300 George L, Streeter 



The lower pair corresponds to the sciatic nerve, and its median bundle 

 constitutes the future tibial nerve, and the lateral the common peroneal. 

 From these larger nerves smaller branches split off and become isolated 

 as discrete nerves, and enter the muscle masses between which the main 

 trunks lie. 



The Sympathetic System, 



The ventral migration of the cells derived from the neural crest, that 

 are destined to become sympathetic ganglion cells, is completed by the 

 end of the first month. The elements derived from the successive seg- 

 ments have by that time fused together, on each side of the body, to form 

 a longitudinal column of proliferating cells, situated lateral to the aorta 

 and directly in front of the developing bodies of the vertebrae. 



This column extends as a rather sharply outlined continuous cellular 

 strand from the occipital region to the level of the lower sacral verte- 

 brae. The differentiation of the individual cells is already under way 

 and nerve fibre formation can be recognized throughout its length, being 

 most pronounced in the thoracic and lower cervical region. This pro- 

 cess however has not advanced far enough to produce a breaking apart 

 of the column into segmental nodes, as is characteristic of the adult 

 ganglionic chain. 



Dorsally the column remains in connection with the nerve roots 

 and spinal ganglia by means of the rami communicantes , which consist 

 of sharply outlined fibrous bundles, varying from one to three bundles to 

 each segment. The rami communicantes are largest and best developed 

 in the thoracic and lumbar regions. In the cervical region they are 

 least well developed, and owing to the slanting course of the upper cervi- 

 cal nerves they could not be satisfactorily traced in sagittal sections for 

 the upper three segments. 



Ventrally in the region of the sixth to the eleventh thoracic segment 

 a definite fibro-cellular plexus extends forward to form the splanchnic 

 nerves and cceliac plexus. The splanchnic nerves extend almost directly 

 forward. As is the case with the phrenic nerve their caudal 

 course is brought about later by the subsequent caudal displacement 

 of the viscera relative to the bodies of the vertebrae. At this time the 

 diaphragm and caudal surface of the heart lie opposite the first and 

 second thoracic vertebrae, and the stomach is opposite the fifth to 

 tenth thoracic vertebrae so that the coeliac region is directly ventral 

 to the origin of the splanchnic nerves from the ganglionated cord. 



