THE NERVOUS SYSTEM 



469 



grows away from the surface towards the underlying muscles. By branch- 

 ing into terminal telodendria, such a neurite may increase the number of 

 its connexions. 



A second evolutionary step is taken when the body of the neurosensory 

 cell sinks below the surface into the underlying connective tissue, but 

 retains connexion with the superficial epithelium by means of a process 

 with branched terminations or dendrites attached to the skin. 



Further advance appears when the dendrites, instead of ending freely 

 among the epithelial cells, become connected with special receptor or 



EPITHELIAL CELL 



rail I jj^^rnm 



^MUSCLI 



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.NEUROMUSCULAR SECONDARY 



NEUROSENSORY CELL CELL 1 SENSORY CELL 



SENSORY NEURONE. 



MUSCLE CELL 



V"-SENSORY CELL—- 



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SECONDARY SENSORY CELL T _SENSORY NEURONE 



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SECONDARY 



SENSORY MOTOR 



NEURONE 



Fig. 386. — A diagram showing hypothetical stages in the evolution of the reflex arc of 

 the higher animals. In A the series begins with the neuromuscular cell of coelenterates. 

 In B the neurosensory cell becomes differentiated from the muscle fiber. In C the 

 body of the neurosensory cell recedes from the surface. In D the neurosensory cell 

 becomes a sensory neuron and a secondary sensory cell conveys external stimuli to the 

 sensory neuron. In £ a motor neuron is interpolated between the sensory neuron and 

 the muscle fiber. Finally, in F, by means of several motor neurons, the connexion of a 

 sensory neuron with several muscle fibers is effected. (Redrawn after Kahn's "Das 

 Leben Des Menschen," W. Keller & Co.) 



secondary sense cells in the skin. The somatic sensory cells of verte- 

 brates are at this evolutionary stage. 



The so-called primitive ganglion cells of coelenterates exhibit another 

 line of differentiation. That these cells derive from neurosensory cells 

 is a conclusion supported by considerable evidence. That they are more 

 differentiated than neurosensory cells is indicated by the fact that they 

 contain tigroid substance and neurofibrillae characteristic of the nerve 

 cells or neurons of the higher animals. Physiologically, however, they 

 are simpler than neurosensory cells, since they transmit nervous impulses 

 in any direction, while neurosensory cells are definitely polarized and 

 transmit impulses in a single direction only. They are, therefore, inter- 

 preted as neurosensory cells which have lost both their primary connexion 

 with the skin and their functional polarity. (Fig. 388) 



