206 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XIII. 

 THE SYMPATHETIC SYSTEM. 



To understand the sympathetic sys.tem it is necessary to begin 

 with the study of its development. At a time when the spinal 

 ganglia and the dorsal and ventral nerve roots are formed there is 

 noticed on the mesial side of the composite ventral ramus of each 

 nerve a collection of ganglion cells which later forms the sympa- 

 thetic ganglion. A stage a little earlier than this has been recog- 

 nized in mammals. The beginning of the development of the 

 sympathetic is the outgrowth of fibers from the ventral root and also 

 from the dorsal root ganglion, in the direction of the aorta. Then 

 cells from the spinal ganglion are seen to migrate along these 

 fibers. These constitute the group of cells first mentioned. As 

 development proceeds the group of cells moves away from the 

 spinal ganglion toward the aorta, but remains connected with 

 the spinal nerve by the strand of fibers which grew out first. There 

 are thus formed a pair of ganglia in each segment, lying below the 

 notochord and lateral to the aorta, and connected with the spinal 

 nerves by the rami communicantes. The ganglia are those known 

 as the ganglia of the chain but at this stage they are not yet con- 

 nected longitudinally into a chain. 



The process of growth of fibers and the migration of cells along 

 them continues beyond these chain ganglia and results on the one 

 hand in joining the ganglia together by longitudinal cords, and on 

 the other hand in the formation of additional ganglia. The cells 

 which migrate from the chain ganglia form first certain median 

 prevertebral ganglia or plexuses. These are in man the cardiac, 

 solar and hypogastric plexuses. Further migration of cells 

 carries them to or even among the tissues of several of the organs 

 innervated by the sympathetic, where peripheral ganglia are 

 formed. Examples of these are the small ganglia in the heart, 

 the plexuses of Auerbach and Meissner in the wall of the digestive 



