'Responses of Higher Animals: 

 The Nervous System 



THE RECEPTORS and effectors in higher 

 animals are interconnected by the cells of the 

 nervous system, which relay excitations from 

 parts of the body where stimulation occurs 

 to other parts where responses take place. 

 Thus the nervous system coordinates activi- 

 ties throughout the body in such a way that 

 each animal displays an integrated behavior 

 as it confronts the problems of survival. 

 Most animal responses are performed with- 

 out benefit of previous training, because ex- 

 citations that originate in a certain group 

 of receptors tend to be transmitted by the 

 nervous system to some specific and localized 

 group of effectors. Such unconditioned re- 

 sponses (p. 468) are generally useful to the 

 organism in that they are specifically adapted 

 to the conditions of some particular habitat. 



NERVE IMPULSES 



The transmission of excitations is the spe- 

 cialty of nerve cells, and nerve cells transmit 

 excitations at far greater speed than other 



tissues. Accordingly, excitations traversing 

 the nervous system are specifically referred to 

 as nerve impulses. 



The speed of the nerve impulse varies in 

 different animals, but it is always great com- 

 pared to the transmission of excitations by 

 unspecialized tissues. An activated egg cell, 

 for example, propagates an excitation at the 

 rate of only 1 cm per hour, and thus it is 

 possible to watch the lifting of the fertiliza- 

 tion membrane, which accompanies the 

 spreading of the excitation over the surface 

 of the egg. Likewise the sponge, which also 

 lacks specialized nerve cells, transmits excita- 

 tions at the comparatively slow rate of 1 cm 

 per minute. But with the appearance of even 

 primitive nerve cells, such as are found in 

 jellyfish (p. 633), the rate of transmission 

 climbs sharply to 10 cm per second; and in 

 the well-developed nervous system of the 

 earthworm, the speed is ten times greater. 

 The maximum velocity is reached, however, 

 in the nerves of warm-blooded vertebrates. 

 In the nerves of man, for example, the larg- 



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