CHAPTER 13 

 THE NERVOUS SYSTEM 



Of the two agencies which integrate the various functions of the body, 

 the nervous system is the more important. In addition, however, to this 

 integrative function the nervous system, with the intermediation of the 

 sense organs, serves to bring the organism into relation with its environment. 



The general protoplasmic properties upon which the actions of nerves 

 depend are merely the irritability and conduction which are characteristic 

 of all cells. An Amoeba, for example, responds to a stimulus by contract- 

 ing. If one of its pseudopodia is touched, all pseudopodia withdraw. 

 Obviously, both irritability and conduction are involved in this reaction. 



FOOD VACUOLE 



PSEUDOPODIUM 



AMOEBA. 



Fig. 299. — Amoeba. Amoeba shows primitive responsiveness to stimulation. 



All cells of higher animals presumably retain these two powers, but they 

 become the special functions of nerve and sense cells. 



ELEMENTS OF THE NERVOUS SYSTEM 



In simple colonial animals such as Volvox, nerves are wanting and 

 impulses are transmitted from cell to cell by means of intercellular bridges 

 or plasmodesms. Special nervous cells first appear in coelenterates, in 

 the form of neurosensory cells located in the skin. Each neurosensory or 

 receptor cell is connected with deeper tissues, such as muscle fibers, by an 

 elongated process or neurite, which carries nervous impulses to the effector 

 cell. In a characteristic reflex action in worms, a ganglion or transmittor 

 cell, comparable with the motor cell of vertebrates, is interpolated between 

 the receptor and effector cells. A similar reaction in vertebrates usually 

 involves four cells: — i a receptor cell in the skin or sense organ; 2, an 



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