CHAPTER LXXXVIII 



THE PROPERTIES OF EACH PART OF THE REFLEX 

 ARC (Cont'd) 



THE NERVE NETWORK 



In all animals above the Celenterates, no direct protoplasmic continuity 

 exists between the various neurons, the transmission of the nerve impulse 

 depending on contiguity; rather than continuity of the elements that con- 

 stitute the reflex arc. This transmission may be effected through a syn- 

 apsis coming in contact either with dendrites or with nerve cells. It is 

 extremely difficult to know whether there is really any anatomic con- 

 tinuity between the various fibers which form the network in the gray 

 matter of the central nervous system. We shall not attempt to discuss 

 this vexed question here, but in order that we may learn something of the 

 possible functions of a nerve network, we may consider that present in 

 the walls of the intestine (plexus of Auerbach and Meissner.) This plexus 

 seems to have an important function to perform in connection with the 

 myenteric reflex (see page 466). At least it has been shown by Meek 3 that 

 after transsection of the intestine the muscular and epithelial structures be- 

 come regenerated considerably earlier than the nervous plexus, but that 

 the myenteric reflex, which, it will be remembered, is characterized by a 

 wave of inhibition preceding one of contraction does not occur until after 

 the plexus has been regenerated. 



NETWORK ON SKIN NERVES 



A very important type of nerve network, from the medical viewpoint, 

 is that which is produced close, to their receptor endings by the branch- 

 ing of the afferent fibers of the skin. Through these branches the vas- 

 cular reactions following the application of an irritant to the sensory 

 surface take place without the intervention of any nerve cells. It used 

 to be thought that such reflex vasodilatation depended upon the trans- 

 mission of an impulse along an afferent neuron to a'n efferent vaso- 

 dilator neuron, a view strictly in consonance with the neuron hypothesis. 

 That such is not the case, however, is shown by the fact observed by 

 Ninian Bruce 4 that irritants such as mustard oil applied to the skin 

 or cornea continue to produce their usual reaction for some time after 



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