SIMILARITY OF NERVE-FIBRES. 263 



another, and the last class of nerv^es, which are called 

 7'etardatory nerves (Hemmungs-nerven). It is com- 

 mon knowledge that the heart beats ceaselessly during 

 life. Now, if a certain nerve which enters the heart 

 is irritated the heart ceases to beat, recommencing 

 when the irritation of the nerve is discontinued. This 

 remarkable fact was discovered by Edward Weber, who 

 spoke of the phenomenon as retardation. It is curious 

 that a nerve can by its activity still a muscle which 

 is in motion. 



2. Before we endeavour to determine this and the 

 other points raised, we must note whether any differ- 

 ences can be shown in these various nerves, which act 

 in such entirely different ways. In the previous chap- 

 ters we have observed so many peculiarities in nerves, 

 and among these, qualities which can be examined 

 without the intervention of the muscle, that it seems 

 not altogether unjustifiable to hope that we may be 

 able to observe differences also in nerves if any such 

 occur. But if this is impossible, if all nerve-fibres, 

 though examined in every possible way, seem to be 

 quite homogeneous, then we shall be justified in con- 

 sidering them really homogeneous, and must look for 

 an explanation of the variety in their actions in other 

 circumstances. 



It may at once be said that it is quite impossible 

 to show differences in the different kinds of nerves. 

 Microscopic observation shows no -differences ; for the 

 difference, to which allusion has already been made, 

 between medullary and medulla-less fibres does not 

 affect the point in question. We are obliged to infer 

 that the medullary sheath is of entirely subordinate 

 significance in the activity of the nerve. At any rate. 



