392 INNERVATION. [CHAP. XII. 



an irritation could be communicated from one to the other in their 

 course. The nerve-fibres only lie in juxtaposition, but do not com- 

 municate; and there is an evident provision in the tubular membrane 

 and white substance of Schwann for the insulation of the central 

 axis, which is probably the effective substance in the nervous action. 

 We must seek, therefore, in the nervous centres for such a communi- 

 cation between these nerves as may explain the excitability of one by 

 the other. In the present state of our knowledge,, we can do no more 

 than state it as in the highest degree probable that nerves implanted 

 in the centre immediately contiguous to each other can exert an influ- 

 ence upon the vesicular matter of the centre, and upon each other. 



But there are certain facts which demonstrate, beyond all doubt, 

 that, in such actions as we refer to, the integrity of the centre forms 

 a necessary condition. First, in many of the instances, it is plain 

 that there can be no connexion between the affected nerves else- 

 where than in the centre; for they are so distinct from each other, 

 that there is not even that apparent connexion which results from 

 the anastomosis of a fasciculus of fibres of the one with a portion 

 of the other. Secondly, the removal of the portion of the nervous 

 centre with which any one of the nerves concerned in the sympa- 

 thetic action is connected, will prevent the development of the phe- 

 nomenon, although the nerves themselves remain uninjured in 

 their peripheral distribution, or in their connexion with each other. 

 Thirdly, if there were any peripheral communication between 

 nerves, it would be most likely to take place in the plexuses. Ex- 

 periments, however, upon the nerves which lead to these, shew that 

 each nerve-tube, in its passage through them, retains its isolation as 

 distinctly as in any other part of its course. The three nerves which 

 supply the lower extremity in the frog, says Miiller, form a plexus 

 from which two nervous trunks issue : if one of these latter be 

 divided and isolated from all its connexions with muscles, and the 

 portion of it connected with the plexus irritated, the impression 

 will be transmitted in the centripetal direction by the sensitive 

 fibres of the nerve ; but the motor fibres of the other nerve arising 

 from the plexus are not affected, and excite no contractions in the 

 muscles to which they are distributed.* 



In applying these principles to the explanation of the instances 

 which we have quoted, we shall find it difficult to determine the 

 central connexion in some, although in others such a connexion is 

 highly probable. It remains, therefore, for future anatomical re- 

 search to ascertain what that connexion is which enables one nerve 

 * Baly's Miiller, vol. i. p. 756. 



