Classics in the History of Psychology

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Christopher D. Green
York University, Toronto, Ontario

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By Ivan P. Pavlov(1927)
Translated by G. V. Anrep (1927)

[Classics Editor's note: Pavlov used both square and round brackets in his texts. These have been preserved but can lead to confusions as to which insertions are the author's and which are the editor's. Page numbers, reference numbers, and the occasional "sic" have been inserted in square brackets by the Classics editor. All other insertions (e.g., on p. 31) are by Pavlov. -cdg-]


The formation of conditioned reflexes by means of conditioned and direct stimuli. -- Agencies which can be used as conditioned stimuli. -- Inhibition of conditioned reflexes: external inhibition.

In the previous cases a conditioned reflex was obtained by linking up the action of a new stimulus with an unconditioned reflex. It is possible, however, to obtain a conditioned reflex less directly, by linking up yet a further stimulus with a conditioned stimulus which is already firmly established. Let us refer again to our experiment with the metronome. The sound of the beats was a conditioned stimulus so firmly and powerfully established as readily to admit of demonstration in face of the large audience present at my lecture. The effect of the metronome even under such unfavourable conditions was complete and precise. With the help of this strong conditioned stimulus it has been found possible to give still another stimulus conditioned properties like the first. For if some new and more or less neutral stimulus is applied in conjunction with the metronome alone -- i.e. not at the same time giving food -- this new stimulus also acquires the character of an alimentary conditioned stimulus [Drs. Zeljony, Foursikov, Frolov].

Conditioned reflexes established in this manner are termed secondary conditioned reflexes, and I shall bring to your attention certain precautions which must be observed for the successful establishment of reflexes of this order. The essential condition is that the new stimulus should be withdrawn some seconds before the primary stimulus is applied. With new stimuli of a medium physiological strength this lapse of time must be not less than ten seconds, while for stronger stimuli the interval must be considerably increased. Any shortening of the indicated minimal interval of time leads to a quite different result, typifying a group of extremely delicate and interesting phenomena in the physiology of the hemispheres, which will be treated in the fifth lecture.

I shall describe first an experiment conducted by Dr. Frolov illustrating the development of a secondary conditioned reflex: A [p. 34] dog has two primary alimentary conditioned stimuli firmly established, one to the sound of a metronome and the other to the buzzing of an electric bell. The appearance of a black square in the dog's line of vision is now used as yet a further stimulus, which is to be given the character of a secondary conditioned stimulus. The black square is held in front of the dog for ten seconds, and after an interval of fifteen seconds the metronome is sounded during 30 seconds. In the table given below the square is presented for the tenth time.

Prior to these experiments the appearance of the black square had no secretory effect at all. As seen from the above table the conditioned reflex of the second order is measured even at this early stage of its development by 5 drops (2.5 plus 3) during 25 seconds.

It was found impossible in the case of alimentary reflexes to press the secondary conditioned stimulus into our service to help us in the establishment of a new conditioned stimulus of the third order. Conditioned reflexes of the third order can however be obtained with the help of the second order of conditioned reflexes in defence reactions such as that against stimulation of the skin by a strong electric current. But even in this case we cannot proceed further than a conditioned reflex of the third order.

I shall describe the results of an experiment of Dr. Foursikov illustrating a conditioned reflex of the third order: The unconditioned stimulus to a defence reaction in a dog was given by the application of an electric stimulus to the skin over the front paw. A mechanical stimulation of the skin over the hind paw to which the dog was formerly quite indifferent had been converted by the usual procedure into a conditioned stimulus of the first order, while for the establishment [p. 35] of the second order of reflex a sound of bubbling water had been employed. By combining with the sound of bubbling water a tone, previously indifferent, of 760 double vibrations per second, a conditioned stimulus of the third order was now established. In these conditioned reflexes, passing from the first to the third order, the latent period progressively increases. In the same order we pass from the strongest to the weakest conditioned defence reflex. All these conditioned reflexes were maintained by Dr. Foursikov by means of appropriate reinforcement for over a year, but here again many attempts to combine a still further stimulus with the conditioned stimulus of the third order were quite unsuccessful.

So far we have discussed two distinct modes of formation of a conditioned reflex, one in which it is based directly upon an unconditioned reflex and the other in which it is based upon another conditioned reflex which has already been firmly established. There is, however, yet another method of establishing conditioned reflexes. We were led a considerable time ago to perform experiments of the following type: A dog was given a small dose of apomorphine subcutaneously and after one or two minutes a note of a definite pitch was sounded during a considerable time. While the note was still sounding the drug began to take effect upon the dog: the animal grew restless, began to moisten its lips with its tongue, secreted saliva and showed some disposition to vomit. After the experimenter had reinforced the tone with apomorphine several times it was found that the sound of the note alone sufficed to produce all the active symptoms of the drug, only in a less degree [experiments of Dr. Podkopaev]. Unfortunately, Dr. Podkopaev was prevented from pursuing his experiments and extending them by modifcations in technique. However, quite recently, Dr. Krylov, of the Tashkent Bacteriological Laboratory, has made some interesting observations bearing on this matter, in the course of certain serological investigations, when he had occasion repeatedly to inject morphine into dogs hypodermically. It is well known that the first effect of a hypodermic injection of morphine is to produce nausea with profuse secretion of saliva, followed by vomiting, and then profound sleep. Dr. Krylov, however, observed when the injections were repeated regularly that after 5 or 6 days the preliminaries of injection were in themselves sufficient to produce all these symptoms -- nausea, secretion of saliva, vomiting and sleep. Under these circumstances the symptoms are now the effect, not of the morphine acting through [p. 36] the blood stream directly on the vomiting centre, but of all the external stimuli which previously had preceded the injection of morphine. The connection between the morphine itself and the various signals may in this instance be very remote, and in the most striking cases all the symptoms could be produced by the dogs simply seeing the experimenter. Where such a stimulus was insufficient it was necessary to open the box containing the syringe, to crop the fur over a small area of skin and wipe with alcohol, and perhaps even to inject some harmless fluid before the symptoms could bc obtained. The greater the number of previous injections of morphine the less preparation had to be performed in order to evoke a reaction simulating that produced by the drug. Dr. Krylov was able to demonstrate these facts quite easily in my laboratory. In a series of experiments specially adapted to the purpose he showed that the phenomena described are absolutely identical with conditioned reflexes. The experiments readily lend themselves for lecture demonstration.

Demonstration. -- The dog has repeatedly been injected with morphine on previous occasions, and is now held quietly on the table by an attendant who has never had anything to do with injecting the morphine. When the experimenter approaches, the dog gets restless and moistens its lips, and as soon as the experimenter touches the animal, severe nausea and profuse secretion of saliva begin.

This experiment provides a clue to the well-known fact that dogs will eat meat the first time it is offered them, after removal of their parathyroids, or after an Eck fistula and tying of the portal vein, but on all subsequent occasions refuse it. Evidently in these cases the appearance and smell of meat produce of themselves a reaction identical with that produced through direct pathological action in the absence of the parathyroids or the portal circulation, by those toxic substances deriving from digestion of the meat.

All this brings us to the important question of the intimate mechanism by which new nervous connections are established in the hemispheres. It is easy to suggest an explanation on the basis of the actual facts as they are known at present. Any unconditioned, or any firmly established conditioned, stimulus undoubtedly evokes a state of nervous activity in some definite part of the brain. Using the generally accepted terminology, let us refer to such areas of the brain as centres, not however thereby implying any idea of anatomical localization. During the period of excitation of such centres [p. 37] all other external stimuli which happen to affect the animal are conducted to these centres, and the paths by which they are conducted through the hemispheres become thereby specially marked out. This is the only possible interpretation of the facts, and upon this interpretation was planned the series of experiments with apomorphine which have just been described as corroborating so thoroughly the experiments of Dr. Krylov with morphine. Arguing that excitations set up in the cells of the cortex were constantly transmitted to the salivary centre when this was reflexly [sic] excited to activity by external agencies, we had to expect a precisely similar phenomenon to take place when the centre was stimulated directly (i.e. automatically) by changes in the internal environment due to alterations in the composition of the blood. Excitations set up in the cortex would now also be transmitted to the salivary and to the vomiting centres. This assumption was fully justified in the sequel.

The facts dealt with so far reveal another important feature of this mechanism, namely, that such external stimuli as have been from the very birth of the animal transmitted to a definite centre, can, notwithstanding, be diverted and made to follow another route, becoming linked up by the nervous connection to another centre, provided always that this second centre is physiologically more powerful than the first.

This linking up of impulses in different areas of the brain, by the formation of new nervous connections, is the first nervous mechanism we have encountered in our study of the physiology of the hemispheres. The question as to the site where this new nervous connection occurs has not yet been clearly answered. Is it within the cortex exclusively, or does it take place between the cortex and the subcortical areas? Both possibilities are conceivable. In the latter case it must be assumed that when two points, one in the cortex, the other in a subcortical area, are simultaneously excited by different stimuli, a path is established for the transmission of the excitation direct from the former point to the latter. If on the other hand the connection takes place entirely within the cortex, it is necessary to assume either that all the receptor organs (including internal receptors) of the organism are represented in the cortex, in which case impulses originating in different organs during their activity would be transmitted to the corresponding cortical point, which would then enter into connection with a point excited by the external stimulus, or else that stimuli which lead to activity of an organ gain [p. 38] direct representation in the cortex independently of the simultaneous excitation of a subcortical area. Of the two last mentioned alternatives I have reason to believe that the latter represents what probably takes place in the intact brain when the hemispheres are in a state of alertness. In any case it appears that the cells predominantly excited at a given time become foci attracting to themselves the nervous impulses aroused by new stimuli -- impulses which on repetition tend to follow the same path and so to establish conditioned reflexes.

We must now take some account of the agencies which can be transformed into conditioned stimuli. This is not so easy a problem as appears at first sight. Of course to give a general answer is very simple; any agent in nature which acts on any adequate receptor apparatus of an organism can be made into a conditioned stimulus for that organism. This general statement, however, needs both amplification and restriction. We can, in the first place, divide up natural agencies into their ultimate component parts as regards their properties as physiological stimuli. Even a very small single component of such agency may acquire in itself the properties of a conditioned stimulus. Such, for example, may be a very small variation in loudness of a tone, or s small and barely distinguishable variation in luminosity, and so on. In this way alone the number of potentially effective stimuli in nature is extended almost indefinitely, although it is also obvious that a limit is set to the fineness of gradation of such stimuli by the degree of sensitivity and perfection of the peripheral receptor organs of the organism. On the other hand, the animal may he affected by the sum total of numerous elementary stimuli acting together as a whole. For example, in distinguishing facial appearance we take info account simultaneously form, dimensions, shades, colours. We behave very much in the same way when making out our direction in a more or less familiar neighbourhood. Such examples of compound stimuli can be multiplied indefinitely, and when we consider the unlimited possibilities of grouping the very great number of single elementary stimuli, we shall arrive at a very formidable figure. Yet in this case also a limit is undoubtedly imposed by the intrinsic structure of the cerebral hemispheres themselves. But I wish at present only to give some idea of the possible number of conditioned stimuli, leaving a, more detailed discussion of this important matter to a subsequent lecture.

So far we have considered only one broad group of conditioned [p. 39] stimuli, namely those derived from the appearance of any natural agency. But the disappearance also of such an agency may become the stimulus to a conditioned reflex. Let us take the following example as an illustration. A metronome is sounding continuously in the experimental laboratory when the dog is brought in. The sound of the metronome is now cut out, and immediately an unconditioned stimulus, say food or a rejectable substance, is introduced. After several repetitions of this procedure it is found that the disappearance of the sound has become the stimulus to a new conditioned reflex [Dr. Zeliony and Dr. Makovsky].

Not only can the cessation of a stimulus be made the signal to a conditioned reflex, but also a diminution in its strength, if this diminution is sufficiently rapid. The effect of rate of change of some property of a stimulating agent is well brought out in the following experiment by Dr. Zeliony. The sudden stoppage of a powerful pneumatic tuning fork D-2 had been made into the signal for a conditioned alimentary reflex, which was measured by 32 drops of salivary secretion during the test interval; but a gradual damping down of this tone over a period of 12 minutes, though ending in complete extinction, did not yield a single drop of saliva. So we see that not only can the appearance of some external agency act as a conditional stimulus, but its disappearance also, or the rapid weakening of its strength. Thus the number of potential stimuli to a conditioned reflex is once more greatly increased.

The next group of conditioned stimuli can be regarded as a variation, or further development, of the type of stimuli with deferred action which has just been discussed. The stimulus this time is not the actual disappearance of an external agent, but the trace left by the action of this agent on the central nervous system after the agent itself has been removed. The procedure of development of such reflexes is as follows. Any convenient external stimulus is applied to the animal and continued for to 1 minute. After another definite interval of 1-3 minutes, food or a rejectable substance is introduced into the mouth. It is found thst after several repetitions of this routine the stimulus will not itself evoke any reaction; neither will its disappearance; but the appropriate reaction will occur after a definite interval, the after-effect of the excitation caused by the stimulus being the operative factor. We have to distinguish this type of reflex from the type described previously, in which the unconditioned stimulus coincided for part of its duration [p. 40] with the conditioned syimulus. The type described in this paragraph is termed a trace conditioned reflex.

I shall describe here an experiment by Dr. Grossman with trace conditioned reflexes : A tactile stimulation of the skin was employed as a stimulus for the trace reflex to injection of acid. The tactile stimulus was applied during one minute and acid was introduced into the mouth after a further period of one minute.

Trace reflexes may be of different character, depending on the length of pause between the termination of the conditioned stimulus and the appearance of the unconditioned stimulus. When the pause is short, being a matter of only a few seconds, then the trace left by the conditioned stimulus is still fresh, and the reflex is what we may term a short-trace reflex. On the other hand, if a considerable interval, one minute or more, is allowed to elapse between the termination of the conditioned and the beginning of the unconditioned stimulus we have a long-trace reflex. It is important to distinguish all these cases on account of the essential peculiarities exhibited by long-trace reflexes.

We come now to the last of those agents which can be transformed into a conditioned stimulus, and this one is peculiar in that it originates apparently quite regularly and spontaneously. Its operation depends on the fact that every stimulus must leave a trace on the nervous system for a greater or less time-a fact which has long been recognized in physiology under the name of after effect. The further agency I wish to introduce to you is no less real, but the clear apprehension of its nature is apt to present some difficulty. [p. 41]

First of all I shall describe an experiment of a general nature. One dog was given food at regular intervals of time; another had acid introduced into the mouth at the same intervals. After this had gone on for a little time it was found that food or acid was no longer necessary to produce the alimentary or mild defence reflex, but that these reflexes appeared spontaneously at the regular intervals of time. This may be illustrated by the following detailed experiment of Dr. Feokritova: A dog is placed in the stand and given food regularly every thirtieth minute. In the control experiments any-one feeding after the first few is omitted, and it is found that despite the omission a secretion of saliva with a corresponding alimentary motor reaction is produced at about the thirtieth minute. Sometimes this reaction occurs exactly at the thirtieth minute, but it may be one or two minutes late. In the interval there is not the least sign of any alimentary reaction, especially if the routine has been repeated a good number of times. When we come to seek an interpretation of these results, it seems pretty evident that the duration of time has acquired the properties of a conditioned stimulus.

The experiment just described may be performed with the following modification. The animal can be given food regularly every thirtieth minute, but with the addition, say, of the sound of a metronome a few seconds before the food. The animal is thus stimulated at regular intervals of thirty minutes by a combination of two stimuli, one of which is the time factor and the other the beats of the metronome. dl this manner a conditioned reflex is established to a compound stimulus consisting of the sound plus the condition of the hemispheres at the thirtieth minute, when both are reinforced by food. Further, if the sound is now applied not at the thirtieth minute after the preceding feeding, but, say, at the fifth or eighth minute, it entirely fails to produce any alimentary conditioned reflex. If it is applied slightly later it produces some effect; applied at the twentieth minute the effect is greater; at the twenty-fifth minute greater still. At the thirtieth minute the reaction is of course complete. If the sound is never combined with food except when applied at the full interval, in time it ceases to have any effect even at the twenty-ninth minute and will only produce a reaction at the thirtieth minute -- but then a full reaction. An illustration is given from experiments by Dr. Feokritova: A dog is given food every hall-hour, each feeding being preceded by the sound of a metronome, [p. 42] which is continued for thirty seconds; the effect of the metronome is tested at the twenty-ninth minute.

Of course in the establishment of a conditioned reflex of this type any length of time interval can be employed. No experiments, however, were made with longer intervals than half an hour.

What is the physiological meaning of these time intervals. In their rôle as conditioned stimuli? Only a tentative approach can be made to a definite answer to such a, question at present: Time is measured from a general point of view by registering different cyclic phenomena in nature, such for instance as the rising and setting of the sun or the vibration of the pendulum of a clock. But many cyclic phenomena take place inside the animal's body. In the course of 24 hours the brain receives a very considerable number of stimuli, becomes fatigued, and again restored through sleep. The alimentary canal is periodically filled or emptied; and, in fact, changes in practically all the component tissues and parts of the organism are capable of influencing the cerebral hemispheres. This continuous cycle of direct and indirect influences upon the nervous activity constitutes the physiological basis for the estimation of duration of time. We may consider the following simple case of physiological registration of short intervals of time by the hemispheres. It is well known that a fresh stimulus -- we will take for example an olfactory stimulus -- produces a very definite nervous excitation. This, however, gradually and progressively weakens. The physiological state of the nervous elements, under the influence of the continued stimulation, without doubt undergoes a series of simultaneous and successive changes. The same is true for the reverse condition: When a stimulus is withdrawn the change is perceived for some time very acutely, but quite soon its influence diminishes until we are no longer aware of it. Here again the physiological [p. 43] condition of the nerve cells is, doubtless, undergoing the reverse series of changes. From this point of view we can give an interpretation of the establishment of conditioned reflexes to an interruption of a stimulus, and to a trace of a stimulus, as well as, apparently, to a duration of time. In the experiment described the administration of food was accompanied and followed by a definite activity in a large number of organs, which all underwent e series of definite cyclic changes. All these changes were reflected in the cerebral hemispheres where they fell on appropriate receptive fields, and a definite phase of these changes acquired the properties of the conditioned stimulus.

To conclude this part of our discussion I shall suggest the following modification and amplification of our definition of agencies which can become conditioned, viz. that innumerable individual fluctuations in the external and internal environment of the organism may, each and all of them, singly or collectively, being reflected in definite changes in the cells of the cerebral cortex, acquire the properties of a conditioned stimulus.

We shall now follow out another important group of phenomena. Hitherto we have been dealing with reflexes of a positive character -- i.e. reflexes which ultimately gave rise to positive reactions, both motor and secretory, all of them associated with various processes of excitation in the nervous system. There is, however, another manifestation of nervous activity which is in no way inferior in physiological and vital importance to that positive manifestation which has just been considered. I refer to nervous inhibition. When we come to investigate the highly complex functions of the cerebral hemispheres, we naturally expect to come across inhibitory phenomena, for these are very constantly and very intimately mixed up with the positive phenomena of nervous excitation. But before dealing with these I consider it advisable to give a brief description of inhibition of centres as observed in the field of unconditioned reflexes.

The present state of physiological knowledge enables us to recognize under normal conditions two types of central inhibition. These may be termed direct and indirect, or internal and external respectively. On the one hand we are familiar with the direct inhibitory action of certain afferent nerves, or of certain physical and chemical properties of the blood, upon definite nervous centres controlling respiration, circulation, locomotion and so on. On the [p. 44] other hand, the central nervous system supplies numerous cases of indirect inhibition. In the latter cases inhibition of the activity of a given centre is brought about as the result of an activity of some other centre, which activity has arisen in its turn as a result of excitation from some afferent nerve or from some change in the composition of the blood. Instances of other kinds of inhibition are provided by those complex unconditioned reflexes generally known as "instincts." Thus many insects, especially when in the larval stage, become immobilized and drop down at the slightest touch. This is obviously a case of direct inhibition of the entire nervous apparatus of locomotion. Another example may be taken from the newly hatched chick, which manifests straightway a pecking reflex to the visual stimulus of small objects or patches of light and shade. If, however, a strongly irritant and injurious substance gets taken up, the pecking reflex becomes inhibited immediately and is replaced by a defence reaction leading to a rejection of the irritating substance. This is a result of an interaction of two active centres, and is an example of external inhibition.

Conditioned reflexes are also subject to these two types of central inhibition. Since the indirect or external inhibition of conditioned reflexes does not differ in the least from the corresponding inhibition of unconditioned reflexes I shall take this first. The following is a very simple case, and one of common occurrence in our earlier experiments. The dog and the experimenter would be isolated in the experimental room, all the conditions remaining for a while constant. Suddenly some disturbing factor would arise -- a sound would penetrate into the room; some quick change in illumination would occur, the sun going behind a cloud; or a draught would get in underneath the door, and maybe bring some odour with it. If any one of these extra stimuli happened to be introduced just at the time of application of the conditioned stimulus, it would inevitably bring about a more or less pronounced weakening or even a complete disappearance of the reflex response, depending on the strength of the extra stimulus. The interpretation of this simple case does not present much difficulty. The appearance of any new stimulus immediately evokes the investigatory reflex and the animal fixes all its appropriate receptor organs upon the source of disturbance, pricking up its ears, fastening its gaze upon the disturbing agency, and sniffing the air. The investigatory reflex is excited and the conditioned reflex is in consequence inhibited. [p. 45]

These extra stimuli have also another important influence. Every stimulus, however rapidly it may disappear, is effective not only while it lasts, but also for some time after its cessation while its after-effect lasts. On account of this, if the conditioned stimulus is applied shortly after the extra stimulus, the reflex reaction will be partly inhibited. Different extra stimuli, whether accidental or deliberate, produce after-effects of different length. Some may still have a powerful after-effect after 2 or 3 minutes, others after 10 or 12 minutes, and still others after several days. These longer after-effects occur especially with alimentary, and gustatory stimuli in general, and in many experiments they have to be taken seriously into account. The effect of exciting an extra reflex will, of course, vary according as the conditioned reflex has only freshly been formed or has already been firmly established. It is obvious that an old-established reflex is not likely to be so easily inhibited as a recent one. In our old laboratory the neglect to provide against external stimuli often led to a curious complication when I visited some of my co-workers. Having by himself established a new conditioned reflex, working in the room with the dog, the experimenter would invite me for a demonstration, and then everything would go wrong and he would be unable to show anything at all. It was I who presented the extra stimulus: the investigatory reflex was immediately brought into play: the dog gazed at me, and smelled at me, and of course this was sufficient to inhibit every recently established reflex. Another example is very similar. If one experimenter had worked with a dog and established some firm and stable conditioned reflexes, conducting numerous experiments with them, when he handed the animal over to another experimenter to work with, all the reflexes disappeared for a considerable time. The same happened when the dog was changed over from one research room to another.

In the case of extra stimuli, which awaken specialized reflex responses, the resulting inhibition is extremely profound. Examples of such extra stimuli are, the sight or sound of game for sporting dogs, of cats for some dogs, rustling under the floor for others, and so on. Not only such specialized stimuli, but also any very strong or unusual stimulus exhibits a prolonged and pronounced inhibitory after-effect. With respect to strong or unusual stimuli dogs can be divided into two groups. Some respond in a manner which may be termed positive, i.e. aggressively -- barking furiously and baring [p. 46] their teeth. Others exhibit a defence reaction of a passive nature -- they try to get free and run away or they stand like lumps of stone absolutely motionless; and sometimes they will shiver violently and crouch down in the stand; or they may urinate, a most unusual occurrence for dogs while in the stand. In these dogs inhibition predominated. This case can also be classified as a type of indirect or external inhibition of conditioned reflexes, for the inhibition originates primarily in other parts of the brain than that where the reflex response is initiated.

All the cases of external inhibition which have just been discussed are of a temporary nature; and on this account the stimuli producing these effects are termed temporary inhibitory stimuli. If they act upon the animal repeatedly and are not reinforced with any unconditioned or conditioned stimulus, sooner or later they become lost upon the organism and lose all their inhibitory properties.

An experiment providing an admirable illustration of this condition occurred during the first two lectures, without the audience being aware of it. The dog used for the experiment with the metronome (p. 22) had been kept in the lecture theatre during the whole of one lecture previous to the demonstration, while one of my co-workers repeatedly tried the experiment. As a matter of fact the experiment was at first a failure, the reflex being inhibited, and the animal only freed itself graduadly from the inhibitory influence of the unfamiliar surroundings.

Some years ago, when I was delivering a series of lectures on conditioned reflexes, I had to proceed in a similar manner for demonstration of experiments. The dogs were placed in the lecture theatre from the very beginning of the course of lectures, and my co-workers constantly repeated the experiments preliminary to the actual lecture demonstration, and only with this precaution could the experiments be successfully conducted. In this series of lectures, unfortunately, the present unavoidable and unforeseen circumstances have deprived me of these facilities; I shall have to restrict myself to occasional demonstrations, keeping chiefly to a description of experiments and researches conducted in our laboratories.

In addition to the temporary external inhibition, a second type of external inhibition can be distinguished. This may be termed permanent external inhibition, since it does not weaken through repetition, but constantly maintains its strength. An example may be found in the reaction to acid. If the dog is given food, even [p. 47] some considerable time before the conditioned stimulus to the acid is applied, this reflex cannot be obtained. What happens is that the alimentary centres inhibit the reaction to acid for a considerable time after they themselves are set into activity, and this happens always, no matter how often the experiment may be repeated. It is therefore a permanent external inhibition which does not undergo any sort of spontaneous extinction. Such cases are not at all rare. If a conditioned reflex is established to introduction of acid, any careless administration -- too much acid, or too concentrated, or applied too often -- produces a severe irritation of the buccal mucous membrane and produces a permanent inhibition of conditioned reflexes which lasts until the pathological condition is removed. Again, the dog may have some laceration of the skin which may be constantly irritated by the supporting loops of the stand. The defence reflex will then become dominant while the conditioned reflexes, and especially the mild defence reflex to acid, will be inhibited. Many more examples of this nature could be quoted. I will give one or two more. In one case an experiment may be running quite smoothly, and suddenly all conditioned reflexes begin to fail, and finally disappear altogether. The dog is taken out, allowed to urinate, and then all the reflexes return to normal. Evidently stimulation of the centre for urination had inhibited the conditioned reflexes. Another example may be chosen in the season when the females are in heat. If the males have been housed near the females before the experiment, it is found that all their conditioned reflexes are inhibited in greeter or less degree. It is obvious in this case that the inhibition derives from the sexual centres in the hemispheres. In face of such numerous potential sources of inhibition we see that our term of "conditioned" reflexes is very appropriate. Yet these conditions can readily be controlled, and the disquieting factors can be eliminated. All of them produce external inhibition, and I should like to give a final summing up of their character: No sooner does any extra nervous excitation occur in the central nervous system than it immediately makes its presence felt in diminishing or abolishing conditioned reflexes, but temporarily only, as long as the causative stimulus or its after-effect is present.