TRANSMISSION IN THE NERVE-NET 133 



that the polarity of these organs, as evidenced in the 

 processes just mentioned, is dependent upon the prox- 

 imal direction taken by the sensory fibrils in these struc- 

 tures whereby the nervous impulses are led to flow pre- 

 dominantly toward the base of the tentacle. This ana- 

 tomical interpretation of the polarity of the tentacle is 

 supported by the observation made by Chester (1912) 

 that after two tentacles of Metridium have been grafted 

 together base to base there is no change in their neuro- 

 muscular polarity. 



Signs of polarization occur not only in the nerve-nets 

 of the lower animals, but they are also evident in those 

 of the higher forms. The rhythmic segmentation of the 

 small intestine, already mentioned, may at times be a 

 purely local process unconcerned with the progress of 

 the food. Under such conditions the intestinal nerve- 

 net must be acting as a diffuse mechanism. Commonly, 

 however, segmentation is associated with peristalsis and 

 the food is not only churned but moved along through the 

 intestine. The direction of this motion is regularly 

 toward the anus and the polarity thus exhibited is not 

 unlike that seen in the actinian tentacle. That intestinal 

 polarity, like tentacular polarity, probably rests on a 

 structural basis is seen from an experiment by Mall (1896 

 b) in which a section of small intestine was cut free, 

 turned end for end and healed into its old position re- 

 versed (Fig. 40). After recovery the animals began to 

 show serious digestive complications, and on killing and 

 examining them it was found that the food had accumu- 

 lated in the intestine at the stomach end of the reversed 

 piece, a state of affairs indicative of marked and persis- 

 tent polarity in the intestinal nerve-net. 



The heart of the lower vertebrates also exhibits pro- 



