Comparative Physiology of the Brain and Comparative Psychology

The results of some investigations carried on by Garrey and myself showed that if a constant current be sent through a trough in which are larvae of Amblystoma, peculiar changes may be observed in the postures of the animals (i). If the current passes through them longitudinally from head to tail (Fig. 37) the back becomes convex and the ventral side concave. This change of position is occasioned by the muscles of the ventral side (the flexors of the spinal column) becoming more tense than the dorsal

muscles (the extensors of the spinal column) from the passage of the current. On the other hand, if the current goes through the animal in the direction from tail to head both head (Fig. 38), and tail are raised. The body becomes more concave on the dorsal side and convex on the ventral side. The extensors of the spinal column become more tense than the ventral muscles. A pronounced opisthotonus exists. In order to show the phenomenon clearly the animal must be brought into the current gradually. If we continue to raise the intensity of the current, changes of position also take place in the legs. The changes in the hind-legs are more easily described than those in the fore-legs. If the current passes from head to tail the hind-legs are braced backward (Fig. 37), making the forward movement (to the anode) easier. If the current passes from tail to head the hind-legs are braced forward (Fig. 38), making the backward movement (to the anode) easier. How can these phenomena be explained ? The current has two kinds of effects. A conduction of the current takes place through ions. Wherever the progress of ions is blocked in the central nervous system, an increase in their concentration will occur and this must be followed by physical or chemical alterations of the colloids. The progress of ions may be blocked by semipermeable membranes at the external limit of neurons or some- where inside the neurons. Wherever anions are blocked different effects (anelectrotonus) will be pro- duced than at places where the progress of kations is

blocked (katelectrotonus). The action of the various ions on nerve-elements is as yet unknown. The other effect of the current may consist in the migration of certain colloids in one direction and of water in the other direction. If in the larvae of Amblystoma the tension of the flexors of the spinal column predominates in a de- scending current (from head to tail) and the tension of the extensors of the spinal column predominates in an ascending current, this proves that the nervous ele- ments of the flexors and extensors in the central ner- vous system, whicJi are affected by the ions, possess an opposite orientation. Maxwell and I have developed more definite ideas concerning the orientation of these elements, but such details are for the time being of minor importance. I only wish to state that the rela- tive orientation of these elements must be the same in every segment of the spinal cord ; for when the spinal cord in the larvae of Amblystoma is severed or the whole animal cut into several pieces the effects of the current remain the same. Since the article mentioned above was published I have found that crayfish (young and small specimens were used for these experiments) behave toward the current like Amblystoma larvae. If the median-plane of the crayfish is in the direction of the lines of the current (which are all straight and parallel in these experi- ments) and the head is turned toward the anode, the flexors of the body contract and the crayfish rolls it- self into a complete ring, provided that the density of

the current .is exactly right. The back is convex, the ventral side concave. But if the current passes from tail to head, the back becomes entirely straight, the extensors being contracted to the utmost limit.1 The dorsal side cannot become concave because the exo- skeleton of the crayfish does not allow it. Thus in the body of the crayfish the motor elements of the extensors and flexors which are affected by the cur- rent must have the same orientation as in the body of Vertebrates. This holds good not only for the flexors and extensors of the body but generally, as we shall at once see.

We have already mentioned the fact that a con- stant current passing through Amblystoma larvae in the longitudinal direction affects not only the tension of the flexors and extensors of the body, but also the muscles of the extremities. The tension too is changed in such a way, as has been already intimated, that it renders movement toward the anode easy, movement toward the kathode difficult. If, for in- stance, the current passes through the animal from head to tail the hind-le^s are braced backward and

the position of the fore-legs is changed correspond- ingly, so that the progressive movement of the ani- mal is made easy, the backward movement dif- ficult. If, however, the current passes from tail to head the hind-legs are braced forward and the posi- tion of the fore-legs is changed correspondingly ; the 1 It is necessary that in this experiment the intensity of the current be increased very slowly. animal can go backwards easily, while forward move- ment is difficult. In fact it is apparent that if the animals attempt to move in any direction while the current is passing through them they go toward the anode.1

From this fact it follows that, for the nervous appa- ratus of the progressive movements of Amblystoma, a close relation must exist between the orientation of the determinative elements of the motor nerves and their function : in fact the nervous elements which cause the progressive movements must have with re- gard to the longitudinal direction of the animal an orientation which is opposite to the orientation of those elements which cause the backward movement. Garrey and I had called attention to the fact that the observations of Blasius and Schweitzer show that other Vertebrates, for instance, young eels, behave like Amblystoma. The same also holds good for the shrimp. This last assertion is based upon a series of experiments made by Maxwell and myself (2). These experiments were made chiefly on Palaemonetes. This Crustacean uses the third, fourth, and fifth pairs of legs for its locomotion. The third pair pulls in the for- ward movement and the fifth pair pushes. The fourth pair generally acts like the fifth and requires no further attention. If a current be sent through the animal longitudinally, from head to tail, and the strength in- creased gradually, a change soon takes place in the

1 This explains the galvanotropic gatherings observed by Hermann, Blasius and Schweitzer, and others. position of the legs. In the third pair the tension of the flexors predominates, in the fifth the tension of the extensors. The animal can thus move off easily with the pulling of the third and the pushing of the fifth pairs of legs, that is to say, the current changes the tension of the muscles in such a way that the forward movement is rendered easy, the backward difficult. Hence it can easily go toward the anode, but only with difficulty toward the kathode. If a current be sent through the animal in the opposite di- rection, namely, from tail to head, the third pair of legs is extended, the fifth pair bent ; that is, the third pair can push and the fifth pair pull. The animal will thus go backward easily and forward with difficulty. We see here again that the nervous elements of the central nervous system which bring about the for- ward movements have the opposite orientation as re- gards the longitudinal axis of the animal from the nervous elements which bring about the backward movement. But we can go still further in the devel- opment of this law. Palsemonetes can not only walk, but is also a good swimmer, and it can swim back- wards as well as forwards. In swimming forwards

the swimming appendages, among which the tail fin must be counted, push backwards forcibly and for- wards gently ; in swimming backwards the opposite occurs. If the current be sent through Palaemonetes in the direction from head to tail J the swimming 1 The tail fin behaves toward the current like the abdominal swimming ap- pendages and not like the body. This must be taken into consideration in galvanotropic experiments. appendages, and the tail also, are stretched backward or dorsad to their fullest extent. This proves that the tension of the muscles that move those organs back- wards is greater than that of their antagonists. The shrimp can thus swim forwards toward the anode easily under the influence of such a current, but back- wards only with difficulty. If the current passes through in the opposite direction (from tail to head), however, the tail and the ventral appendages are turned forward. The tension and the development of energy now predominate in those muscles which move the swimming appendages backwards. In this way the animal can swim backwards easily, while it is difficult or impossible for it to swim forwards. Hence the nervous elements, which determine the forward- swimming, must also have the same orientation in re- gard to the longitudinal axis of the animal as those elements which determine the walking forwards, while the nervous elements for swimming backwards have the opposite orientation.

The fact yet remains to be considered that Palse- monetes, like many other Crustacea, can also move sideways. This movement is produced by the pulling of the legs on the side toward which the animal is moving (contraction of the flexors), while the legs of the other side push (contraction of the extensors). If a current be sent transversely, say from right to left, through the animal, the legs of the right side assume the flexor-position, those of the left side the extensor-position. In the legs of the right side the

flexors produce more energy, in the legs of the left side the extensors. The transverse current assists the animal in moving to the right toward the anode and prevents it from moving to the left toward the kath- ode. Hence the nervous elements which produce the sidewise movement of the crayfish toward the right must have the opposite orientation in regard to the longitudinal axis from the nervous elements which produce the sidewise movement to the left.

Maxwell and I had attempted to give a picture of the arrangement of those elements on the assumption that they are the motor neurons. No reason exists, however, for regarding the ganglion-cell in toto as the point of action for the chemical effect of the ions set free. It may be any element inside of the ganglion- cells, or even of the fibre itself ; it is not even neces- sary that this element should be especially noticeable histologically. Life-phenomena are determined by physical and chemical conditions which are outside the realm of histology. But whatever it is, it is certain that those determinative elements in the central ner- vous system whose activity produces movements of the body, have a fixed orientation in the body, which evi- dently stands in some simple relation to the direction of the movement whicJi is produced by it.

The idea of such a simple relation between the orientation of nervous elements and the direction of motion produced by them is no more strange than the facts observed in the stimulation of the horizon- tal canal of the labyrinth. If this canal be slightly touched motions of the eyes or head occur in the plane of this canal. In this case we have to deal with a sim- ple relation between the orientation of the canal and the plane in which the organs or the whole body of the animal move. This fact is just as mysterious as the more general facts mentioned in this chapter. I am inclined to assume that the peculiar relation be- tween the semicircular horizontal canal and the motions produced by the stimulation of this canal finds its explanation through the facts mentioned in this chap- ter. It is possible that the central endings of the nerve of the horizontal canal are connected with the motor elements in the medulla whose activity pro- duces motions in the plane of the horizontal canal. When Flourens made his experiments on the semi- circular canals he found that there was a striking resemblance between the effects of a destruction of the canals and the sectioning of the crura cerebelli. He came to the conclusion that there must be a sim- ple relation between the direction of the fibres of the crura cerebelli and the motion produced by them (3). His observations are not in all points correct ; yet with some modification his fundamental idea remains true. The next chapter, on the cerebellum, will give us some more data about his observations.

It is possible for us to conceive from this how it happens that the same optic stimulus or the same space-perception is able to direct our eyes toward a certain point, to turn our head in that direction, to guide our finger thither, or to bring our legs into such activity that our body arrives at that place. It is possible tJiat the elements of the central nervous system iv hie h become active in this way all have the same ori- entation in each segment, and what we call an innerva- tion may be a process in which tJic orientation of the elements plays a role. The effect of the electric cur- rent might be an example of such a process. This problem of the physiology of coordinated movement which we touch upon here has always seemed to me the most mysterious in the whole physiology of the central nervous system, and the way offered here of reaching a simple solution seems to me worthy of mention. The whole conception can easily be classified under the segmental conception of the central nervous system. Movements of the eyes, head, arms, and legs depend upon as many different segmental ganglia. Each of these ganglia has some features in common with every other ganglion, for instance the orientation and arrangement of the elements (neurons?). If a process of such a nature that it can only stimulate elements oriented in a certain way in each ganglion spreads through the segmental ganglia, it must pro- duce a movement in exactly the same direction in the appendages of each segment. This does away with the necessity of imagining artificial connections of the neurons which would be able to produce such a series of coordinated motions in different limbs and segments.

If the question be raised, however, as to how it happens that a simple relation exists between the orientation of the motor nerve-elements and the movement or progressive movement produced by them, we must again refer to the simple segmerital relations of the first embryonic formation that re- mains better preserved in the central nervous system than in the muscles. The problem with which we have to deal here is ultimately a problem of embryology.

1. LOEB, J., and WALTER E. GARREY. Zur Theorie des Gal- vanotropismus. II. Mittheilung. Versuche an Wirbelthieren. Pflii- gers Archiv, Bd. Ixv., 1896. The experiments on the cerebellum support to a certain extent the observations mentioned in the preceding chapter. The cerebellum, like the cerebral hemispheres, is a structure which clearly expresses inequality of growth. Both may be considered as evaginations and appendages of the segmental nervous system. The cerebellum is connected with the central nervous system by three crura, the crura cerebelli ad medullam oblongatam, the crura cerebelli ad pontem, and the crura cerebelli ad corpora quadrigemina. The latter extend forward in a pretty straight line, the first ex- tend backward, and the peduncles to the pons at right ano-les to both. Ma^endie discovered and Flourens

confirmed the fact that lesion of these tracts possess- ing so characteristic an orientation to the chief axes of the body produces " forced " movements whose di- rection bears a simple relation to the orientation of the severed peduncle. If a peduncle of the pons be severed on one side the animal rolls about its longitudinal axis. If the crura cerebelli that extend forward be severed the animal rushes forward with great force ; if the crura cerebelli ad medullam ob- longatam be severed the animal goes backwards or shows a tendency to turn somersaults backwards. " La direction des mouvements produits par la section des fibres de 1'encephale est done toujours determinee par la direction de ces fibres" (Flourens).

Flourens called attention to the analogy of these phenomena with those he observed after lesion of the semicircular canals. This analogy, however, does not exist just as he states it. He compares the effect of the one-sided division of the pons with the division of a horizontal canal. This is not correct. So far as I know, such a lesion does not produce rolling mo- tions about the longitudinal axis in any animal. On the other hand, destruction of a whole ear, probably in most cases, causes rolling motions. Flourens states further that after destruction of the anterior canals an animal turns somersaults forwards, after de- struction of the posterior canals backwards. Flourens assumes that the nerves of the three canals continue into the corresponding peduncles of the cerebellum, and that this origin of the nerves is the cause of the

phenomena that we observe after lesion of the single semicircular canals (3). But this is probably not cor- rect, since the auditory nerve ends in the medulla. It is possible, however, that the cerebellum is connected with the same motor elements in the medulla with which the acoustic nerve is connected. The cerebel- lum might thus appear as an appendage of the acous- tic segments. This harmonises with the results Ferrier obtained from his stimulation experiments (i). He found that stimulation of the different parts of the cerebellum causes associated movements of the eyes, and that the direction of the movement changes with the posi- tion of the electrodes. The head also moves in the same direction as the eyes. Movements of the limbs were also observed, but it could not be determined whether or not they were associated with the move- ments of the head. From this we may conclude that possibly or probably the movements which are pro- duced by stimulation of the cerebellum are somewhat related to those movements which are produced by stimulation or injury of the semicircular canals, only that the stimulation experiments on the cerebellum, according to Ferrier, often yield no results.

Extirpation of the cerebellum leaves the sensory and psychic functions of the animal undisturbed. It is only in the movements that peculiar disturbances appear, which are described differently by the differ- ent authors. The motions of the animals resemble somewhat those of a patient suffering from St. Vitus's dance, inasmuch as they do not reach the intended aim and are often excessive in character. It is neces- sary that the head of a dog whose cerebellum has been injured be held in the dish when it eats, for if not held every effort sends the head so much too far that the animal is not able to get its food. This disturbance is most pronounced immediately after the operation and may disappear more or less after a certain time.

Such disturbances also show themselves in the limbs. The animal often staggers about like an in- toxicated person and finds difficulty in keeping itself on its legs. All these peculiarities perhaps point ul- timately to a decrease in the tension of the skeletal muscles. The measured movements of the normal animal are only possible if the tension of the antag- onistic muscles is so great that excessive movements cannot take place. But if the muscles of the spinal column are relaxed in the dog without the cerebellum, as has been maintained (and apparently with good reason), every intended movement can go wide of its aim.

According to the results of Luciani's numerous experiments, weakness or relaxation of the muscles seems to form the most constant factor in the effects of operations on the cerebellum. The affected groups of muscles, however, seem to vary with the position of the part of the cerebellum which is destroyed. Flourens, who attributed special functions to each sec- tion of the brain, maintained that the cerebellum was the general centre of coordination, because lesions of the cerebellum bring about the above-mentioned dis- turbances. Luciani (2) has shown, however, that some of the dogs that had lost the cerebellum were still able to perform coordinated swimming motions in the water and even coordinated walking motions. The weakness of all or of certain groups of muscles may lead to ataxic disturbances, but in some cases these may be very slight. Thus we see that Flourens's

theory that the cerebellum is an organ of coordination is not correct. It may be too that a part of the dis- turbances observed after lesion of the cerebellum are due to secondary effects of the operation on the med- ulla or the corpora quadrigemina. This latter conception is supported by comparative physiology. In fishes and frogs, in which the shock- effects are slight, the cerebellum can be removed without producing any disturbance in the behaviour of the animals (Vulpian, Steiner). In sharks, whose cerebellum is strongly developed, I myself have made numerous division-experiments and numerous experi- ments on partial or total extirpation of the cerebel- lum, and no change whatever took place in the behaviour of the animals. It is impossible and un- justifiable in this case to talk of a definite " function " of the cerebellum.

It may be well, in consideration of what has been said in the preceding chapter and of observations which will be discussed later on concerning the results of lesions of the cerebral hemispheres, to remind the reader of a hypothesis made by Magendie. He saw animals walk or fly backwards permanently after les- ion of a certain part of the medulla oblongata. He saw further that a lesion of the corpora striata pro- duces an impulse to run forwards. Finally he ob- served the rolling motions of the animals about their longitudinal axis after one-sided lesion of the pons. He makes the following remark in this connection : " Comme notre esprit a besoin de s'arreter a certaines

images je dirai qu'il existe dans le cerveau quatre im- pulsions spontanees ou quatre forces qui seraient pla- cees aux extremites de deux lignes, qui se couperaient a angle droit, Tune pousserait en avant, la deuxieme en arriere, la troisieme de droit a gauche en faisant rouler le corps, la quatrieme de gauche a droite en faisant executer un mouvement semblable de rotation. Dans les diverses experiences d'ou je tire ces consequences les animaux deviennent des especes d'automates mon- tes pour executer tels ou tels mouvements et incap- ables d'en produire aucun autre." The last statement goes too far, but Magendie's main thought deserves more consideration than it has heretofore received from physiologists. The galvanotropic facts men- tioned in Chapter XI. show most conclusively that in Crustaceans and Vertebrates there exists a relation between the orientation and function of certain motor elements, and a similar relation also finds expression in the experiments on the horizontal semicircular canal.1

1 Dr. Lyon has shown that only the stimulation of the horizontal canal gives rise to motions in the plane of this canal, while the same result cannot be ob- tained with any degree of certainty through a stimulation of the two other canals (4). 4. LYON, E. P. A Contribution to the Comparative Physiology of Compensatory Motions. The American Journal of Physiology, vol. iii., 1900. i. The discrimination between reflex and instinctive actions is chiefly conventional. In both cases we have to deal with reactions to external stimuli or conditions. But while we speak of reflex actions when only a single organ or a group of organs react to an external stimulus, we generally speak of instincts when the animal as a whole reacts. In such cases the reactions of the animal, although unconscious, seem often to be directed towards a certain end. A fly acts instinct- ively when it lays its egg on objects which serve the hatching larvae as food. We call the periodical migrations of animals instinctive. We call it instinct- ive when certain animals conceal themselves in cracks and crevices where they are safe from persecution. But the purposeful character of instincts cannot be used to distinguish them from reflexes, as a great many of the reflexes are also purposeful, for instance, the closing of the eyelid if the conjunctiva be touched or the wiping off of acetic acid that is put on the skin of a decapitated frog. On the other hand, it cannot be said that every instinctive action is purpose-

In many cases the greater complication of instinctive actions compared with simple reflex actions is due to the fact that in the instinctive actions we have to deal with a chain of reflexes in which the first reflex be- comes at the same time the cause which calls forth the second reflex. The taking up of food by the frog is a good illustration of this. The motion of the fly causes an optical reflex which results in the snap- ping motion. The contact of the fly with the mucous membrane of the pharynx sets free a second reflex, the swallowing reflex, which brings the fly into the ceso- phugus. If it be true that the instincts belong to the same class of processes as the reflexes, their relation to the central nervous system should be the same. We have seen that as far as reflexes are concerned, the nervous system only acts as a protoplasmic conductor between the periphery (sense organs) and the muscles. I think it is possible to show that this is also true for instincts. In order to prove this, we shall have to go into an analysis of the instincts. We shall select for our analysis such simple cases of instincts as depend upon tropisms.

2. We have seen that when certain Crustaceans, for instance Palsemonetes, are subjected to the effect of a galvanic current such changes of tension take place in the muscles of the appendages that movement toward the anode becomes easier, and toward the kathode more difficult. The result is that if the current is continued long enough, all the animals collect at the positive pole. When this process is observed with- out a careful analysis, it seems as though these Crus- taceans possessed the instinct to move toward the anode, just as the moths possess the instinct to move into the flame. The flight of the moth into the flame is in reality only the result of a tropism, — heliotropism, which differs from galvanotropism chiefly in that the rays of light take the place of the curves of the current. The reader knows that certain plants when exposed to the light on one side, for instance, when cultivated at a window, bend their tip toward the window until the tip of the stem is in the direction of the rays of light. The tip then continues to grow in the direction of the rays. We call this dependence of orientation on light heliotropism. We speak of positive heliotropism when the organ bends towards the source of light, of negative heliotropism when the organ bends away from it. It is generally assumed that the light has a chemical effect in these cases.

The relations of symmetry in plants and animals play an important part in these phenomena. We will take, by way of illustration, the stem of a hydroid, Eudendrium that is being raised near a win- dow. I have found that it bends toward the window like a positively heliotropic plant under the same conditions. The process may be described as follows : The light strikes the Eudendrium-stem from the side. A contraction of the protoplasm on that side ensues and a greater resistance is thus offered to the increase in length on this side than on the opposite side. The result is that the stem bends and becomes concave on the side toward the light. As soon, how- ever, as the bending has progressed so far that the stem comes into the direction of the rays of light, all the symmetrical elements are struck by the light at the same angle. The intensity of light is thus equal at symmetrical points, and there is no longer occasion for the stem to leave this direction. It thus con- tinues to grow in the direction of the rays of light. Negatively heliotropic elements, roots, for instance, differ from positively heliotropic elements in that the light produces a relaxation of the protoplasm. Hence when the light comes from one side, the resistance to the growth on that side will be less than on the op- posite side, and the tip will bend away from the source of light. As soon as the tip comes into the direction of the rays of light and the symmetrical points are all struck by them at the same angle, the intensity of the light on both sides is the same, and every cause for leaving this direction is removed. It has been known for a long time that many animals are " attracted " by the light and fly into the flame. This was considered a special instinct. It was said that these animals loved the light, that curiosity drove them into it. I have shown in a series of articles, the first of which appeared in January, 1888, that all these actions are only instances of those phe- nomena which were known in plants as heliotropism.

It was possible to show that the heliotropism of animals agreed in every point with that of plants. If a moth be struck by the light on one side, those muscles which turn the head toward the light become more active than those of the opposite side, and correspondingly the head of the animal is turned toward the source of light. As soon as the head of the animal has this orientation and the median-plane (or plane of symmetry) comes into the direction of the rays of light, the symmetrical points of the sur- face of the body are struck by the rays of light at the same angle. The intensity of light is the same on both sides, and there is no more reason why the animal should turn to the right or left, away from the direction of the rays of light. Thus it is led to the source of the light. Animals that move rapidly (like the moth) get into the flame before the heat of the flame has time to check them in their flight. Animals that move slowly are affected by the increas- ing heat as they approach the flame ; the high tem- perature checks their progressive movement and they walk or fly slowly about the flame. The more re- fractive rays are the most effective in animals just as in plants (i).