June, 1920] Origin of Cerebral Ganglia 301 



The next fact historically that is of great significance to this 

 discussion is the development of the doctrine of nerve com- 

 ponents. The first effect of this doctrine on our problem was 

 the establishment of the conception that there were more ganglia 

 in the head than had been suspected, since a given ganglion 

 such as the vagus may contain a number of components, and, 

 further, to emphasize the importance, not of the individual 

 nerves and ganglia, but of the component parts of which they 

 are composed. The significance of this is apparent when 

 your attention is called to the fact that seven out of twelve 

 cerebral nerves, exclusive of the nervus terminalis, possess 

 ganglia; when, however, you enumerate the components of 

 these ganglia, there are found about twenty different ganglionic 

 masses, the origin of which must be accounted for and these 

 are distributed among only four components. Evidently, the 

 mode of attack must be changed from that of individual nerves 

 and ganglia to that of their components. Fortunately, there 

 are not twenty different modes of origin for these components. 

 There are about five, at the most; but the point to be emphasized 

 is that these five are distributed among components rather than 

 among ganglia. 



The general tendency of the nerve component work has been 

 to emphasize the analysis of nerve s rather than of ganglia. This 

 was perfectly natural and inevitable. There is always, in 

 adult types, more or less combination of these various ganglionic 

 components, which renders the identification of the individual 

 ganglionic masses somewhat difficult. By the study of embryos 

 in favorable stages it has been possible to isolate and describe 

 in detail these ganglionic masses and to plot them. This 

 analysis was evidently the second step in the effort to determine 

 the mode of origin of these ganglia. It would have been utterly 

 impossible to write an accurate description of the origin of 

 these ganglia without knowing how many there were and how 

 they were distributed in the various cerebral nerves. The 

 complications that have arisen from ignoring this difficulty 

 are almost innumerable. 



The next step, evidently, was to determine the origin of the 

 ganglia whose number, position and morphological relations 

 are known. The diversity in modes of origin is not nearly so 

 great (as mentioned above) as are the number of ganglia, and, 

 in fact, barring the first and second nerves, not so numerous 

 as the number of sensory components. 



