PHYLOGENETIC ORIGIN OF NERVOUS SYSTEM 141 



cells in all cases in close relationship with the segmental nerves 

 from the posterior root ganglia. Where the cardinal veins fuse 

 together to form the sinus of the heart, the chromaffine cells 

 form a mass on the dorsal wall of the sinus, and extend over 

 the walls of the cceliac artery, which passes through the sinus 

 obliquely. In the branchial region the chromaffine cells are 

 found along the segmental veins up to the second branchial 

 segment, but much more sparsely than in the segments posterior 

 to the heart. Headwards of this limit no chromaffine cells have 

 been found in connexion with any segment. 



Comparative anatomy thus shows most clearly the close 

 relationship between the chromaffine and sympathetic systems, 

 and the remarkable increase in the latter system, apparently at 

 the expense of the former, which has taken place, commencing 

 in the lowermost groups of vertebrates. 



Embryology emphasizes in the most positive manner the 

 close relationship between the two systems. Kohn has shown 

 that the sympathetic ganglia and the chromaffine cells arise in 

 the embryo from a common mass of cells, part of which gives 

 rise to the sympathetic ganglia and the rest form chromaffine 

 cells ; so intimate is the relationship according to his researches 

 that he has given the name of paraganglia to the various groups 

 of chromaffine cells. 



Combining then the evidence of Kohn with that of Onodi 

 it follows that the cells which gave origin to both the sympathetic 

 and chromaffine systems of cells were originally in close contact 

 with the posterior root ganglia, and have travelled out from that 

 position. The truth of the Law of Recapitulation is shown yet 

 again by the relationships found in the lowest vertebrates the 

 Cyclostomata where masses of chromaffine cells still retain that 

 position even in the adult animal. 



The physiological evidence points in the same direction as 

 the embryological, for the work of Elliott and others has shown 

 that a discharge of adrenaline with a consequent rise of blood 

 pressure takes place through the action of the splanchnic nerves ; 

 therefore the activity of these cells in the medulla of the supra- 

 renal glands can be brought about by excitation of the fibres of 

 the splanchnic nerves, which go to the suprarenals. Further 

 he has shown that the fibres which pass into the suprarenals 

 from the splanchnics are all medullated fibres, and are therefore, 



