^32 



NA TURE 



\_7uly 19, 1877 



direct communication between one street and another, but 

 every letter passes first to the central office ; so the trans- 

 mission of stimuli from one member of the body to 

 another is effected exclusively through a centre or 

 ganglion. 



Those among you who are acquainted with Mr. 

 Herbert Spencer's writings are doubtless well aware how 

 strong a case he makes out in favour of his theory re- 

 specting the genesis of nerves. This theory, you will 

 remember, is that which supposes incipient conductile 

 tissues, or rudimentary nerve-fibres, to be difierentiated 

 from the surrounding contractile tissues, or homogeneous 

 protoplasm, by a process of integration which is due 

 simply to use. Thus, beginning with the case of undif- 

 ferentiated protoplasm, Mr. Spencer starts from the fact 

 that every portion of the colloidal mass is equally excit- 

 able and equally contractile. But soon after protoplasm 

 begins to assume definite shapes, recognised by us as 

 specific forms of life, some of its parts arc habitually 

 exposed to the action of forces different from those to 

 which other of its parts are exposed. Consequently, as 

 protoplasm continues to assume more and mora varied 

 forms, in some cases it must happen that parts thus 

 peculiarly situated with reference to external forces will be 

 more frequently stimulated to contract than are other 

 parts of the mass. Now in such cases the relative frequency 

 with which waves of stimulation radiate from the more 

 exposed parts, will probably have the effect of creating a 

 sort of polar arrangement of the protoplasmic molecules 

 lying in the lines through which these waves pass, and for 

 other reasons also will tend ever more and more to con- 

 vert these lines into passages offering less and less resist- 

 ance to the flow of such molecular waves— z>., waves of 

 stimulation as distinguished from waves of contraction. 

 And lastly, when lines offering a comparatively low resist- 

 ance to the passage of molecular impulses have thus been 

 organically established, they must then continue to grow 

 more and more definite by constant use, until eventually 

 they become the habitual channels of communication be- 

 tween the parts of the contractile mass through which 

 they pass. Thus, for instance, if such a line has been 

 established between the points A and B of a contractile 

 mass of protoplasm, when a stimulus falls upon A a mole- 

 cular wave of stimulation will course through that line to 

 B, so causing the tissue at B to contract — and this even 

 though no contractile wave has passed through the tissue 

 from A to B. Such is a very meagre epitome o! Mr. 

 Spencer's theory, the most vivid conception of which may 

 perhaps be conveyed in a few words by employing his 

 own illustration — viz., that just as water continually 

 widens and deepens the channels through which it flows, 

 so molecular v.'aves of the kmd we are considering, by 

 always flowing in the same tissue tracts, tend ever more 

 and more to e.\cavate for themselves functionally dif- 

 ferentiated lines of passage. When such a line of pas- 

 sage becomes fully developed, it is a nerve-fibre, distin- 

 guishable as such by the histologist ; but before it arrives 

 at this its completed stage — i.e., before it is observable 

 as a distinct structure — Mr. Spencer calls it a " line of 

 discharge" 



Such being the theory,'! will endeavour to show how it 

 is substantiated by facts. And here it becomes neces- 

 sary to refer to my own work. You are all, I suppose, 

 acquainted with the general appearance of a Medusa, or 

 jelly-tish. The animal presents the general form of a 

 mushroom. The organ which occupies the same position 

 as the stalk does in the mushroom is the mouth and 

 stomach of the Medusa, and is called the polypite ; while 

 the organ which resembles in shape the dome of the 

 mushroom constitutes the main bulk of the animal, and 

 is called the swimmiiig-bell. Both the polypite and the 

 swirining-bell are almost entirely composed of a thick 

 trans^arejit and non-i;oritraclile jelly ; but the whole sur- 

 face o( the polypite, and the whole concave surface of the 



bell, are overlaid by a thin layer or sheet of contractile 

 tissue. This tissue is not exactly protoplasm and not 

 exactly muscle, but something between the two. It con- 

 stitutes the earliest appearance in the animal kingdom of 

 anything resembling muscular tissue. The thickness of 

 this continuous layer of incipient muscle is pretty uniform, 

 and is nowhere greater than that of very thin paper. The 

 margin of the bell supports a series of highly contractile 

 tentacles, and also another series of bodies which are of 

 great importance for us to-night. These are the so-called 

 marginal bodies, which are here represented, but the 

 structure of which I need not describe. Lastly, it may 

 not be superfluous to add that all the Medusa; are loco- 

 motive. The mechanism of their locomotion is very 

 simple, consisting merely of an alternate contraction and 

 relaxation of the entire muscular sheet which lines the 

 cavity of the bell. At each contraction of this muscular 

 sheet, the gelatinous walls of the bell are drawn togethei ; 

 the capacity of the bell being thus diminished, water is 

 ejected from the open mouth of the bell backwards, and 

 the consequent reaction propels the animal forwards. In 

 these swimming movements systole and diastole follow 

 one another with as perfect a rhythm as they do in the 

 beating of a heart. 



The question as to whether the Medusas possess a 

 nervous system is a question which has long occupied the 

 more or less arduous labours of many naturalists. Until 



Fiff.l 





Iately,'however, there has been so little certainty on the 

 subject that Prof. Huxley — himself one of the greatest 

 authorities on the group— thus defined the standing of 

 the question in his " Classification of the Animal King- 

 dom : " " No nervous system has yet been discovered 

 in any of these animals." The cause of this uncer- 

 tainty is to be found in the fact that the transparent 

 and deliquescent nature of the tissues of the Medusje 

 renders adequate microscopical observation in their 

 case a matter of extreme difficulty ; so much so that, 

 looking to the quantity and quality of the labour which 

 has been bestowed on the question, I doubt whether the 

 latter would ever have been satisfactorily settled by the 

 histological methods alone. But those of you who were 

 present at my lecture last year will no doubt remember 

 that by employing methods other than the histological, I 

 was able to set this long-standing question finally at rest. 

 For you will no doubt remember my having told you that 

 on merely cutting off the extreme marginal rim of the 

 bell I was surprised to find that the previously active 

 motions of the animal suddenly and entirely ceased ; 

 the paralysis caused by this simple operation was instan- 

 taneous, enduring, and complete. On the other hand, 

 you may remember, the severed margin which had just 

 been taken from the suimming-bell invariably continued 

 its rhythmical motions with a vigour and a pertinacity 

 not in the least impaired by its severance from the main 



