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that two spinal nerves are supplied from one ganglion; or two ganglia may supply branches to a single spinal nerve. The gray ramus is distributed along the somatic divisions of the spinal nerves, supplying branches to unstriped muscular fibres (vaso-motor, pilo-motor) and glands (secretory). They also provide small recurrent branches, ending in the membranes enveloping the spinal nerve-roots. Mingled with the non-medullated fibres of the gray rami are a small number of medullated fibres, regarded as afferent fibresaxons passing to the spinal ganglia which are incorporated with the gray rami.
The connecting cords of the sympathetic system are composed of white and gray fibres. The white fibres are: (1) splanchnic efferent fibres, passing to a ganglion above or below the point of entrance into the sympathetic system; (2) splanchnic afferent fibres, guided along the connecting cord and over or through the ganglia. The gray fibres are the axons of sympathetic ganglion cells: (1) true association fibres passing into connexion with the cells of a neighbouring ganglion; (2) fibres passing along the connecting cord for a certain distance upwards or downwards before entering the splanchnic area as peripheral branches.
The peripheral branches of the sympathetic trunk consist of (1) white fibres, which may be either splanchnic afferent fibres on their way from the viscera through the gangliated trunk to the spinal ganglia, or splanchnic efferent fibres which, after traversing the gangliated trunk, proceed to join and end in collateral or terminal ganglia in relation to viscera; (2) gray fibres, efferent branches, the axons of the ganglion cells, distributed on the one hand peripherally to the vessels and viscera of the splanchnic area, and on the other hand centrally through the gray rami communicantes and the somatic divisions of the spinal nerves, to the glands and involuntary muscles in the somatic area, as secretory, and vaso-motor and pilo-motor fibres.
FIG. 636.-SCHEME OF THE CONSTITUTION AND CONNEXIONS OF THE
gangliated trunk is indicated on the right, with the arrangement of the fibres arising from the ganglion cells. On the left the roots and trunks of spinal nerves are shown, with the arrangement of the white ramus communicans above and of the gray ramus below.
Although forming always one continuous cord, the sympathetic system may
or cavernous plexus (pl. cavernosus). Both plexuses supply offsets to the artery and its branches, and form communications with certain cerebral nerves.
Pl. Caroticis Internus.-The internal carotid plexus communicates by fine
branches with (a) the abducens nerve, and (b) the semilunar ganglion, and gives off (c) the great deep petrosal and (d) the carotico-tympanic nerves. The deep
rosal nerve joins the greater superficial petrosal nerve from the genicular ganglion
of the facial, in the foramen lacerum. By their union the pterygoid nerve is formed, which, after traversing the pterygoid canal, ends in the spheno-palatine ganglion. The carotico-tympanic nerves pass to the tympanic plexus. This plexus, formed by the carotico-tympanic nerves, the tympanic branch of the glossopharyngeal, and a twig from the genicular ganglion of the facial nerve, is placed on the labyrinthic wall of the tympanum. It supplies the mucous lining of the tympanum and auditory tube; and the lesser superficial petrosal nerve passes from it to the otic ganglion.
Plexus Cavernosus.-The cavernous plexus communicates with (a) the oculo-motor, (b) the trochlear nerve, and (c) the ophthalmic division of the trigeminal nerve; it also (d) supplies twigs to the hypophysis (pituitary body), and (e) forms the sympathetic root of the ciliary ganglion. This may pass to the ganglion independently, or it may be incorporated in the long root of the ganglion from the naso-ciliary branch of the ophthalmic nerve.
GANGLION CERVICALE MEDIUS.
Central Communicating Branches.-1. Gray rami communicantes arise from the ganglion or from the connecting cord, and join the anterior rami of the fifth and sixth cervical nerves. 2. The ansa subclavia (Vieussenii) is a loop of communication from this ganglion, which, after passing in front of and supplying offsets to the subclavian artery and its branches, joins the inferior cervical ganglion.
Peripheral Branches of Distribution.-1. Heart.-A slender middle cardiac branch descends, either separately or in company with other cardiac nerves, behind the large vessels into the thorax, where it ends in the deep part of the cardiac plexus on each side.
2. Thyreoid Gland.-Branches extend medially along the inferior thyreoid artery to supply the thyreoid gland.
When the middle ganglion is absent the branches described arise from the connecting cord.
GANGLION CERVICALE INFERIUS.
Central Communicating Branches.-1. Gray rami communicantes arise from this ganglion for the anterior rami of the seventh and eighth cervical nerves. 2. The ansa subclavia already mentioned connects the middle and inferior ganglia over the front of the subclavian artery. 3. A communication frequently occurs with the recurrent nerve.
Peripheral Branches of Distribution.-1. Heart.—An inferior cardiac branch is given off on each side to enter the deep cardiac plexus.
2. Vessels. (a) The vertebral plexus is a dense plexus of fibres surrounding the vertebral artery and accompanying its branches in the neck and the cranial cavity. (b) The subclavian plexus is derived from the ansa subclavia (subclavian loop), and supplies small offsets to the subclavian artery. It gives branches to the internal mammary artery, and communicates with the phrenic nerve.
II. PARS THORACALIS SYSTEMATIS SYMPATHICI.
The thoracic part of the sympathetic trunk lies behind the pleura, in front of the necks of the ribs, and the intercostal vessels and nerves. It consists of a number of ganglia of an irregularly angular or fusiform shape, joined together by connecting bands of considerable thickness. The number of ganglia is usually ten or eleven; but the first and sometimes others may be so fused with the neighbouring ganglia as to reduce the number still further.
This part of the sympathetic trunk is characterised by its union with the thoracic spinal nerves. Each thoracic nerve, with the probable exception of the first, sends a visceral branch (white ramus communicans) to join the gangliated trunk in the thorax. These white rami separate into two main streams in relation to the sympathetic trunk. Those of the upper five nerves are for the most part directed upwards to be distributed through the cervical part of the sympathetic trunk
in the manner already described. The white rami of the lower thoracic nerves are for the most part directed downwards in the inferior part of the sympathetic trunk and its branches, to be distributed in the abdomen; at the same time some of their fibres are directly supplied to certain thoracic viscera,-lungs, aorta, cesophagus. These white rami are composed of (1) splanchnic afferent fibres passing from its peripheral branches through the sympathetic trunk into the ganglia of the
Nervus caroticus internus
FIG. 639. THE CARDIAC PLEXUSES.
spinal nerves-medullated nerve-fibres unconnected with sympathetic ganglion cells; and (2) somatic and splanchnic efferent fibres, small medullated nerves which, after a longer or shorter course in the gangliated trunk or its peripheral branches, become connected with the sympathetic ganglion cells, or with the cells of peripheral (collateral or terminal) ganglia, from which again (non-medullated) axons proceed to supply branches to viscera and vessels. The ultimate destination of the upper stream of white rami from the thoracic nerves has been mentioned in the description of the cervical sympathetic.
The peripheral branches supplying thoracic organs contain vaso-motor fibres for the lungs and aorta. The peripheral branches from the lower part of the sympathetic trunk in the thorax, receiving white rami from the lower thoracic nerves, are distributed mainly to structures below the diaphragm. They comprise (a) viscero-inhibitory fibres for the stomach and intestines; (b) motor fibres for part of the rectum; (c) pilo-motor fibres for the lower part of the body; (d) vasomotor fibres for the abdominal aorta and its branches, and for the lower limbs; (e) secretory, and (f) sensory fibres for the abdominal viscera.
The branches from the gangliated trunk are, as in the neck, divisible into two sets (4) Central branches, communicating with other nerves, and (B) peripheral branches, distributed in a plexiform manner to the thoracic and abdominal viscera.
(A) Central Communicating Branches.-The white rami communicantes from the thoracic nerves have already been described. Passing forwards from the anterior rami of the nerves, they become connected with the ganglia or the connecting cord of the sympathetic.
The gray rami communicantes are branches arising irregularly from each thoracic ganglion; passing backwards along with the white rami, they join the anterior rami of the thoracic nerves, and are distributed in a manner already described (p. 754).
(B) Peripheral Branches of Distribution. These branches arise irregularly from the ganglia and the connecting cord. They are composed of non-medullated (splanchnic efferent) fibres derived from the ganglion cells, and medullated fibres (splanchnic efferent and afferent) derived directly from the white rami, without the intervention of the cells of the ganglia.
1. Rami Pulmonales.-Pulmonary Branches. From the gangliated trunk opposite the second, third, and fourth ganglia fine filaments arise which join the posterior pulmonary plexus.
2. Rami Aortici.-Aortic Branches.-The upper part of the thoracic aorta receives fine branches from the upper five thoracic ganglia.
3. Nn. Splanchnici.-Splanchnic Nerves.-Three nerves arise from the inferior part of the gangliated trunk, partly from the ganglia themselves, and partly from the connecting cord between the ganglia. Passing downwards over the bodies of the thoracic vertebræ they pierce the diaphragm, to end in the abdomen.
(a) N. Splanchnicus Major.-The greater splanchnic nerve arises from the gangliated trunk between the fifth and ninth ganglia. By the union of several irregular strands a nerve of considerable size is formed, which descends in the posterior mediastinum, and piercing the crus of the diaphragm, joins at once the anterior end of the cœliac ganglion. In its course in the thorax the splanchnic ganglion is formed upon the nerve. It is more prominent in the foetus than in the adult. From both nerve and ganglion branches arise in the thorax, for the supply of the esophagus and descending thoracic aorta (Fig. 638).
(b) N. Splanchnicus Minor.-The lesser splanchnic nerve arises from the gangliated trunk opposite to the ninth and tenth ganglia. It passes over the bodies of the lower thoracic vertebræ, pierces the diaphragm near or along with the greater splanchnic nerve, and ends in the cœliac plexus (aortico-renal ganglion).
(c) N. Splanchnicus Imus.-The lowest splanchnic nerve arises from the last thoracic ganglion of the sympathetic, or it may be a branch of the lesser splanchnic nerve. It pierces the diaphragm, and ends in the renal plexus.
III. PARS ABDOMINALIS SYSTEMATIS SYMPATHICI.
The abdominal part of the sympathetic trunk is placed upon the bodies of the lumbar vertebræ, medial to the origins of the psoas major muscle, and in front of the lumbar vessels. It is connected with the thoracic portion of the trunk by an attenuated cord which either pierces or passes behind the diaphragm. It is continuous below with the pelvic portion of the trunk by means of a connecting cord, which passes behind the common iliac artery.
It is joined by medullated fibres (white rami communicantes) from the first two