CHAPTER X 



TRANSMISSION IN THE NERVE-NET 



A SECOND peculiarity commonly attributed to the 

 nerve-net as contrasted with the synaptic nervous sys- 

 tem is diffuse transmission. In the synaptic nervous 

 system of the higher animals nervous impulses travel 

 from the receptors in one direction only, over well-cir- 

 cumscribed paths to given effectors; in other words, the 

 synaptic nervous system exhibits a high degree of polar- 

 ity. In the nerve-net, on the other hand, an impulse 

 started at any point is believed to spread freely in all 

 possible directions throughout the structure. This aspect 

 of the nerve-net is best seen perhaps in the subumbrellar 

 system of the medusae. As Romanes (1877, 1878), Eimer 

 (1878), and especially Mayer (1906) have shown, this 

 net may be cut into the most intricate patterns and yet 

 so long as the original organic continuity from, point to 

 point exists a nervous impulse may be started anywhere 

 and will spread throughout the full extent of the tissue. 

 Much the same form of diffuse transmission is to be seen 

 in the sea-anemones, from almost any point of whose 

 surface the whole retractor musculature may be brought 

 into action. 



In other examples of the nerve-net evidences of be- 

 ginning polarization can be seen and these may be re- 

 garded as suggestive of the kind of steps by which the 

 nerve-net was converted into the synaptic system. This 

 is perhaps nowhere better seen than in the tentacles of 

 actinians. When the tip of a tentacle is vigorously stimu- 



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