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7c. The Thoracic Portion of the Sympathetic System
FIG. 846 - Thoracic portion of the sympathetic trunk. (Pars Thoracalis S. Smypathici)
The thoracic portion of the sympathetic trunk (Fig. 846). consists of a series of ganglia, which usually correspond in number to that of the vertebræ; but, on account of the occasional coalescence of two ganglia, their number is uncertain. The thoracic ganglia rest against the heads of the ribs, and are covered by the costal pleura; the last two, however, are more anterior than the rest, and are placed on the sides of the bodies of the eleventh and twelfth thoracic vertebræ. The ganglia are small in size, and of a grayish color. The first, larger than the others, is of an elongated form, and frequently blended with the inferior cervical ganglion. They are connected together by the intervening portions of the trunk.Two rami communicantes, a white and a gray, connect each ganglion with its corresponding spinal nerve. The branches from the upper five ganglia are very small; they supply filaments to the thoracic aorta and its branches. Twigs from the second, third, and fourth ganglia enter the posterior pulmonary plexus. The branches from the lower seven ganglia are large, and white in color; they distribute filaments to the aorta, and unite to form the greater, the lesser, and the lowest splanchnic nerves. The greater splanchnic nerve (n. splanchnicus major; great splanchnic nerve) is white in color, firm in texture, and of a considerable size; it is formed by branches from the fifth to the ninth or tenth thoracic ganglia, but the fibers in the higher roots may be traced upward in the sympathetic trunk as far as the first or second thoracic ganglion. It descends obliquely on the bodies of the vertebræ, perforates the crus of the diaphragm, and ends in the celiac ganglion. A ganglion (ganglion splanchnicum) exists on this nerve opposite the eleventh or twelfth thoracic vertebra. The lesser splanchnic nerve (n. splanchnicus minor) is formed by filaments from the ninth and tenth, and sometimes the eleventh thoracic ganglia, and from the cord between them. It pierces the diaphragm with the preceding nerve, and joins the aorticorenal ganglion. The lowest splanchnic nerve (n. splanchnicus imus; least splanchnic nerve) arises from the last thoracic ganglion, and, piercing the diaphragm, ends in the renal plexus. A striking analogy exists between the splanchnic and the cardiac nerves. The cardiac nerves are three in number; they arise from all three cervical ganglia, and are distributed to a large and important organ in the thoracic cavity. The splanchnic nerves, also three in number, are connected probably with all the thoracic ganglia, and are distributed to important organs in the abdominal cavity.
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