SYMPATHETIC NERVES. 



95 



form the splanchnic nerves. These pass along the sides of the aorta, in 

 front of which they form a large plexus, the coeliac [or solar] plexus, as- 

 sociated with which is the coeliac (or semilunar] ganglion (Fig. 112). 

 A plexus is a net of nerves which allows a transfer of fibers from one 

 bundle to another; the individual nerve fibers probably do not anastomose. 

 In the sympathetic plexuses there are usually nerve cells, called ganglion 

 cells, often found at the angles of the network. In contact with them 

 the nerve fibers may terminate. When these cells are very abundant 

 the plexus becomes a ganglion. From the coeliac ganglion, fibers pass 

 into the intestine and form a ganglionated plexus between the muscle 

 layers, called the myenteric plexus. Branches from it innervate the 

 muscles and pass on to make another plexus under the intestinal epithelium, 

 the submucous plexus. Finally they come very 

 close to the epithelium itself. 



All of the nerve cells of the sympathetic 

 system are believed to be ectodermal, and de- 

 scendants of those which migrated from the 

 spinal ganglia or central nervous system. All 

 the sympathetic nerve fibers are processes of 

 such cells, and they are found forming plexuses 

 around the blood vessels and organs, including 

 those of the intestinal tract, the bladder, kid- 

 ney, suprarenal gland and spleen. Two fea- 

 tures of the sympathetic system seem funda- 

 mental; their fibers supply the viscera, and they 

 are so connected with peripheral ganglion cells, 

 that they act more or less independently of the 

 central nervous system. 



THE CEREBRAL NERVES. The nerves con- 

 nected with the brain are not a series of similar structures like the spinal 

 nerves. Four of them possess only ventral motor roots. Four others have 

 dorsal sensory roots provided with ganglia, and lateral motor roots. Lateral 

 roots emerge just ventral to, or beneath the dorsal roots. Their fibers are 

 the neuraxons of cells, the bodies of which remain within the central ner- 

 vous system. Lateral root fibers occur as far down the cord as the sixth 

 cervical ganglion. Instead of entering the corresponding cervical nerves, 

 however, these fibers unite to form a bundle which passes along just outside 

 of the spinal cord, through the foramen magnum into the skull where it 

 becomes the accessory portion of the vagus nerve. Below the sixth cervical 

 ganglion the lateral root elements have not been demonstrated. (It has 

 been suggested that they pass out in the dorsal roots, and that they form 

 parts of the ventral roots.) 



FIG. 112. 



Diagram of the sympathetic 

 system in its relation to the 

 intestine, int.; A., aorta; sp 

 g., spinal ganglion; w. r.. 

 white ramus; g. t., ganglion 

 of the sympathetic trunk 

 spl., splanchnic nerve; coe. g. 

 coeliac ganglion; my. pi. 

 myenteric plexus; sbm. pi. 

 submucous plexus. 



