THE NEURON 



53 



that at the synapse there must be a reversal of the polarity with 

 reference to the cell body as the nervous impulse passes over 

 from an axon to a dendrite. 



In the simple diffuse form of nervous system found in primi- 

 tive animals like the jelly-fishes and lowest worms (p. 27) the 

 nerve-cells are described as connected by protoplasmic strands 

 to form a continuous network. Here, of course, there are no 



Fig. 17. Plexus of sympathetic neurons in the villi of the small intes- 

 tine of a guinea-pig: a, b, c, d, Neurons of the subepithelial plexus; e, f, 

 neurons of the plexus within the villi; g, fibers of the submucous (Meissner's) 

 plexus. (After Ram6n y Cajal.) 



synapses and the neurons are not polarized. Apparently the 

 nervous impulse may be transmitted equally well in all direc- 

 tions throughout this network. The physiological properties 

 of such an arrangement appear to be very different from those 

 of the synaptic nervous systems of higher animals. A non- 

 synaptic network similar to that mentioned above has. been des- 

 cribed as occurring in some of the diffuse ganglionic plexuses of 

 the human body (Fig. 1.7). 



