834 



CENTRAL NERVOUS SYSTEM 



serve to connect afferent fibers in one segment with motor neurons in some 

 distant segment. They are called association neurons because they asso- 

 ciate the activities of remote parts of the body. 



The fundamental characteristics of the central nervous 

 system as it appears in the worms consists in (1) the 

 individuality of the component nerve cells and their 

 specialization for certain functions, (2) the arrangement 

 of the neurons in such a way that the impulses must 

 pass over definite paths or reflex arcs in order to reach 

 an effector, (3) the introduction of neurons specialized 

 to associate the activities of reflex arcs in remote parts 

 of the body, and (4) the segregation of the greater part 

 of the nervous tissue into a centrally located chain of 

 ganglia, from which fibers pass to peripheral structures 

 in definite nerve trunks. These characteristics persist 

 in the central nervous system in the higher animals, in- 

 cluding man. 



Certain modifications of the simple arrangement found 

 in the worms have been introduced into the nervous sys- 

 tem as the motor activities of organisms became more 

 precise and complex. Of minor importance may be men- 

 tioned the introduction of an afferent nerve cell distinct 

 from the receptor between the latter and the central 

 ganglionic mass, (see the Evolution of Receptors page 

 854) and the introduction of an outlying neuron between 

 the termination of the primary motor neuron and the 

 effector organ (see the autonomic system page 894). Of 

 far greater importance is the great increase in the num- 

 ber of association neurons which become the salient fea- 

 ture of the vertebrate nervous system. In the worms the 

 inter-ganglionic connectives are scarcely more bulky 

 than the peripheral nerves from a single ganglion, 

 whereas in the central nervous system of man the num- 

 ber of the fibers extending from segment to segment ex- 

 ceeds by many times the number of fibers in any single 

 pair of spinal nerves. This increase in the mass of the 

 central portion of the nervous system is not distributed 

 equally in all parts, but tends to concentrate itself par- 

 ticularly at the anterior end. Even in the worms the most 

 anterior ganglion, the supraesophageal ganglion, is larger than the others, 

 while this tendency finds its culmination in the great bulk of the brain in 

 man. This condition is obviously correlated with the development 



Fig. 208. Dia- 

 gram of nervous 

 system of seg- 

 mented inverte- 

 brate; a, supra- 

 esophageal g a n- 

 glion; b, subeso- 

 phageal ganglion; 

 oe, esophagus or 

 gullet. 



