30 



INTRODUCTION TO NEUROLOGY 



(whose structures are very diverse) will not be further consid- 

 ered in this work; the nervous systems of all vertebrates, how- 

 ever, are constructed on a common plan, and, though our prime 

 interest is the analysis of the human nervous system, we shall 

 find that many of the details sought can be seen much more 

 clearly in the lower vertebrates than in man. 



Fig. 3. The nervous systems of four species of flies, to illustrate the 

 various degrees of concentration of the ganglia: A, Chrionomus plumosus, 

 with three thoracic and six abdominal ganglia; B, Empis stercorea, with 

 two thoracic and five abdominal ganglia; C, Tabanus bovinus, with one 

 thoracic ganglion and the abdominal ganglia moved toward each other; D, 

 Sarcophaga carnaria, with all thoracic and abdominal ganglia united into 

 a single mass. (After Brand, from Lang's Text-book of Comparative 

 Anatomy.) 



Correlated with these differences between the structure of 

 invertebrate and vertebrate nervous systems there are equally 

 fundamental differences in the behavior of these animals which 

 require a few words of further explanation. Living substance 

 exhibits as its most fundamental characteristic, as we saw at the 

 beginning, the capacity of adjusting its own activities to con- 

 stantly changing environmental conditions in such a way as to 

 promote its own welfare. This adjustment may be effected 



