August 2, 1877] 



NA TURE 



27T 



why such a wave should ever become blocked at all, still 

 less that the point at which it does become blocked should 

 be so variable in different individuals of the same species. 

 On the other hand, if we suppose the propagation of the 

 ganglionic influence to be more or less dependent on the 

 presence of a more or less integrated nerve-plexus, we 

 encounter no difficulty ; for on the general theory of evo- 

 lution it is to be expected that if such fibres are present 

 in such lowly animals they should not be constant as to 

 position. 



But there is a still stronger argument in favour of 

 nerve-fibres, and it is this. At whatever point in a spiral 

 strip which is being progressively elongated by section the 

 blocl:ing of the contractile wave takes place, such block- 

 ing is sure to take place completely and exclusively at that 

 point. Now I cannot explain this invariable fact in any 

 other way than by supposing that at that point the section 

 has encountered a line of functionally differentiated tissue 

 — has severed an incipient nerve. 



Such, some of you may remember, was the state of the 

 evidence when I last addressed you upon this subject. 

 On the whole I provisionally adopted the view that all 

 parts of the rudimentary muscular sheet of the Medusje 

 are pervaded by a plexus of rudimentary nerves, or " lines 

 of discharge ;" and I explained the fact of the tissues in 

 some cases enduring such severe forms of section without 

 suffering loss of their physiological continuity, by suppos- 

 ing that all the rudimentary nerve-fibres composing the 

 plexus to be capable, in an extraordinarily high degree, of 

 vicarious action. If you were to represent the hypo- 

 thetical nervous plexus by a sheet of muslin, it is clear 

 that however much you were to cut the disc of muslin with 

 such radial or spiral sections as are represented in the 

 diagrams, you could always trace the threads of the mus- 

 lin with a needle round and round the disc without once 

 interrupting the continuity of your tracing ; for on coming 

 to the end of a divided thread you could always double 

 back on it and choose another thread which might be 

 running in the required direction. And this is what I 

 last year stated to be my opinion as to what took place in 

 the fibres of the hypothetical nervous plexus ; — whenever 

 a stimulus wave arrives at a cut, I imagined it to double 

 back and pass into the neighbouring lines of dis- 

 charge, which I thus supposed to act vicariouily for the 

 divided line. 



Such, then, when last I addressed you, was the stand- 

 ing of this question as to the character of these highly 

 remarkable contractile waves. On the whole I decided in 

 favour of a rudimentatary nervous plexus, notwithstand- 

 ing the improbability that such a plexus should be capable 

 of vicarious action in all its parts to so almost unlimited 

 a degree. ' I am glad to say that this decision has now 



* This antecedent improbability is not so ovenvbeImin» as it is at first 

 sight apt to appear ; for we must remember that in a peripheral nervous 

 plexus as we meet with it in the higher animals — i.e., in the fully evolved form 

 of such a structure— each of the constituent nerve-tibres is provi^'ed with an 

 insulating coat for the very purpose of preventing vicarious action among 

 these fibres and the consequent confusion among the reflex mechanisms 

 which such vicarious action would mani'*estly occasion. But because insu- 

 lation of peripheral nerve-fibres is thus an obvious neces-ity in the case of 

 a fully evolved nervous plexus, it by no means follows that any high degree 

 of insulation shou'd be required in the case of an inc"pient nervous p'exns. 

 On the conirary, any hypothesis as to the manner in which nerve-fibres first 

 begin ro be differentiated fiom protoplasm must suppose that the condnctile 

 function of the incipient nervous tracts precedes any structure, such as that 

 of nerve-coats, whereby this function is strictly confined to particular tracts. 

 The antecedent probability being thus in favour of the view that insulating 

 structures are a prodrct of later evolution than are the essential nervous 

 structures which they insulate, it would clearly be very hazardous to draw any 

 analogy between an incipient nervous plexus such as I suppose to be present in 

 the Medusa;, and a fully-evolved peripheral plexus of any of the higher ar,i- 

 mals. A less hazardous analogy would be furnished by the fibres which occur 

 in the central nervous system of the higher animals ; for here it may be said, 

 both A priori Uoxn Mr. Spencer's theory and (i/t7j^<r//^w^' from histological in- 

 dications, that the nerve-fibres occur in various degrees of differentiation. And 

 that vicarious action is possible to some considerable extent through a bridge of 

 the grey matterof the cord, has been shown bythe double hemi-section experi- 

 ments of Brown-Sequard. -Moreover, the admirab'eexperiments of Goltz would 

 seem to indicate that vicarious action is also possible to a large extent among the 

 ultimate elements of the brain- I may add that recent research has tended to 

 suggest a novel interpretation of the way in which certain poisons, such as 

 strychnia, act upou the cord ; for whereas it has hitherto been supposed that 



been further justified by some additional observations 

 which are of the first importance. For since my last lecture 

 I have noticed the fact that reflex action takes place be- 

 tween the marginal ganglia of the Medusa: and aU the con- 

 tractile tissues of the animal. Thus, for instance, if you 

 seize the polypite withapair of forcep=, the marginal ganglia 

 almost immediately set the swimming-bell in violent mo- 

 tion — thereby showing that the stimulus must have coursed 

 up the polypite to its point of insertion in the bell, and then 

 down the sides of the bell to the ganglia, so causing them 

 to discharge by reflex action. Again, suppose that seven of 

 the eight ganglia have been removed from the margin of 

 Aiirclia, and that any part of the contractile disc is stimu- 

 lated too gently to start a contractile wave from the point 

 immediately stimulated, a contractile wave will nevertheless 

 shortly afterwards start from the ganglion — thus showing 

 that a stimulus wave must have passed through the con- 

 tractile sheet to the ganglion, and so caused the ganglion 

 to discharge. Indeed in many cases the passage of this 

 stimulus wave admits of being actually seen. For it is a 

 peculiarity of the numberless tentacles which fringe the 

 margin of this Medusa, that they are more excitable than is 

 the contractile tissue of the bell. Consequently a stimulus 

 m.ay be applied to the contractile tissue of the bell which is 

 not strong enough to start a contractile wave in the bel!- 

 tissue itself, and is yet strong enough to start a contractile 

 wave in the tentacles — one tentacle after anolher contract- 

 ing in rapid succession until the wave of stimulation has 

 passed all the way round the disc. The latter, of course, 

 remains quite passive until the tentacular wave, or wave 

 of stimulation, reaches one of the ganglia (or the single 

 remaining ganglion, if the disc has been prepared by 

 removing seven of the ganglia), when, after an interval of 

 half a second for the period of latency, the ganglion is 

 sure to discharge, and so to cause a general wave of 

 contraction. 



Now these facts prove, in a singularly beautiful manner 

 — for this optical expression of the passage of a wave of 

 stimulation is a sight as beautiful as it is unique— these 

 facts, I say, conclusively prove that the whole contractile 

 sheet of the bell presents not merely the protoplasmic 

 qunlities of excitability and contractility, but also the 

 essentially nervous quality of conducting stimuli to a 

 distance irrespective of the passage of a contractile wave. 

 So I conclude there can be no longer any question that 

 we have here to deal with a tissue already so far dif- 

 ferentiated from primitive protoplasm, that the distin- 

 guishing function of nerve has become fully established. 



JI 



THE NORWEGIAN ATLANTIC EXPLORING 

 EXPEDITION 



Tromso, July 13, 1877 

 HE expedition met at the beginning with several 

 unfavourable circumstances. In the last week of 

 May Capt. Wille went out to Huso with the Varingen,'yc\. 

 order to determine the magnetical constants of the ship. 

 After his arrival a flaw was discovered in the shaft, so 

 that he went back to Bergen, where there was fortimately 

 a new shaft lying ready. A few days later the ship was 

 again at Huso, and was swung, not without some difficulty 

 owing to rough weather. Itii Absolute magnetical observa- 



tlie abnormal reflex exc'tability which these poisons engender is ("ue to the'r 

 exerting a stimulating ii-flue ce on the cord, the researches in question have 

 fii-ly well proved that the very reverse is true, viz . that the action of these 

 poisons is to depress the vitality of the cord. For a number of facts go to 

 prove that the abnormal reflex excitability is due to the impairment of some 

 function which has been provisionally termed *' resistance of the cord, ' a 

 function which in health prevents the undue spread of a stimulus through 

 the substance of the cord, and the impairment of which by the poison con- 

 sequently admits of a stimulus spreading to an undue extent, so giving rise 

 to the abnormal reflex excitability in question. As bearing on ihissubjec, 

 1 may observe thatwhile the action of strychnia on the iVIedusje is the same 

 as it is on the higher animals, viz.. that of causing paroxysmal convulsions, 

 it certainly seems to exercise a depressing infl.ience on the tissues ; for an 

 extremely weilc sea-water snlut'on has the effect of blocking contractile 

 waves inany part ofaspiralstrip ihatissubmittedtoitsinRuence.—G. J R. 



