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. 

 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 netirosensory 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 transmitter 

 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 

 afferent or sensory cell; 3. an efferent or motor nerve cell, which is con- 

 nected with 4. an effector cell by a neurite. Further complications arise 

 by the chaining together of additional nervous units within a central 

 nervous system, until, in vertebrates, so few cells are devoid of nervous 

 connexions that, if all were destroyed except the nervous tissues, the 

 general form of the body would still be preserved. 



The steps in the evolution of a reflex nervous system such as that of 

 worms and vertebrates involve, first, the differentiation of a neurosensory 

 cell in the ectoderm. The body of such a cell remains in the ectoderm, 

 and one or more protoplasmic hairs may extend above the surface. The 

 most characteristic feature of such a cell, however, is the neurite, which 



467 



