small plexus is formed here, from which two or three
cardiac branches are detached. These pass back beneath the trachea and
ramify on the atria and ventricles.
(_b_) On the left side there are commonly three cardiac nerves. One of
these is distributed to the great vessels in the anterior mediastinum.
The largest passes back at first with the vagus beneath the arch of the
left brachial artery, inclines downward, perforates the pericardium, and
divides into two branches. One branch passes beneath the bifurcation of
the pulmonary artery and is distributed to the left atrium. The larger
branch dips in between the aorta and the pulmonary artery, gives twigs
to these vessels, and ramifies on the ventricles, especially along the
course of the right coronary artery. The third nerve crosses the deep
face of the left brachial artery, passes back below the trachea, and
unites with filaments of a right cardiac nerve. It passes around the
aorta and ramifies chiefly along the course of the left coronary artery
on the left face of the ventricles.
3. The =thoracic part= (Pars thoracalis systematis sympathici) extends
below the costo-vertebral joints from the inferior cervical ganglion to
the upper border of the diaphragm, and passes between the latter and the
psoas minor to be continued by the abdominal part.
The =trunk= is concealed in the first part of its course by the outer
border of the longus colli, but further back it is visible under the
pleura.
The =ganglia= are arranged segmentally at each intercostal space and
partly on the heads of the ribs. They are flattened and are small and
fusiform, with the exception of the first. This (G. thoracale primum) is
extensive, irregularly quadrilateral in outline, and is united with the
inferior cervical ganglion. The ganglia are connected with the thoracic
nerves by white and gray rami communicantes.
The =visceral branches= comprise aortic, cardiac, pulmonary, and
œsophageal branches, and the splanchnic nerves. The =aortic branches=
ramify on the thoracic artery, forming around that vessel the =thoracic
aortic plexus= (P. aorticus thoracalis). The =cardiac branches= concur
with those of the vagus in forming the =cardiac plexus= (P. cardiacus).
From this branches go to form the =coronary plexuses= (P. coronarii)
along the course of the vessels of like name. The =pulmonary branches=
join with corresponding branches of the vagus and filaments from the
cardiac plexus in forming the =pulmonary plexus= (P. pulmonalis) at the
root of the lung. Branches of the plexus, on which are minute ganglia,
ramify with the bronchi in the substance of the lung.[211] The
=œsophageal branches= join with those of the vagus in the formation of
the =œsophageal plexus= (P. œsophageus).
The =great splanchnic nerve= (N. splanchnicus major) arises by a series
of roots derived from the sixth or seventh to the fifteenth or sixteenth
thoracic ganglia inclusive. It extends at first along the inner side of
the thoracic trunk, then crosses the latter ventrally, passes back
between the crus of the diaphragm and the outer border of the psoas
minor, and joins the cœliac ganglion. It is small at its origin, but
becomes considerably larger than the sympathetic trunk. Near its
termination it bears the small =splanchnic ganglion=, from which and
from the nerve, filaments go to the aorta, the œsophagus, and the
vertebræ.
The =small splanchnic nerve= (N. splanchnicus minor) is formed by roots
derived from the last two or three thoracic ganglia. It runs back with
the great splanchnic nerve, but ends in the cœliac or in the renal and
adrenal plexuses.
This nerve is not always present and is variable in arrangement. The
roots communicate by filaments with the great splanchnic.
4. The =abdominal part= of the sympathetic trunk (Pars abdominalis
systematis sympathici) lies along the inner border of the psoas minor,
above the aorta on the left side and the posterior vena cava on the
right (Fig. 524). The trunk is smaller than the thoracic part, and
presents usually six small fusiform =lumbar ganglia= (G. lumbalia),
which are connected by rami communicantes with the ventral divisions of
the lumbar nerves. Visceral branches go to the aortic and hypogastric
plexuses, and to the cœliac and mesenteric ganglia.
5. The =pelvic part= of the sympathetic trunk (Pars pelvina systematis
sympathici) begins at the last lumbar ganglion and extends along the
pelvic surface of the sacrum internal to the emergence of the ventral
branches of the sacral nerves (Fig. 451). At the third segment of the
sacrum the trunk divides into internal and external branches. The
internal branch inclines to the median plane and unites with the
opposite branch. At the junction there is often found the small
=coccygeal ganglion= (G. impar), which lies on the coccygeal artery at
the joint between the first and second coccygeal vertebræ. A filament
from the ganglion accompanies the artery. The external branch
communicates with the last two sacral nerves and joins the ventral
coccygeal nerves. Ganglia occur near each of the first three sacral
foramina, and are connected by gray rami communicantes with the adjacent
spinal nerves. The visceral branches are distributed through the
hypogastric plexus. They supply motor fibers to the longitudinal and
inhibitory fibers to the circular muscular coat of the rectum, motor
fibers to the bladder and uterus, and the vasodilator fibers (Nn.
erigentes) to the penis.