An Introduction to the Study of Experimental Medicine

cease as soon as we restore the normal proportion of water. Complete removal of water from living bodies invariably leads to death in large organisms provided with delicate histological units ; but it is well known that in small inferior organisms, removal of water invariably suspends life. Cases in point are the return to life of rotifera, of tardigrades and the small eels in mildewed wheat. There are numberless cases of latent life in vegetables and animals, due to removal of the organism's water.

Temperature has a considerable influence on life. Eaising the temperature makes vital phenomena as well as the manifestation of physico-chemical phenomena more active. In the outer cosmic environment, variations of temperature create the seasons which are characterized only by variations in the behavior of animal and vegetable life on the surface of the earth. These variations take place only because the inner environment or organic atmosphere of plants and certain animals remains in equilibrium with the outer atmosphere. If we put plants in hothouses, the influence of winter no longer makes itself felt ; the case of cold-blooded and hibernating animals is the same. But warm-blooded animals keep their organic units, as it were, in a hothouse ; so they do not feel the influence of hibernation. However, since this is only a special resistance of the inner environment to falling into heat equilibrium with the outer environment, this resistance can be overcome in certain cases, and in some circumstances warm-blooded animals can warm or cool themselves. The maximum temperature compatible with life does not generally rise above 75 degrees. The minimum does not go below the freezing point of the organic animal or vegetable fluids. However, these extremes may vary. In warm-blooded animals the temperature of the inner atmosphere is normally from 38 to 40 degrees : it cannot go above 45 to 50 degrees nor below 15 to 20 degrees, without causing physiological disturbance or even death if the variations are rapid. In hibernating animals, the gradual change of temperature which occurs may go much lower, causing the progressive disappearance of manifestation of life, to the point of lethargy or latent life, which may sometimes last a very long time, if the temperature does not vary.

Air is necessary to the life of all vegetable and animal beings; air therefore exists in the inner organic atmosphere. The three gases of the outer air, oxygen, nitrogen and carbonic acid gas, are in solution in the organic fluids in which the histological units breathe directly, like fish in water. Cessation of life through removal of these gases, especially oxygen, is what we call death by asphyxiation. In living beings, there is a constant interchange of gases between the inner and the outer environment; yet vegetables and animals, as we know, differ in respect to the changes which they cause in the surrounding air.

Pressure exists in the outer atmosphere; we know that on the surface of the earth the air exerts on living beings a pressure that lifts a column of mercury to a height of about 76 centimeters. In the inner atmosphere of warm-blooded animals, the nutritive fluids circulate under the influence of a pressure greater by about 150 mm. than the outer atmospheric pressure, but this does not necessarily indicate that the histological units really support such pressure. The influence of variations of pressure on the manifestations of life of organic units, moreover, is little known. We learn, however, that life cannot appear in too highly rarefied air, not only because the gases of the outer air cannot then dissolve in the alimentary fluid, but also because the gases dissolved in it escape. This is what we observe when we place a small animal under an air pump; its lungs are obstructed by the gases liberated in the blood. The articulates bear this removal of air much better, as various experiments have shown. Fishes in the depths of the sea sometimes live under considerable pressure.

The chemical composition of the cosmic or outer environment is constant and very simple. Its composition is that of the air, which remains the same, except for the proportions of water vapor and a few electric and other conditions which vary. The chemical conditions of the inner or organic environments are much more complex; and complication increases as the animal itself becomes higher and more complex. Organic environments, as we have said, are always aqueous ; they hold in solution definite saline and organic substances ; they show fixed reactions ; the lowest animal has its own organic environment; infusoria have an environment belonging to them, in this sense that they are more permeated than is a fish with the water in which he swims. In the organic environments of the higher animals, the histological units are like veritable infusoria, that is to say, they are provided with an environment proper to themselves which is not the general organic environment. Thus a corpuscle of blood is permeated with a fluid different from the serum in which it floats.

3. Organic Conditions. — Organic conditions are those which correspond to the evolution or change of the vital properties of organic units. Variation of these conditions necessarily leads to changes whose principal features we must recall here. Manifestations of life grow more varied, delicate and active in proportion as heings rise in the scale of organization. But susceptibility to disease also increases at the same time. Experimentation, as we have already said, is necessarily more difficult as organization becomes more complex.

Animal and vegetable species are separated by peculiar conditions which prevent them from mingling, so that fecundation, grafting and transfusion cannot be performed between beings of different species. These are problems of the greatest interest, which I believe may be attacked and reduced to differences in the physicochemical properties of environment. In the same species of animals, breeds may still show a certain number of very interesting differences. In different breeds of dogs and horses I have noted very peculiar physiological characteristics related, in different degrees, to the properties of certain histological units, particularly of the nervous system. Finally, among individuals of the same breed we find physiological peculiarities which are also connected with special variations in the properties of certain histological units. These, therefore, are what we call idiosyncrasies.

The same individual is unlike himself at some periods in his evolution; this leads to differences connected with age. After birth, the phenomena of life are of slight intensity ; soon they become very active, to slow down again toward old age. Sex and the physiological state of the genital organs may lead to changes which are sometimes very profound, especially in the lower animals, among which the physiological characteristics of the larvae, in certain cases, completely differ from the characteristics of the animal when full grown and endowed with genital organs.

Molting at times leads to such profound organic changes that experiments performed on animals in different stages yield by no means the same results. Hibernation also leads to great differences in the phenomena of " See L. Ziegler, Ueler die Brunst und den Embryo der Rehe. Hanover, 1843. life ; and operating on a frog or a toad is by no means the same thing in summer as in winter. -^^ States of digestion or fasting, of health or disease, also cause great changes in the intensity of vital phenomena and so in the resistance of animals to the influence of certain toxic substances and in their susceptibility to one or another parasitic or virulent disease.

Habit is yet another condition potent in changing organisms. This condition is one of the most important to keep in mind, especially when we intend to experiment on the action of toxic or medicinal materials. The size of animals also involves important changes in the intensity of vital phenomena. In general, vital phenomena are more intense in small animals than in large, which means, as we shall see further on, that we cannot rigorously measure physiological phenomena in proportion to weight.

To sum up, after all that we have previously said, we see what huge complexity inheres in animal experimentation because of the numberless factors with which the physiologist must reckon, i^evertheless, we may succeed if we make proper distinctions and subordinations, as we have just said, among the various factors, and if we seek to connect the factors in question with definite physico-chemical conditions. VII. The Choice of Animals ; the Usefulness to Medicine of Experiments on Various Species of Animals

Among the objections that physicians have offered to experimentation is one which must be seriously considered because it throws doubt on the usefulness of animal experiments to human physiology and medicine. It has been said, indeed, that experiments performed on a dog or a frog may be conclusive in their application to dogs and frogs, but never to man, because man has a physiological and pathological nature proper to himself and different from all other animals. It has been further stated that to be really conclusive for man, experiments would have to be made on man or on animals as near to him as possible. It was surely with this idea that Galen chose a monkey for his experiments, and Vesalius a pig, as subjects more closely

" See Stannius, Beobachtungen uher Verjiingungsvorgdnge im thierischen Organismus. Rostoch and Scliwerin, 1853. resembling man in his omnivorous capacity. Even to-day, many people choose dogs for experiments, not only because it is easier to procure this animal, but also because they think that experiments performed on dogs can more properly be applied to man than those performed on frogs. How well founded are these opinions ? How much importance should we ascribe to the choice of animals in relation to the usefulness of the experiment to physicians?

As far as direct applicability to medical practice is concerned, it is quite certain that experiments made on man are always the most conclusive. 'No- one has ever denied it. Only, as neither the moral law nor that of the state permits making on man the experiments which the interests of science imperatively demand, we frankly acclaim experimentation on animals: from the theoretic point of view, experiments on all sorts of animals are indispensable, while from the immediately practical point of view, they are highly useful to medicine. In fact, as we have already often expressed it, two things must be considered in the phenomena of life: first the fundamental properties of vital units which are general, then arrangements and mechanisms in organizations, which give each animal species its peculiar anatomical and physiological form. Now, among all the animals on which physiologists and physicians may experiment, some are better suited than others to the studies depending on these two points of view. Here we shall merely say in general that, for the study of tissues, cold-blooded animals or young mammals are more appropriate, because the properties of their living tissues vanish more slowly and so can better be studied. There are also experiments in which it is proper to choose certain animals which offer favorable anatomical arrangements or special susceptibility to certain influences. For each kind of investigation we shall be careful to point out the proper choice of animals. This is so important that the solution of a physiological or pathological problem often depends solely on the appropriate choice of the animal for the experiment so as to make the result clear and searching.

General physiology and pathology are necessarily based on the study of tissues in all animals, for a general pathology that did not ultimately rest on considerations drawn from the comparative pathology of animals at all stages of organization could build up a collection of generalities about human pathology, but never a general pathology in the scientific sense of the word. Just as an organism can live only by the normal manifestation of its properties or the help of one or more of its vital units, so the organism can become diseased only by abnormal manifestation of the properties of one or more of its vital units. Now the vital units, being of like nature in all living beings, are subject to the same organic laws. They develop, live, become diseased and die under influences necessarily of like nature, though manifested by infinitely varying mechanisms. A poison or a morbid condition, acting on a definite histological unit, should attack it in like circumstances in all animals furnished with it ; otherwise these units would cease to be of like nature ; and if we went on considering as of like nature units reacting in different or opposite ways under the influence of normal or pathological vital reagents, we should not only deny science in general, but also bring into zoology confusion and darkness that would absolutely block its advance; for the quality which should be placed in the front rank of the science of life and should dominate all the rest is vitality. The vital quality may doubtless offer great diversities in degree and kind of manifestation, according to peculiar circumstances of environment or mechanism shown by healthy or diseased organisms. The lower organisms have fewer distinct vital units than do the higher organisms ; whence it follows that these beings are less easily attacked by morbid or mortal influences. But in animals of the same class, order or species there are also constant or variable differences which medical physiologists must absolutely know and explain, because these differences, though resting on delicate distinctions, give phenomena an essentially different aspect.

The problem of science will consist precisely in this, to seek the unitary character of physiological and pathological phenomena in the midst of the infinite variety of their particular manifestations. Experimentation on animals is therefore one foundation of comparative physiology and pathology ; and we shall later quote examples to prove how important it is not to lose sight of the above ideas. In special questions of physiology and pathology, experimentation on the higher animals yields daily results, which are applicable in practice, that is, in hygiene or in medicine ; studies of digestion made on animals are evidently comparable with the same phenomena in man, as W. Beaumont's observations on his Canadian, compared with those he made by means of a gastric fistula in a dog, have superabundantly proved. Experiments made with animals, whether on the

cerebrospinal nerves or on tbe vasomotor and secretory nerves of the large sympathetic (like experiments on circulation), are applicable at every point to the physiology and pathology of man. Ex- periments on animals, with deleterious substances or in harmful circumstances, are very useful and entirely conclusive for the toxicology and hygiene of man. Investigations of medicinal or of toxic auhstances also are wholly applicable to man from the therapeutic point of view; for, as I have shown,^^ the effects of these substances are the same on man as on animals, save for differences in degree. In pathological physiology, investigations of the formation of callus, the production of pus, etc., in animals are incontestably useful to human medicine.

But aside from all the connections to be found between man and animals we must recognize that there are differences also. Thus from a physiological point of view, experimental study of sense organs and cerebral functions must be made on man, on the one hand, because man is made higher than the animals by faculties which animals lack, and, on the other hand, because animals cannot directly account to us for the sensations which they experience. From the pathological point of view, we also note differences between man and animals; thus animals have parasitic and other diseases unknown to man, and vice versa. Among these diseases some are trans^ missible from man to animals and from animals to man; others, not. Finally, certain susceptibilities to inflammation of the peritoneum and other organs are not developed to the same degree in man as in animals of various classes or species. But, far from being motives to hold us back from experimenting and from applying conclusions from pathological investigation made on animals to differences observed in man, these differences provide convincing reasons to the contrary. Different species of animals show numerous and important differences in pathological tendencies. I have already said that there are breeds and varieties among domestic animals, such as asses, dogs and horses, which present wholly individual physiological and pathological susceptibilities; I have even noted individual differences that were often rather marked. Only experimental studies of these diversities can furnish an explanation of the individual differences observed in man, either in different races or in different in-

^* Claude Bernard, Recherches sur Vopium et ses alcaloides { Comptes rendus de VAcad^mie des sciences, 1864). dividuals of the same race, differences which physicians call predispositions or idiosyncrasies. Instead of persisting as indeterminate states of the organism, predispositions, when studied experimentally, will be classed in due time as particular cases of a general physiological law, which will thus become the scientific foundation of practical medicine.

To sum up, I not only conclude that experiments made on animals from the physiological, pathological and therapeutic points of view have results that are applicable to theoretic medicine, but I think that without such comparative study of animals, practical medicine can never acquire a scientific character. In this connection I shall finish with the words of Buffon, to which we might ascribe a different philosophic meaning, but which are scientifically very true for this occasion : "If animals did not exist, man's nature would be still more incomprehensible. "

In animals, and especially the higher animals, experimentation is so complex and liable to so many sources of error, both foreseen and unforeseen, that we must proceed most circumspectly to avoid them. To bring experimentation to bear on parts of the organism that we wish to explore, we must often do considerable tearing down and produce direct or indirect disturbances which must change or destroy our experimental results. These very real difficulties have often vitiated experimental investigations on living beings and furnished arguments to the detractors of experimentation. But science would never progress if we thought ourselves justified in renouncing scientific methods because they were imperfect ; in this case, the one thing to do is to perfect the methods. Now perfecting physiological experimentation consists not only in improving instruments and operative methods, but above all and still more in study and well-regulated use of comparative experimentation.

We have elsewhere said (p. 55) that experimental counterproof must not be mistaken for comparative experimentation. Counterproof has not the slightest reference to sources of error that may be met in observing facts; it assumes that they are all avoided and is concerned only with experimental reasoning; it has in view only judging whether the relation established between a phenomenon and its immediate cause is correct and rational. Counterproof is therefore only a synthesis verifying an analysis or an analysis controlling a synthesis.

Comparative experimentation, on the contrary, bears solely on notation of fact and on the art of disengaging it from circumstances or from other phenomena with which it may be entangled. Comparative experimentation, however, is not exactly what philosophers call the method of differences. When an experimenter is confronted with complex phenomena due to the combined properties of various bodies, he proceeds by differentiation, that is to say, he separates each of these bodies, one by one in succession, and sees by the difference what part of the total phenomenon belongs to each of them. But this method of exploration implies two things : it implies, first of all, that we know how many bodies are concerned in expressing the whole phenomenon, and then it admits that these bodies do not combine in any such way as to confuse their action in a final harmonious result. In physiology the method of differences is rarely applicable, because we can never flatter ourselves that we know all the bodies and all the conditions combining to express a collection of phenomena, and in numberless cases because various organs of the body may take each other's place in phenomena, that are partly common to them all, and may more or less obscure the results of ablation of a limited part. Suppose, for instance, that we paralyze the whole body, a single muscle at a time. The disturbance produced by each paralyzed muscle will be more oi: less compensated and replaced by neighboring muscles, and we should finally come to the conclusion that each particular muscle contributed little to the movements of the body. The nature of this source of error has been very well expressed by comparing it with what would happen to an experimenter who removed, one after another, every brick in the foundation of a column.

He would see, indeed, that removing in succession one brick at a time does not make the column totter, and he would come to the logical but false conclusion that not one of these bricks helps to support the column. In physiology, comparative experimentation depends upon quite another idea; for its object is to reduce the most complex investigation to unity, and its result is to eliminate by a single stroke all known and unknown sources of error.

Physiological phenomena are so complex that we could never experiment at all rigorously on living animals if we necessarily had to define all the other changes we might cause in the organism on which we were operating. But fortunately it is enough for us completely to isolate the one phenomenon on which our studies are brought to bear, separating it by means of comparative experimentation from all surrounding complications. Comparative experimentation reaches this goal by adding to a similar organism, used for comparison, all our experimental changes save one, the very one which we intend to disengage.

If, for instance, we wish to know the result of section or ablation of a deep-seated organ which cannot be reached without injuring many neighboring organs, we necessarily risk confusion in the total result between the effects of lesions caused by our operative procedure and the particular effects of section or ablation of the organ whose physiological role we wish to decide. The only way to avoid this mistake is to perform the same operation on a similar animal, but without making the section or ablation of the organ on which we are experimenting. We thus have two animals in which all the experimental conditions are the same, save one, — ablation of an organ whose action is thus disengaged and expressed in the difference observed between the two animals. Comparative experimentation in experimental medicine is an absolute and general rule applicable to all kinds of investigation, whether we wish to learn the effects of various agents influencing the bodily economy or to verify the physiological role of various parts of the body by experiments in vivisection.

At times comparative experimentation may be done on two animals of the same species in condition as closely comparable as possible ; again the experiment must be made on the same animal. When working on two animals, as we have just said, we must place them in the same conditions, save one, the one that we wish to compare. This implies that the two animals compared are so much alike that differences noted in them after the experiment cannot be attributed to a difference depending on the individuals themselves. For experimenting on organs or tissues whose properties are definite and easily perceived, comparison of two animals of the same species will suffice ; but, on the other hand, when we wish to compare delicate and fugitive qualities, we must make our comparison on the same animal, whether the nature of the experiment permits experimenting on him repeatedly at different times, or whether we have to act at one and

the same time on similar parts of the same specimen. Differences, in fact, are harder to grasp in proportion as the phenomena that we wish to study grow more fugitive or more delicate ; in this respect no animal is ever absolutely comparable with another, and, as we have already said, neither is the same animal comparable with himself at different times when we examine him, whether because he is in different conditions, or because his organism has grown less sensitive, by getting used to the substance given him or to the operation to which he is subjected.

Finally, it sometimes becomes necessary to extend comparative experimentation outside of the animal, since sources of error may also be met in the instruments used for experimentation. I shall limit myself here to pointing out and defining the principle of comparative experimentation ; it will be explained, a propos of special cases, in the course of this work. In the third part of this Introduction I shall cite examples chosen to show the importance of comparative experimentation, which is the true foundation of experimental medicine ; it would be easy, in fact, to prove that almost all experimental errors come from neglecting comparative judgment of facts or from thinking cases comparable which are not so.

In the experimental sciences, measurement of phenomena is fundamental, since their law can be established by quantitatively determining an effect in relation to a given cause. In biology, if we wish to learn the laws of life, we must therefore not only observe and note vital phenomena, but moreover must also define numerically the ratios of their relative intensity one to another. The application of mathematics to natural phenomena is the aim of all science, because phenomenal law should always be mathematically expressed. To this end, data used in calculations should be results of well-analyzed facts, so that we may be sure that we fully know the conditions of the phenomena between which we wish to establish an equation. Now, I think that efforts of this kind are premature in most vital phenomena, precisely because these phenomena are so complex that we must not only assume, but are in fact certain that, beside the few among their conditions which we know,