154 THE HUMAN BODY 



nucleus; (4) the medulla; all the gray matter of the medulla is 

 contained within its nuclei. They constitute the so-called deep- 

 origins of those cranial nerves which arise in the medulla. 



All nerve-ganglia in the Body, using the term ganglia in the 

 restricted sense suggested above, fall into two groups: (1) Those 

 which contain the cell-bodies of sensory neurons; in this group 

 belong all dorsal root-ganglia of spinal nerves (see p. 143), like- 

 wise the ganglia which are found on some of the cranial nerves; 

 (2) the so-called sympathetic ganglia which are described in the 

 next paragraph. 



The Sympathetic or Autonomic System. The ganglia which 

 form the main centers of the sympathetic nervous system lie in 

 two rows (s, Fig. 2), one on either side of the bodies of the vertebrae. 

 Each ganglion is united by a nerve-trunk with the one in front of 

 it, and so two great chains are formed reaching from the base of 

 the skull to the coccyx. In the trunk region these chains lie in the 

 ventral cavity. The ganglia on these chains are forty-nine in 

 number, viz., twenty-four pairs, and a single one in front of the 

 coccyx in which both chains terminate. They are named from the 

 regions of the vertebral column near which they lie; there being 

 three cervical, twelve thoracic, four lumbar, and five sacral pairs. 



Each sympathetic ganglion is united by communicating branches 

 with the neighboring spinal nerves, and near the skull with various 

 cranial nerves also ; while from the ganglia and their uniting cords 

 arise numerous trunks, many of which, in the thoracic and abdom- 

 inal cavities, form plexuses, from which in turn nerves are given 

 off to the viscera. These plexuses frequently possess numerous 

 ganglia of their own; two of the most important are the cardiac 

 plexus which lies on the dorsal side of the heart, and the solar 

 plexm which lies in the abdominal cavity and supplies nerves to 

 the stomach, liver, kidneys, and intestines. Many of the sympa- 

 thetic nerves finally end in the walls of the blood-vessels of various 

 organs. To the naked eye they are commonly grayer in color 

 than the cerebro-spinal nerves. 



