28 



INTRODUCTION TO NEUROLOGY 



of the different parts of the body. Thus arose the central ner- 

 vous system. (Some works dealing with the evolution of the 

 nervous system are cited at the end of this chapter.) 



The aggregations of nervous tissue to which reference has just 

 been made, containing the bodies of the nerve-cells, are called 

 ganglia, 1 and in all invertebrate animals the central nervous 

 system is a series of such ganglia, variously arranged in the body 

 and connected by strands containing nerve-fibers only, that is, 

 by nerves. 



Superior ganglia 

 Pharynx 

 Inferior ganglia 



Ventral ganglia 



Fig. 2. The anterior end of an earthworm (Lumbricus) laid open from 

 above with all of the organs dissected away except the ventral body wall 

 and ventral ganglionic chain. 



The central nervous systems of all but the lowest forms of 

 animals are developed in accordance with two chief structural 

 patterns, represented in typical form by the worms and insects 

 on the one hand, and by the back-boned animals or vertebrates 

 on the other hand. 



In the segmented worms (such as the common earthworm, 

 Fig. 2) the central nervous system consists of a chain of ganglia 

 connected by a longitudinal cord along the lower or ventral wall 



1 On the ganglia of the vertebrate nervous system, see page 108. 



