BEGINNING OF NERVES. 311 



gain nothing- on the side of simplicity by assuming that the contractile 

 waves are merely muscle-waves, so long as the undoubtedly nervous 

 waves are equally able to pass round sections interposed in their path. 

 Briefly, then, I find that the nervous waves of stimulation are quite as 

 able to pass round these interposed sections as are the waves of con- 

 traction. Thus, for instance, in this specimen (Fig. 5), the tentacular 

 wave of stimulation continued to pass as before, even after I had sub- 

 mitted the parallelogram of tissue to the tremendously severe form 

 of section which is represented in the diagram. And this fact, I am 

 not afraid to say, is one of the most important that has ever been 

 brought to light in the whole range of invertebrate physiology. For 

 what does it prove ? It proves that the distinguishing function of nerve, 

 where it first appears upon the scene of life, admits of being performed 

 vicariously to almost any extent by all parts of the same tissue-mass. 

 If we revert to our old illustration of the muslin as representing the 

 nerve-plexus, it is clear that, however much we choose to cut the sheet 

 of muslin with such radial or spiral sections as are represented in the dia- 

 gram, one could always trace the threads of the muslin with a needle 

 round and round the disk, without once interrupting the continuity of 

 the tracing; for, on coming to the end of a divided thread, one could 

 always double back on it and choose another thread which might be run- 

 ning in the required direction. And this is what we are now compelled 

 to believe takes place in the fibres of this nervous network, if we assume 

 that these visible fibres are the only conductive elements which are pres- 

 ent. Whenever a stimulus-wave reaches a cut, we must conclude that 

 it doubles back and passes into the neighboring fibres, and so on, time 

 after time, till it succeeds in passing round and round any number of 

 overlapping cuts. 



Now, it was in view of this almost unlimited power of vicarious 

 action on the part of the fibres composing *the (then) hypothetical 

 nervous plexus, that I was in the first instance inclined to suppose these 

 nerve-fibres to be of a non-fully differentiated character ; and although 

 the above detailed experiments, and others of a similar kind, proved 

 that an intimate network of such channels was present, I scarcely ex- 

 pected that they would admit of being distinguished by the microscope. 

 But, not to give an inference the value of a fact, I was careful to state 

 in the publication where this inference was adduced — viz., in the printed 

 abstract of a Royal Institution lecture — that this position was only 

 " provisional," and that, until I should have had " time to conduct a 

 systematic inquiry concerning the histology of the Medusm^'' the infer- 

 ence in question must be regarded as premature and uncertain." ' Such 

 a systematic inquiry has now shown that this provisional inference was 



' I guarded the inference in this way, lest the fibres in question should afterward 

 prove to be nerves ; and it will therefore be observed that, supposing them to be nerves, 

 the above inference cannot be negatived until it is shown that there are no other nervous 

 channels present of a less differentiated character. 



