1008 THE NERVOUS SYSTEM 



THE THORACIC PORTION OF THE SYMPATHETIC TRUNK 



The thoracic part of the trunk runs downwards on the heads of the ribs from the 

 first to the tenth, and then passes a little ventralwards on the sides of the bodies 

 of the lower two vertebra?. Above it is continuous with the cervical portion at 

 the root of the neck, behind the vertebral artery. Below it leaves the thorax dor- 

 sad to the medial lumbo-costal arch (arcuate ligament), or semetimes to the 

 lateral lumbo-costal arch, and continues into the lumbar portion of the trunk. 

 It lies behind the costal pleura and crosses over the aortic intercostal arteries. 



The number of ganglia in this part of the trunk is variable. There are usually 

 ten or eleven, but the first is sometimes fused with the inferior cervical ganglion and 

 occasionally other ganglia fuse. The ganglia are irregularly angular or fusiform in 

 shape, and'lie on the head of the ribs, on the costo-vertebral articulations, or on the 

 bodies of the vertebra'. The portions of the trunk connecting the ganglia usually are 

 single, but sometimes they are composed of two or three small cords in juxtapo- 

 sition. Each ganglion, with the possible exception of the first, receives a white 

 ramus communicans from a thoracic nerve and all give off grey rami communicantes 

 to these nerves. 



The white rami communicantes, as they approach the sympathetic trunk, 

 quite often appear double, due to the separation of a large portion of their fibres 

 into two main streams, one passing upwards in the sympathetic trunk, and one pass- 

 ing downwards. Of the white rami from the upper five thoracic nerves, the upward 

 stream of fibres is much larger than the downward, due to the fact that a greater 

 part of the efferent splanchnic fibres from these nerves are distributed through the 

 cervical portion of the sympathetic trunk, as noted above in the construction of that 

 portion. Usually the white rami from the nerves pass directly to the corresponding 

 ganglia of the trunk, and thus lie in company with the corresponding grey rami. Some- 

 times, however, they may join the intermediate portions of the trunk, and in the lower 

 thoracic region especially, a ramus may pass from a nerve to the ganglion corre- 

 sponding to the nerve above or below. The fibres of the white rami from the lower 

 thoracic nerves are in greater part directed downwards in the sympathetic trunk, and 

 also in its peripheral branches, to be distributed to the abdominal viscera. In all 

 cases, however, some of the fibres of the thoracic white rami terminate in the 

 ganglia nearest their junction with the trunk, while others pass into the nearest 

 peripheral branches. In this way the white rami from all the thoracic spinal 

 nerves, especially those of the mid-region, are directly concerned in the innervation 

 of the thoracic viscera, lungs, oesophagus, aorta, etc. 



The first thoracic ganglion is larger than the other ganglia of this region and is 

 irregular in form. It may be narrowly ovoid or semilunar. It lies in front of the 

 neck of the first rib, behind the pleura, and on the inner side of the costo-cervical 

 trunk (superior intercostal artery); this vessel separates it from the prolongation 

 of the portion of the first thoracic nerve which passes to the brachial plexus. It 

 sometimes fuses with the inferior cervical ganglion, and, on the other hand, sometimes 

 extends to the upper part of the second rib to fuse with the second thoracic ganglion. 

 The result of the latter fusion resembles the stellate ganglion of the carnivora, and 

 when it occurs, is sometimes referred to as. the ganglion stellatum. When largely 

 developed, the first ganglion sends a branch to the cardiac plexus, the fourth 

 cardiac nerve of Valentin. 



The second thoracic ganglion, triangular in shape and almost as large as the 

 preceding, is sometimes.placed on the costo-vertebral articulation, and is sometimes 

 partly concealed by the first rib. 



The third to the ninth thoracic ganglia are usually placed opposite the 

 heads of the corresponding ribs, but the tenth and eleventh may lie on the bodies of 

 the vertebrsE. 



The fibres passing from the ganglia form two groups of branches, the central and 

 the peripheral. 



The central branches are the grey rami communicantes, which pass from the 

 ganglia to the corresponding spinal nerves. After they have united with the an- 

 terior primary divisions of the nerves, the fibres of these rami divide into four groups: 

 (1) Fibres which pass inwards along the roots of the nerves to supply the mem- 

 branes of the spinal cord, or enter a meningeal or recurrent branch for the same 

 purpose; (2) fibres which enter the spinal ganglion and terminate there (sensory 



