THE NERVOUS SYSTEM 463 



the relation of prse- and post-trematic branches to their cor- 

 responding gill-slits. 



Later researches have modified these diagrams somewhat, 

 as, for example, the discovery of transitory sensory roots for 

 the hypoglossal elements, and indications of other nerves an- 

 terior to the Ophthalmicus profundus; the spino-occipital 

 nerves, although of unknown value, seem indicative of still 

 other somites in the occipital region and need to be thoroughly 

 explained before the problem of the segmentation of the head 

 can receive its final solution. 



There remains to be mentioned an auxiliary system of gan- 

 glia and nerve fibers, not directly under the control of the will, 

 but often of great importance in regulating the physiological 

 activities of certain of the internal organs. This is the sym- 

 pathetic system, often erroneously treated as originally a dis- 

 tinct nervous system coordinate with the cerebro-spinal, thus 

 far the subject of this chapter. As a matter of fact, however, 

 the sympathetic system is an integral portion of the latter, and 

 its differentiation from this may be followed both in the race 

 history as well as during individual development. It attains 

 its greatest degree of individuality only among the higher 

 forms. It consists primarily of a series of ganglia, segmented 

 off from the sensory ganglia of the spinal nerves, although re- 

 taining connection with their places of origin through commu- 

 nicating branches. The ganglia of the two sides, which come 

 to lie ventral to the spinal nerves on either side of the ver- 

 tebral column, may become secondarily connected with one an- 

 other by longitudinal connectives, thus forming two lateral 

 trunks. 



This appearance of metameric ganglia connected in two lon- 

 gitudinal series, and especially their ventral position, has led 

 the sympathetic system to be compared to the ventral chain 

 of ganglia found in articulates (e.g., insects, crustaceans), a 

 suggestion of homology that is sufficiently disproved by the 

 mode of origin and the fact that the similarity is most perfect 

 in the higher forms. Indications suggest that the separation 

 of sympathetic ganglia began historically in the head, since in 



