522 



NATURE 



{Sept. 26, 1889 



jcars bclure, were demonstrated experimentally by Ludwii^, at 

 the very time when Helmholtz was giving definite form to the 

 great natural philosopher's theory of colour perceptions. 



'liie effect of these discoveries was to produce a complete 

 revolution in the ways of thinking and speaking about the 

 phenomena of life. The error of the past had been to believe 

 that, although the heart resembled a pump, although digestion 

 could be imitated in the laboratory, and comparisons of vital 

 with physical processes cou'd be used for illustration, it was 

 always wrong to identify them. But, inasmuch as it had been 

 learned that sensation is propagated along a nerve just as sound 

 is propagated through the air, only with something like a tenth of 

 the velocity, that the relations between the work done, the heat 

 produced, and the fuel used, can be investigated in the living body 

 just as they are in the steam-engine, it now came to be felt that, in 

 other similar cases, what had been before regarded as peculiarly 

 vital might be understood on physical principles, and that for, 

 the future the word " vital " as distinctive of physiological pro- 

 cesses might be abandoned altogether. In looking back, we 

 have no difficuliy in seeing that the lines of investigation which 

 were then initiated by such men as Helmholtz, Ludwig, Biiicke, 

 du Bois-Reymond, Bonders, Bernard, are those along which, 

 during the succeeding generation, the science of physiology 

 advanced ; nor can anyone vho is acquainted with the literature 

 of that time doubt that these leaders of physiological thought 

 knew that they were the beginners of a new epoch. But such 

 an epoch cannot occur again. We have adopted once for all 

 the right, i.e. the scientific method, and there is not the least 

 possibility of our recurring to the wrong. We have no new 

 departure, no change of front in prospect ; but cten limes 

 which are not epochal have their tendencies, and I venture to 

 submit to you, that in physiology the tendency of the present 

 time is characterized by the concentration of the best efforts of 

 the best minds on what I have already referred to as elementary 

 questions. The work of investigating the special functions of 

 organs, which during the last two decades has yielded such 

 splendid results, is still proceeding, and every year new ground 

 is being broken and new and fruitful lines of experimental 

 inquiry are being opened up ; but the further the physiologist 

 advances in this work of analysis and differentiation, the more 

 frequently does he find his attention arrested by deeper questions 

 relating to the essential endowments of living matter, of which 

 even the most highly differentiated functions of the animal or 

 plant organism are the outcome. In our science the order of 

 progress has been hitherto and will continue to be the reverse of 

 the order of Nature. Nature begins with the elementary and 

 ends with the complex (first the amoeba, then the man). Our 

 mode of investigation has to begin at the end. And this not 

 merely for the historical reason that the first stimulus to physio- 

 logical inquiry was man's reasonable desire to know himself, but 

 because differentiation actually involves simplification. For just 

 as in manufactures it is the effect of division of labour that less 

 is required of each workman, so in an organism which is made 

 up of many organs, the function of each is simpler. 



Physiology, therefore, first studies man and the higher animals 

 and proceeds to the higher plants, then to invertebrates and 

 cryptogams, ending where development begins. From the 

 beginning her aim has b en to correlate function with structure, 

 at first roughly, afterwards, when, as I have explained, her 

 methods of observation became scientific, more and more ac- 

 curately — the principle being that every appreciable difference 

 of strucHire corresponds to a difference of function ; and con- 

 versely that each endowment of a living organ must be explained, 

 if explained at all, as springing from its structure. 



It is not difficult to see whither this method must eventually 

 lead us. For inasmuch as function is more complicated than 

 structure, the result of proceeding, as Physiology normally does, 

 from structure to function, must inevitably be to bring us face to 

 face with functional differences which have no structural differ- 

 ence to explain them. Thus, for example, if the physiologist 

 undertakes to explain the function of a highly differentiated 

 organ like the e)e, he finds that up to a certain point, provided 

 that he has the requisite knowledge of dioptrics, the method of 

 correlation guides him straight to his point. He can mentally 

 or actually construct an eye which will perform the functions of 

 the real eye, in so far as the formation of a real image of the field 

 of vision on the retina is concerned, and will be able thereby to 

 understand how the retinal picture is transferred to the organ 

 of consciousness. Having arrived at this point he begins to 

 correlate the known structure of the retina with what is re- 



quired of it, and finds that the number of objects which he can 

 discriminate in the field of vision is as numerous as, but not 

 more numerous than, the parts of the retina, i.e. the cones- 

 which are concerned in discriminating them. So far he has no 

 difficulty ; but the method of correlation fails him from the 

 moment that he considers that each object point in the field of 

 vision is coloured, and that he is able to discriminate not merely 

 the number and the relations of all the object points to each 

 other, but the colour of each separately. He then sees at once 

 that each cone must jDossess a plurality of endowments for which 

 its structure affords no explanation. In other words, in the 

 minute structure of the human retina, we have a mechanism 

 which would completely explain the picture of which I am 

 conscious, were the objects composing it possessed of one 

 objective quality only, being colourless, but it leaves us without 

 explanation of the differentiation of colour. 



Similarly, if he is called upon to explain the function of a 

 secreting gland, such, e.g., as the liver, there is no difficulty in 

 understanding that, inasmuch as the whole gland consists of 

 lobules which resemble each other exactly, and each lobule is 

 s milarly made up of cells which are all alike, each individual 

 cell must be capable of performing all the functions of the whole 

 organ. But when by exact experiment we learn that the liver 

 possesses not one function but many — when we know that it is a 

 storehouse for animal starch, that each cell possesses the power 

 of separating waste colouring-matter from the blood, and of 

 manufacturing several kinds of crystallizable products, some of 

 which it sends in one direction and others in the opposite — we 

 find again that the correlation method fails us, and that all that 

 our knowledge of the minute structure has done for us is to set 

 before us a question which, though elementary, we are quite 

 unable to answer. 



By multiplying examples of the same kind, we should in each 

 case come to the same i^sue, namely, plurality of function -with 

 unity of structure, the unity being represented by a siinple 

 structural element — be it retinal cone or cell — possessed of 

 numerous endowments. Whenever this point is arrived at in 

 any investigation, structure mu>t for the moment cease to be our 

 guide, and in gci.eral two courtes or alternatives are open to us. 

 One is to fall back on that worn-out Deus ex machind, proto- 

 plasm, as if it afforded a sufficient explanation of everything 

 which cannot be explained otherwise, and accordingly to defer 

 the consideration of the functions which have no demonstrable 

 connection with structure as for the present beyond the scope of 

 investigation ; the other is, retaining our hold of the funda- 

 mental principle of correlation, to take the problem in reverse, 

 i.e. to use analysis of function as a guide to the ultra-microscopical 

 analysis of structure. 



I need scarcely say that of these two courses the yJr^/ is wrongs 

 the jf^cwr/ right, for in following it we still hold to the fundamental 

 principle that living material acts by virtue of its strtictiire, pro- 

 vided that we allow the term structure to be used in a sense 

 which carries it beyond the limits of anatomical investigation, 

 i.e. beyond the knowledge which can be attained either by the 

 scalpel or the microscope. We thus (as I have said) proceed 

 from function to structure, instead of the other way. 



The departure from the traditions of our science which 

 this change of direction seems to imply is indeed more 

 apparent than real. In tracing the history of some of the 

 greatest advances, we find that the recognition of function has 

 preceded the knowledge of structure. Haller's discovery of 

 irritability was known and bore fruit, long before anything was 

 known of the structure of muscle. So also, at a later period, 

 Bichat was led by his recognition of the physiological differ- 

 ences between what he termed the functions of organic and 

 animal life, to those anatomical researches which were the 

 basis of the modern science of Histology. Again, in much 

 more recent times, the investigation of the function of gland 

 cells, which has been carried on with such remarkable results 

 by Prof Heidenhain in Germany, and with equal success by 

 Mr. Langley in this countrj-, has led to the di?covery of the 

 structural changes which they undergo in passing from the 

 state of repose to that of activity : nor couli;l I mention a better 

 example than that afforded (among many others relating to 

 the physiology of the nervous system) by Dr. Gaskell's recent 

 and very important discovery of the anatomical difference 

 between cerebro-spinal nerves of different functions. We 

 may therefore anticipate that the future of physiology will differ 

 from the past chiefly in this respect — that whereas hitherto the 

 greater part of the work has consisted in the interpretation ol 



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