Sensory end -organs of retina 



r*r 



End plate of nerve in muscle fiber 

 Nerve endings 



in gland 



Motor 



Sensory 



NERVE CONNECTIONS 



Associative 



Nerve cells are connected with sensory receptive organs (such as eyes or ears), 

 with muscles or glands, and with other nerve cells. The end branchings of a nerve 

 cell form intimate connections with the branchings of another nerve cell or with other 

 tissue cells. Nerve impulses pass through a nerve cell in one direction only, although 

 on electric current can be made to pass through a nerve cell in either direction 



main nerve runs the length of the body. This has side branches which con- 

 nect with the skin and special sense organs and also with muscles (see illus- 

 tration above). These connections have been definitely traced in many kinds 

 of animals, including man. Moreover, experiments show clearly that the 

 parts of the structure behave in complete agreement with the idea of a 

 "reflex" (see page 262). There is a definite nerve connection between the 

 point of stimulation and the acting muscle. This path consists of at least two 

 parts: (1) an afferent or incoming neuron, the sensory portion; and (2) an 

 efferent or outgoing neuron, die motor portion. Most reflexes involve one or 

 more intermediary associative neurons. The entire path makes up the reflex 

 arc (see illustration, p. 282). 



In all animals with a central nervous system the axis contains fibers that 

 run, so to say, forward and backward, connecting ganglia in the various 

 segments. Through these nerve connections stimuli acting upon receptors 

 in one part of the body can produce effects in other segments, both in front 

 of and behind the stimulated region. 



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