784 



THE CENTRAL NERVOUS SYSTEM 



(such as the earthworm, crayfish, lobster, etc.,) much more definite spe- 

 cialization of the adjuster occurs, for now this intermediate nervous tis- 

 sue becomes collected into so-called ganglia, a pair existing for each 

 segment and the various pairs being connected by definite 

 nerve structures, constituting the ganglion chain. It is in 

 this group of animals that we have, for the first time, def- 

 inite evidence of the existence of the 'neuron, which may be 

 considered as the elementary unit of which the nervous sys- 

 tem of all the higher animals is built. A neuron may be 

 either sensory or motor, and in both cases it consists of 

 a cell with a nucleus, one long process, called the axon, 

 and several short branching processes, called the den- 

 drites. The axon in its course may give off a branch, 

 or more, at right angles, these are sometimes called 

 collaterals, and at its end it may break up into very fine 

 branches called a synapsis. In a sensory neuron the im- 

 pulse is transmitted from the end of the axon to the 

 nerve cell, whereas in a motor neuron it is transmitted 

 in the opposite direction from the cell to the end of the 

 axon (Fig. 203). 



The simplest arrangement of sensory and motor neu- 

 rons to constitute the nervous system is seen in the 

 earthworm, in which it forms the simplest type of reflex 

 arc (Fig. 201, No. 3). The sensory neuron has its cell 

 body in the skin, and its axon proceeds to one of the 

 segmental ganglia, in which are large nerve cells whose 

 thick axons pass out from the ganglion as motor fibers 

 to the muscles of the body wall. The dendrites of the 

 motor neuron and the branching of the termination of 

 the sensory neuron cause a very fine interlacement of 

 nerve fibers in the ganglia, forming a network known 

 as the neuropile. The sensory impulse, on reaching the 

 ganglion, is transmitted by the synapsis to the den- 

 drites, probably without the fibers actually joining to- 

 gether; that is, the nerve impulses pass from the one 

 to the other set of branches by contact rather than by 

 transmission through continuous tissue. 



By such an arrangement it is evident that the nervous 

 apparatus in each segment could cause a contraction of 

 the muscles of its own neighborhood, but that a stimulus applied to one re- 

 ceptor would be incapable of calling forth a contraction of the muscles of a 

 far distant segment, much less a coordinated contraction of the musculature 



Fig. 202. Dia- 

 gram of nervous 

 system of seg- 

 mented inverte- 

 brate; a, supra- 

 esophageal g a n- 

 glion; b, subeso- 

 phageal ganglion ; 

 oc, esophagus or 

 gullet. 



