Comparative Physiology of the Brain and Comparative Psychology

be no doubt that the connection between the morpho- genetic functions and the central nervous system is much more slight than between this organ and the sensory and motor functions. I am inclined to believe that the simultaneous dis- appearance of gills and tail-fins is due to some change occurring in the blood, — e.g., the appearance of certain enzymes, or possibly changes in the number of red blood corpuscles, etc. It has been urged that in this experiment the sympa- thetic system transmitted the connection between the two halves of the animal. The sympathetic has always been used as a bridge across the gulf between preconceived notions and facts. I am pretty certain that at least in a number of my Amblystomas the sym- pathetic was cut. But as I did not make sure of this I will not urge this point. But I may at least point out the true reliability of this bridge. It had been a generally accepted belief that the secretory activity of the milk glands during and after pregnancy was caused by the stimulation of the nerve-endings of the uterus. Goltz severed the spinal cord in the pectoral region of a female dog which afterwards became preg- nant and gave birth to young ones. It turned out that the mammary glands in front and behind the place of the section began to secrete milk equally well. Goltz concluded that the secretion was not due to a nervous influence. As was to be expected, those who try to explain everything by the omnipotence of the central nervous system at once pointed out that the

sympathetic connected the two halves of the spinal cord in Goltz's dog. Recently Ribbert made an ex- periment which, if correct, does away with these mysterious sympathetic influences (8). He trans- planted a milk gland to the ear of a guinea-pig. The guinea-pig became pregnant and the gland on the ear began to secrete. It is evident that a change in the blood or lymph must be responsible for the secretion of milk glands during pregnancy, possibly the appear- ance of certain enzymes.

Schaper has added an experiment that speaks for the lack of dependence of the morphogenetic devel- opment on the central nervous system. In a tadpole six mm. long he extirpated the brain and the medulla oblongata. When the animal was killed seven days later, the spinal cord seemed to have vanished. Nevertheless the healing of the wound, growth, and development continued during the seven days (2). In face of the fact that the first processes of de- velopment precede the formation of the central ner- vous system in every animal, these results need not surprise us. They suffice, however, to convince us that the processes of development and the formation of organs are less closely connected with the central nervous system than the sensory and motor processes. For this reason we cannot well decide in favour of the assumption that every impression on the central nerv- ous system must impart itself to the egg, with which it is, moreover, not connected.

mental diseases are hereditary harmonise with this view? It is, perhaps, not impossible that those men- tal diseases that are hereditary are, in reality, chem- ical diseases caused by poisons that are formed in the body just as special substances, for instance, alcohol, hashish, and other intoxicating substances, produce temporary mental diseases (3). The delirium of fever as well as certain other mental diseases may owe their origin to poisons which are formed in the body. It is quite possible that these poisons are also formed in the normal body. It is only necessary that they be formed in somewhat larger quantities or destroyed in somewhat smaller quantities in the body of the in- sane than in the normal man. It is further not at all necessary that these hypothetical poisons which cause mental diseases be formed in the central nervous sys- tem. They may be formed in any organ of the body. It is only necessary that they affect the central nervous system - - in other words, that they be nerve-poisons. Nothing is better qualified to make this view clear than the result which the destruction of the thyroid gland has on the mental and physical development of children. We know that in case of degeneration of the thyroid gland the growth and mental development of children are retarded. Idiocy may result from the de- struction of the thyroid gland. It has been found that an improvement or even a cure can be attained by feeding patients afflicted with this trouble with the thyroid substance of animals. Baumann found that the thyroid gland contains an element which is

contained in no other organ of the body, namely, io- dine. It is thus conceivable that hereditary mental diseases are chemical diseases. The germ-cells may in these diseases also be influenced by the poisons circulating in the blood. 4. If we thus deny the immediate influence of the central nervous system on the germ, and assume a chemical theory of heredity, it might still be possible that the central nervous system could influence hered- ity indirectly, in so far as it can affect the chemical processes of the body. As illustrations of a chemical effect of the nerves, the fact is mentioned that stimu- lation of the nerves of certain glands produces a secre- tion. Mathews has shown, however, that in cases where stimulation of the sympathetic produces a secre- tion, the glands contain muscular fibres which contract when stimulated, and in this way press a liquid out of the ducts (4). (Conditions seem to be different in the case of the secretion produced by stimulation of the chorda, but it is also possible that in this case the secre- tion is only an indirect effect of the stimulation caused by changes in the circulation.) There are other cases of an apparent chemical effect of the nerves. The fact that herpes zoster follows the nerves has led many to assume that this disease is caused by a trophical influence of the nerves. But we know that in the case of rabies the micro-organism or the poison creeps along the nerves. Goltz has found that ulcer- ations and suppurations occur on the skin behind the cut after division of the spinal cord, which are so sym-

metrical that it is impossible to attribute them solely to external injuries. They occur only during the first weeks after the operation, disappearing later on (5). It is conceivable that the cause of these phenomena is to be sought in abnormal chemical processes which are perhaps caused by the vasomotor nerves in so far as disturbances in the supply of oxygen, etc., are de- termined by them. These disturbances occasionally fail to appear. Physicians are familiar with these phenomena of bed-sores which ensue after lesion of the spinal cord. One fact that Goltz and Ewald found is especially interesting for the theory of these pro- cesses. When they severed the spinal cord of animals, these phenomena of ulceration of the skin were very pronounced. But if they afterward operated on the spinal cord behind the cut, the disturbances were much less severe or failed to appear. Thus the separation of a part of the spinal cord from the brain is accom- panied by more serious consequences than the sub- sequent destruction of the spinal cord itself (5).

An inflammation of the cornea occurs generally after the division of the trigeminus of the same side. This inflammation is naturally caused by bacteria, but the fact that these bacteria affect the cornea whose sensory nerve is severed might have two causes : either the animal on account of the lack of sensibility might not notice the foreign bodies (dust, etc.) that get into the eye and cause a wound, or as a result of the division of the nerve changes take place in the cornea which render it more susceptible to inflam-

mation. The latter might be the case if Gaule's statement is correct, namely, that histological changes can be shown in the cornea ten minutes after the division of the trigeminus (6, 7). In this case it can only be that the power of resistance or, more accurately speaking, the chemical nature of the tissue is changed as a result of the lesion of the nerve. If this be true, it does not force us to the assumption of specific trophic nerves ; if it is true that the influence of every nervous im- pulse on the affected tissue is chemical, all nerves are in one sense trophic, and it would be quite erroneous to maintain that certain nerves serve trophic func- tions exclusively while others are sensory and motor. There are no specifically trophic nerves, but it is pos- sible that many nerves produce indirectly (for instance, through disturbances of the circulation and limitation of the supply of oxygen) such extensive chemical changes that morphological changes of the tissue ensue.

If this is in reality the case, a possibility still exists that the central nervous system also affects the sex- ual cells indirectly, in so far as disturbances of circu- lation and hence chemical changes are produced, which may modify the sexual cells contained in the testes and ovaries chemically. Thus there might be a very remote chance that brain-activity of one generation might lead to the formation of chemical substances which affect the sexual cells. It is difficult to under- stand, however, what should cause these sexual cells

to produce descendants with greater intellect. The intellect is not proportional to chemical changes, like muscular activity. In the brain of an idiot and of a genius the same chemical changes may occur. The difference between the two, however, is that the idiot fails to notice valuable associations of ideas while the brain of the genius retains them. We arrive thus at the conclusion that a transmission of hereditary char- acteristics through the egg is only possible in the form of specific chemical substances, and that the central nervous system could only influence heredity, if it could bring about the formation of special sub- stances in the egg (by influencing metabolism). It would, of course, first have to be proved that the cen- tral nervous system has such an influence upon the sexual cells, and this is extremely doubtful. For this reason we should not be justified in maintaining that the activity of a generation can produce an heredi- tary increase of the ability and tendencies in the same direction. Herbert Spencer gives as a proof of this last possibility the fact that the circles of touch in the tip of our toncrue are the smallest. He believes that

this is due to the fact that from time immemorial man had the tendency to examine the spaces between the teeth with the tongue, and this is supposed to have caused an hereditary increase in the nerve-endings of the tongue. Spencer overlooks the fact that in the tip of the nose the circles of touch are also a comparative minimum, and it is certain that this organ has not been used for such a purpose since time immemorial. It is more probable that the relative number of the nerve-endings or, more correctly speaking, the rela- tive size of the circles of touch in the tip of the tongue and the tip of the nose is determined by the relatively small radius of curvature or the minimal areal growth of these tips.

1. LOEB, J. Hat das Centralnervensystem einen Einfluss auf die Vor gauge der Larvenmetamorphose 1 Archiv fiir Entwickelungs- mechanik, Bd. iv., 1896. 2. SCHAPER, A. Experimental Studies on the Influence of the Central Nervous System upon the Development of the Embryo. Journal of the Boston Soc. of Medical Science, Jan., 1898. 3. MEYER, ADOLF. A Short Sketch of the Problems of Psy- chiatry. Am. Jour, of Insanity, vol. liii., 1897. 5. GOLTZ and EWALD. Der Hund mit verkurztem Riicken- mark. Pfliiger's Arch., Bd. Ixiii., 1896.

6. GAULE, J. Der Einfluss des Trigerninus auf die Hornhaut. Physiologisches Centralblatt, Bd. v., 1891. 7. GAULE, J. Wie beherrscht der Trigeminus die Erndhrung der Hornhaut. Physiologisches Centralblatt, Bd. vi., 1892. 8. RIBBERT, H. Ueber Transplantation von Ovarium, Hoden und Mamma. Arch. f. Entwickelungsmechanik^ vol. vii., 1898. i. The most important problem in the physiology of the central nervous system is the analysis of the mechanisms which give rise to the so-called psychic phenomena. The latter appear, invariably, as a func- tion of an elementary process, namely, the activity of the associative memory. By associative memory I mean the two following peculiarities of our central nervous system : First, that processes which occur there leave an impression or trace by which they can be reproduced even under different circumstances than those under which they originated. This pecu- liarity can be imitated by machines like the phono- graph. Of course, we have no right to assume that the traces of processes in the central nervous system are analogous to those in the phonograph. The sec- ond peculiarity is, that two processes which occur simultaneously or in quick succession will leave traces which fuse together, so that if later one of the pro- cesses is repeated, the other will necessarily be re- peated also. The odour of a rose will at the same

time reproduce its visual image in our memory, or, even more than that, it will reproduce the recollection of scenes or persons who were present when the same odour made its first strong impression on us. By associative memory we mean, therefore, that mechan- ism by means of which a stimulus produces not only the effects which correspond to its nature and the specific structure of the stimulated organ, but which produces, in addition, such effects of other causes as at some former time may have attacked the organism almost or quite simultaneously with the given stimu- lus (2). The chief problem of the physiology of the brain is, then, evidently this : What is the physical character of the mechanism of associative memory ? As we said in the first chapter, the answer to this question will probably be found in the field of physi- cal chemistry.

I think it can be shown that what the metaphys- ician calls consciousness are phenomena determined by the mechanism of associative memory. Mach has pointed out that the consciousness of self or the ego is simply a phrase for the fact that certain constitu- ents of memory are constantly or more frequently produced than others (i, 1 1). The complex of these elements of memory is the " ego " or the "soul," or the personality of the metaphysicians. To a cer- tain extent we are able to enumerate these con- stituents. They are the visual image of the body so far as it lies in the field of vision, certain sensations of touch which are repeated very frequently, the

sound of our own voice, certain interests and cares, a certain feeling of comfort or discomfort according to temperament or state of health, etc. (i, n). An inventory of all the memory-constituents of the ego-complex of different persons would show that the consciousness of self is not a definite unit, but, as Mach maintains, merely an artificial separation of those constituents of memory which occur most fre- quently in our perceptions. These will necessarily be subject to considerable variation in the same per- son in the different periods of life.

If we speak of loss or an interruption of conscious- ness, we mean a loss or an interruption of the activity of associative memory. If a faint is caused directly by lack of oxygen or indirectly by a disturbance in the circulatory system, the activity of associative memory ceases. This was proved by Speck's experiments on the effects of a low pressure of oxygen. When he breathed air with less than eight per cent, of oxygen, he soon fainted. In these experiments, he had to count the number of respirations. Before he fainted, he became confused in his counting and forgot what happened. When this disturbance in counting began to appear, he knew it was time to discontinue the ex- periment. When a loss of consciousness is produced by narcotics or anaesthetics, we have again to deal with an interruption in the activity of the associative memory. It is the same in the case of a deep sleep.

The metaphysician speaks of conscious sensations and conscious will. That the will is only a function of the mechanism of the associative memory can be proved. We speak of conscious volition if an idea of the resulting final complex of sensations is present before the movements causing it have taken place or have ceased. In volition three processes occur. The one is an innervation of some kind which may be caused directly or indirectly by an external stimulus. This process of innervation produces two kinds of effects. The one effect is the activity of the associa- tive memory which produces the sensations that in former cases accompanied or followed the same inner- vation. The second effect is a coordinated muscular activity. It happens that in such cases the reaction- time for the memory-effect of the innervation is shorter than the time for the muscular effect. When some internal process causes us to open the window, the activity of the associative memory produces the idea of sensations which will follow or accompany the opening of the window sooner than the act of opening really occurs. As we do not realise this any more than we realise the inverted character of the retina-image, we consider the memory-effect of the innervation as the cause of the muscular effect. The common cause of both effects, the innervating pro- cess, escapes our immediate observation as our senses do not perceive it. The will of the metaphysicians is then clearly the outcome of an illusion due to the necessary incompleteness of self-observation. Our conception of will harmonises with Miinsterberg's and James's views on this subject (6, 12). I think

that we are justified in substituting the term activity of associative memory for the phrase consciousness used by the metaphysicians. 2. We have spoken of associative memory because the word memory is often applied in quite a different sense scientifically, namely, to signify any after-effect of external circumstances. For instance, the term memory has been used to account for the fact that a plant which had been cultivated in the tropics will often not endure low temperatures so well as a plant of the same species which was raised in the north. It is true in this case that preceding conditions influence the ability of the plant to react, but the process differs from the one which we have called associative memory in the lack of associative processes. No definite stim- ulus produces in a plant, in addition to its own effects, those of another entirely different stimulus which at some former time occurred simultaneously with the given stimulus. It is probable that the tropical plant is somewhat different chemically from the plant raised in the north. This would account for its smaller power of resistance. Further illustrations of a differ- ent use of the word memory can easily be given.

Many moths sleep during the day and wake in the evening when it becomes dark. If kept for days in a dark room, they will continue at first to do the same thing. The same is true of certain plants. One might also say in this case that the moth or the plant " remembers " the difference between day and night. It is probable, however, that internal changes take place in the organism, corresponding to the periodic change of day and night, and that these changes con- tinue for a time in the same periodicity, when the ani- mal is kept in the dark.

3. We will then consider the extent of associative memory in the animal kingdom instead of the extent of consciousness among animals. How can we deter- mine whether an animal possesses the mechanism necessary for associative memory ? The criteria for the existence of associative memory must form the basis of a future comparative psychology. It will require more observations than we have made at present to give absolutely unequivocal criteria. For the present, we can say that if an animal can learn, that is, if it can be trained to react in a desired way upon certain stimuli (signs), it must possess associative memory. The only fault with this criterion lies in the fact that an animal may be able to remember (and to associate) and yet may not yield to our attempts to train it. In this case other experiments must be substituted which will prove that the animal does associate or remember.

We may conclude that associative memory is pre- sent when an animal responds upon hearing its name called, or when it can be trained upon hearing a certain sound to go to the place where it is usually fed. The optical stimulus of the place where the food is to be found and the sensations of hunger and satiety are not qualitatively the same, but they occur simultaneously in the animal. The fusion or growing together of heterogeneous but by chance simultaneous

processes is a sure criterion for the existence of as- sociative memory (2). Associative memory probably exists in most mam- mals. The dog which comes when its name is called, which runs away from the whip, which welcomes its master joyfully, has associative memory. In birds, it is likewise present. The parrot learns to talk ; the dove finds its way home. In lower Vertebrates, mem- ory is also occasionally found. Tree-frogs, for ex- ample, can be trained, upon hearing a sound, to go to a certain place for food. In other frogs, Rana escu- lenta, for instance, no reaction is as yet known which proves the existence of associative memory. Some fishes evidently possess memory ; in sharks, however, its existence is doubtful. With regard to the Inver- tebrates, the question is difficult to determine. The statements of enthusiasts who discover consciousness and resemblance to man on every side should not be too readily accepted.

4. In my experiments on the tropismsof animals, it became clear to me how easy it is for an observer who is inclined to think anthropomorphically to re- gard machine-like effects of external .stimuli on lower animals as the expression of intelligence. He needs only to neglect the analysis of the external stimuli. I have protested against the anthropomorphisms of Romanes, Eimer, Preyer, and others in a series of ar- ticles (2, 3). Bethe has recently published a paper on the psychic qualities of ants and bees in which he took special pains not to fall into the gross anthropo-

morphisms which have characterised this field here- tofore (4). But I am afraid that he went too far and that he overlooked the fact that bees and ants possess associative memory. Bethe assumes associa- tive memory as the criterion for the existence of con- sciousness, as I had done before. (He has evidently overlooked, or at least does not mention, my work on this subject.) According to him : " An animal that is able to do the same things the first day of its exist- ence which it can do at the end of its life, that learns nothing, that always reacts in the same way upon the same stimulus, possesses no consciousness." This statement is not sufficient. It is possible that an ani- mal at birth, or just after hatching, may not be fully developed. In this case it may be able later to per- form actions which would have been impossible on the first day, without possessing associative mem- ory. Yet according to Bethe's definition such actions would indicate associative memory.

It is a well-known fact that if an ant be removed from a nest and afterwards put back it will not be attacked, while almost invariably an ant belonging to another nest will be attacked. It has been customary to use the words memory, enmity, friendship, in de- scribing this fact. Now Bethe made the following experiment. An ant was placed in the liquids (blood and lymph) squeezed out from the bodies of nest companions and was then put back into its nest ; it was not attacked. It was then put in the juice taken from the inmates of a " hostile " nest and was at once

attacked and killed. Hence chemical stimuli of certain volatile substances will excite the ants. In this case we do not need to assume intelligence any more than we do in the case of the tentacles of Ac- tinians which, as we have seen, will immediately carry a piece of filter paper soaked in meat-juice to the mouth while they ignore a piece of paper soaked in sea-water. The assumption of machine-like irritable structures is quite sufficient here to explain the reaction. Mem- ory is quite unnecessary. Possibly the behaviour of the ant may be explained in the same way. Bethe was able to prove by special experiments that these reactions of ants are not learned by experience, but are inherited. The "knowing "of "friend and foe " among ants is thus reduced to different reactions, depending upon the nature of the chemical stimulus and in no way depending upon memory.

Memory and intellect are supposed to be responsible for the fact that an ant is able to find its way back to the nest and that when " foragers " have discovered honey or sugar the other ants of the nest soon go to it in great numbers. The ability to communicate in- formation was assumed in this case. Bethe, however, was able to determine by means of ingenious experi- ments that an ant, when taking a new direction from the nest for the first time, always returns by the same path. This shows that some trace must be left be- hind which serves as a guide back to the nest. If the ant returning by this path bear no spoils, Bethe found that no other ants try this direction. But if it

bring back honey or sugar, other ants are sure to try the path. Hence something of the substances carried over this path by the ants must remain on the path. These substances must be strong enough to affect the ants chemically. I can prove by the following obser- vation, which must surely have been made before me by many breeders of butterflies, that Bethe is justified in the assumption that insects are affected by ex- tremely weak chemical stimuli. I placed a female butterfly of a certain species in a cigar-box, and closed the box. The box was then suspended half way be- tween the ceiling and floor of the room and then the

window was opened. At first no butterfly of this species was visible far or near. In less than half an hour a male butterfly of the same species appeared on the street. When it reached the height of the window, its flight was retarded and it came gradually toward the window. It flew into the room and soon up to the cigar-box, upon which it perched. During the afternoon, two other males of the same species came to the box. Thus we see that butterflies and certainly many other insects possess a delicacy of chemical irritability which, if possible, is finer than that of the best blood-hound. Plateau maintains that insects are attracted to the flowers by the odour rather than by the colour and marking. The dioptric apparatus of insects is very inferior to that of the human eye, while their chemical irritability is much superior to that of our olfactory epithelium. I believe that both odour and colour may influence insects.