SYMPATHETIC NERVOUS SYSTEM IN MAN 181 



gators, according to whom the primordia of the entire sympa- 

 thetic nervous system are derived from the cerebrospinal gangHa 

 by the peripheral displacement of cells of neural-crest origin. 



SYMPATHETIC TRUNKS 



The primordia of the sympathetic trunks arise in human em- 

 bryos about 5 mm. in length as small groups of cells lying along 

 the dorsolateral aspects of the aorta in the lower thoracic and 

 upper abdominal regions. These cells may be recognized among 

 the mesenchymal cells by the slightly larger size and more 

 intense staining reaction of their nuclei. In embrj^os 6 mm. in 

 length (nos. 676, 242)- the sympathetic primordia are present 

 from the lower cervical to the sacral region. By reason of the 

 strong curvature of the embryo at this stage, the aggregates of 

 cells constituting the primordia of the segmental ganglia lie in 

 such close proximity with each other that the entire primordium 

 of the sympathetic trunk appears as a continuous column of 

 loosely aggregated cells. This condition obtains until the em- 

 bryos have reached a length of 9 to 10 mm. and the primordia of 

 the sympathetic trunks are present from the upper cervical to 

 the sacral region. Longitudinal fibers may not be observed in 

 the primordia of the sympathetic trunks until the segmental 

 character of the latter has become apparent. In the upper 

 thoracic and cervical regions the sympathetic primordia arise 

 along the dorsal aspects of the descending aortae. Doubtless 

 the position of these primordia is in part determined by the posi- 

 tion of the great arterial trunks. Inasmuch as the paired de- 

 scending aortae lie at an appreciable distance from the median 

 plane and converge toward the unpaired dorsal aorta, the pri- 

 mordia of the sympathetic trunks lie farther from the median 

 plane in the cervical and upper thoracic than in the abdominal 

 region. 



When the primordia of the sympathetic trunks first appear, 

 the cells are very loosely aggregated and the fibers of the com- 



- Numbers inserted in this manner are the serial numbers of human embryos 

 in the Carnegie Embryological Collection. 



