282 PHYSIOLOGY OF MUSCLES AND NERVES. 



that is, is due to an increase in the resistance by 

 which the motion is temporarily suspended.^ 



Retardatory nerves have been recognised in ahnost 

 all automatic apparatus, and all are accounted for by 

 the above explanation. The same explanation may also 

 be applied at once to the retardation of reflex action ; 

 for even in the passage of the excitement from the 

 sensory to the motor nerves very great resistance has 

 to be overcome, and an increase in this resistance 

 must prevent the passage of the excitement and thus 

 hinder reflex action. Our acquaintance with this sul)- 

 ject is, however, not yet by any means complete, and 

 a final opinion on the matter is therefore for the time 

 impossible. 



I will only mention further that the opposite effect, 

 the facilitation of the passage of the excitement from 

 the nerve-cells in which it originates, to the peripheric 

 nerve-courses, appears to occur. 



Finally, it is sometimes observable that when those 

 portions of the nerves which contain nerve-cells are 

 continually and regularly irritated, a rhythmic or even 

 an irregular movement results, instead of a regular 

 tetanic contraction of the muscles concerned, — a cir- 

 cumstance which is evidently to be explained in the 

 same way as rhythmic automatic activity. The regu- 

 lar excitement having to pass through nerve-cells is 

 modified by the great resistance present in these, and 

 is transformed into a rhythmic motion, while when the 

 nerve and the muscle are directly connected, the latter 

 responds to a continuous excitement of the nerve with 

 a regular and continuous contraction. 



' See my account of theautomatic nerve-centres, to which refer- 

 ence has already been made. 



