CHAPTER XL 



THE PHYLOGENETIC ORIGIN OF THE SYMPATHETIC NERVOUS 



SYSTEM. 



IN all the higher vertebrates we find three characteristic groups 

 of motor nerve cells, which have travelled out from the central 

 nervous system to form the vagus, sympathetic and pelvic 

 groups ; but in the lowest fishes, the Cyclostomata, the only 

 one of these three groups which has been clearly recognized 

 is that belonging to the vagus. Along the whole of the 

 intestine of Ammoccetes the vagus nerve can be traced, and 

 in connexion with its fibres nerve cells are found in the walls 

 of the intestine itself, right up to the end of the intestine proper. 

 The intestine is characterized by the presence of the so-called 

 spiral valve, and terminates in a chamber, the hind gut or cloaca, 

 into which the segmental duct from the pronephric organ opens 

 on each side. Upon this cloaca Dohrn has seen scattered nerve 

 cells lying close on its surface. These he calls sympathetic, but 

 clearly he uses the term vaguely and not as it is used in physio- 

 logical writings. Dohrn's cells correspond in position to the cells 

 of the pelvic plexus in higher vertebrates, and in my opinion in- 

 dicate that group, and not any of the sympathetic group of cells. 

 In the Ammocoetes then the cloacal and intestinal cell groups 

 are both present in the same positions as in the higher verte- 

 brates, and therefore in all probability the sympathetic system of 

 cells must be present also in this animal. 



It is impossible to consider the origin of the sympathetic 

 nervous system in vertebrates, without taking into account 

 the distribution of the chromaffine system of cells ; for the 

 two systems are closely bound up together. My son, J. F. 

 Gaskell, has been investigating this relationship both in verte- 

 brates and invertebrates. In the adult mammal the chromaffine 

 system is confined to the medulla of the suprarenals, but in the 

 embryo chromaffine cells are found in close connexion with 



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