THE SYMPATHETIC TRUNK 1039 



ganglia (fig. 787). It is usually formed by branches from the fifth to the tenth. The superior 

 branch, usually the largest, receives smaller inferior branches from the lower ganglia as it passes 

 downward on the sides of the bodies of the vertebrae in the posterior mediastinum. The nerve 

 enters the abdominal cavity by passing through the crus of the diaphragm, and joins the upper 

 end of the coeliac (semilunar) ganglion of the cceliac (solar) plexus. Near the disk between the 

 eleventh and the twelfth thoracic vertebra there is formed on the nerve the splanchnic ganglion. 

 Filaments from the nerve and from this ganglion pass along the intercostal arteries to the aorta, 

 oesophagus, and the thoracic duct, and some fibres from the right side pass to the vena azygos 

 (major). Sometimes this nerve divides into two cords, giving off numerous branches which 

 anastomose with each other and with the lesser splanchnic nerve to form a plexus, in the meshes 

 of which are found some small ganglia. 



(2) The lesser splanchnic nerve receives fibres from the ninth and tenth ganglia. Its 

 course is similar to that of the great splanchnic nerve (fig. 787), but on a more dorsal plane, and 

 it terminates in the cceliac (solar) and renal plexuses. 



' (3) The least splanchnic nerve, not always present, arises from the last thoracic ganglion 

 or sometimes from the small splanchnic nerve. It passes through the crus of the diaphragm 

 and ends in the renal plexus. 



Construction of the thoracic portion of the cord. — The majority of the visceral efferent 

 fibres which pass from the central nervous system enter the thoracic portion of the sympathetic 

 trunk; some end there in ramifications around the cells of its ganglia, while others merely pass 

 through on their way to more distant terminations. With regard to those which terminate 

 in the gangha, it has been showoi that in the dog and cat many end in the ganghon stellatum 

 which corresponds with the last cervical and the upper three or four thoracic ganglia in man. 

 Among these are the fibres conveying secretory impulses to the sweat-glands of the upper limb, 

 which emerge from the spinal cord in the thoracic nerves from the sixth to the ninth, and, in 

 the dog, those which convey and transfer vaso-constrictor impulses to the sympathetic neurones 

 supplying the pulmonary blood-vessels. These visceral efferent fibres leave the spinal cord 

 in the second to the seventh thoracic nerves. Other fibres which terminate around the thoracic 

 sympathetic ganglion-cells in the dog and cat are the vaso-constrictor fibres for the upper limbs 

 and some of the vaso-constrictor fibres for the lower limbs. 



Of the fibres which traverse the thoracic portion of the sympathetic trunk to gain more 

 distant terminations, some ascend to the cervical region (p. 1033), others descend to the lumbar 

 region, and many pass by the immediate peripheral branches to the splanchnic nerves. 



Among those which descend to the lumbar region are pilo-motor fibres, vaso-motor fibres, 

 and secretory fibres to the lower limb, some vaso-constrictor fibres to the abdominal blood- 

 vessels, motor fibres to the circular, and inhibitory fibres to the longitudinal muscle of the 

 rectum. The latter enter the sympathetic trunk by the lower thoracic nerves and pass in the 

 lumbar peripheral branches to the aortic plexus, and terminate around the cells of the inferior 

 mesenteric ganglion. 



The visceral efferent fibres which pass through the thoracic ganglia to the splanchnic nerves 

 are mainly vaso-motor fibres to the abdominal blood-vesse!s; the majority of them probably 

 terminate around the cells of the ganglia in the coelic (solar) plexus, but those for the renal blood- 

 vessels no doubt end in the renal ganglia. In addition to all the above-mentioned fibres there 

 are in the thoracic part of the sympathetic trunk afferent fibres of both sympathetic and cere- 

 bro-spinal type, passing toward the spinal ganglia and the latter, greatly predominating, pass 

 into the dorsal roots of the thoracic spinal nerves. 



THE LUMBAR PORTION OF THE SYMPATHETIC TRUNK 



The lumbar portion of each trunk lies on the fronts of the bodies of the verte- 

 brae along the anterior border of the psoas muscle, and nearer to the median line 

 than the thoracic portion. It is connected with the thoracic portion of the 

 sympathetic trunk by a slender intermediate portion of the trunk that may pass 

 through the diaphragm or dorsal to it (fig. 787). The continuation of the 

 lumbar into the sacral portion is also slender, and descends dorsal to the common 

 iliac artery. The right trunk is partly covered by the vena cava inferior and the 

 left by the aorta. 



The ganglia, which are small and oval, vary in number from three to eight, but 

 are usually four. Rarely they are so fused as to form one continuous ganglion. 



White rami communicantes pass to the ganglia from the first two or three 

 lumbar nerves only. This portion of the sympathetic trunk also receives visceral 

 eft^erent and afferent fibres which are derived from the white rami communicantes 

 of the lower thoracic nerves and continue downward in the trunk. 



Branches. — As in the thoracic region, the branches from the ganglia are central and per- 

 ipheral. The central are grey rami communicantes. There may be two branches to a nerve 

 or one ramus may divide so as to join two adjacent spinal nerves. Sometimes a spinal nerve 

 may receive as many as five grey rami from the sympathetic trunk. 



The peripheral branches include: — (a) Branches passing to the aorta and taking part in 

 the formation of the aortic plexus; (b) branches which descend in front of the common ihac 

 artery to the hypogastric plexus; and (c) branches to the vertebrae and ligaments. 



