Comparative Physiology of the Brain and Comparative Psychology

Differences like those found between the behaviour of normal Nereis and of Nereis from which the brain has been removed do not appear in earthworms under the same conditions. What causes this difference ? Is the supracesophageal ganglion in the earthworm a segmental ganglion, while in Nereis it is a " control- ling ganglion," a brain in the sense of the anthropo- morphic nerve-physiology ? I am inclined to believe that we have to deal with differences of the same character as those found between Acalephae and Hydromedusae. In addition it should be said that there is a much higher degree of differentiation of the head-organs in Nereis than in the earthworm. We have already seen in preceding chapters that the apparent functions of the brain or of the ganglia are chiefly determined by the peripheral organs. In Nereis the differentiation of the head- segments is carried much farther than that of the other segments (Fig. 30). In the earthworm, on the other hand, the difference is much less (Fig. 27).

In Vertebrates the head contains special sense- organs, mouth-organs, which are lacking in the other segments. In judging of the relation of the brain- ganglia to the other segmental ganglia of the body this fact should not be overlooked. Not infrequently physiologists have ascribed to a ganglion what in reality was due to the higher differentiation of the peripheral organs of the segment. We desire now to touch briefly upon the behaviour of the muscles after extirpation of the ganglion, for the phe- nomena will occupy our attention repeatedly.

In the case of loss or a congenital lack of a piece of the spinal cord, the skeletal muscles belonging to the corresponding segment atrophy. Nothing of the kind occurs in leeches or earthworms from whose ventral chain a piece has been removed. I believe that the dif- ference is determined as follows : In worms direct impulses flow from the neighbouring muscles to the muscles that have been deprived of their ganglion, while in Vertebrates, as soon as the spinal cord is destroyed, the protoplasmic connection between the skeletal muscles and the rest of the body is destroyed

and it is not possible for stimuli to be transmitted. In the muscles of the blood-vessels of the Vertebrates, however, a conduction of the stimulation from element to element is possible. For this reason their reactions remain intact even in higher animals after destruction of the corresponding segments of the spinal cord. In Nereis, after division of the ganglion-chain, a phenomenon may be observed that reminds one of the Brondgeestian phenomenon. The back part of the body becomes more flat, while the part that is connected with the brain remains round. This indi- cates a relaxation of the ring-muscles in the part of the animal that is situated behind the point of division.

The results of our physiological analysis of the functions of the central nervous system in Annelids are in perfect harmony with Professor C. O. Whitman's investigations on the morphological structure of the central nervous system in Annelids. He arrives at the conclusion that the brain of Annelids is not of a higher 1. LANG, A. Untersuchungen zur vergleichenden Anatomie und Histologie des Nervensystems der Plathelminthen. Mittheil. aus der Zoolog. Station zu Neap el, Bd. I.

2. LOEB, J. Beitrdge zur Gehirnphysiologie der Wiirmer, Pfluger's Archil*, Bd. 56, 1894 ; and Ueber kiinstlichc Umivand- lung positiv helistropischer Thicre in negativ heliotropische und um- gekehrt, P finger's Archiv, Bd. 54, 1893. Farbensinns der Thiere. Prag und Leipzig, 1884. Verlag von Tempsky & Freitag. 4. FRIEDLANDER, BENEDICT. Ueber das Kriechen der Regen- wurmer, Biologisches Centralblatt, Bd. 8 ; and Zur Beurtheilung und Erforschung der thicrischen Beu<egungen, Biolog. Centralblatt, Bd. 1 1 ; and Beit rage zur Physiologic des Centralnewensy stems und des Bewegungsmechanismus der Regenunirmer, Pfliigers Archiv, Bd.

6. VAN DUYNE, JOHN. Ueber Heteromorphose bei Planarien. Pflugers Archiv, Bd. 64, 1897. 7. WHITMAN, C. O. The Metamerism of Clepsine. Festschrift fur Leuckart, Leipzig, 1892. i. Experiments on the lowest animal forms have taught us that the peculiar reactions of animals are determined, first, by the different forms of irrita- bility of the elements composing the tissues, and, second, by the arrangement of the muscle-fibres. The central nervous system does not control response to stimulation : it merely serves as a conductor from the point of stimulation to the muscle through which weaker stimuli may pass, and pass more rapidly than would be possible if the muscle were stimulated directly.

In the Annelids each ganglion is the relay station for the sensory and motor nerves of the correspond- ing segment. If the head exercises a stronger in- fluence upon the behaviour of the animal than any other segment, as in Nereis, for instance, I believe it is due to the fact that in the oral end more kinds of irritability are present and more peripheral organs are differentiated (sense-organs, mouth, etc.) than in the other segments. The fact that in this case the sympathetic nervous system takes its origin from the

supracesophageal ganglion also helps to increase the predominance of the head-segments. Hence it is not the presence of the supracesophageal ganglion which determines the greater number of reactions and their more complicated nature in the oral seg- ments of some Annelids, but it is the presence of the greater number of irritabilities and the greater number of specific organs in the forward end of the body. In addition there may exist chemical differences between the various segments of an animal.

We shall now see that this conception of the central nervous system also holds good for the Ar- thropods. We will begin the analysis of the brain- functions of these forms with Limulus polypJicmus If tenacity of life favours the age of the species as it does the age of individuals, this assertion can be readily understood, for it is difficult to conceive of a tougher animal. At my suggestion, Miss Ida Hyde made ex- periments on the functions of the single parts of the central nervous system of Limuhis polyphemus, with special attention to the respiratory centres (i). Con- cerning these centres, Faivre had made assertions

which did not harmonise with the apparent segmental arrangement of the central nervous system. He as- sumes that the subcesophageal ganglion which is located in the head has a coordinating influence on the respiratory movements, but in forms like these with the respiratory organs (gills) in the abdomen, the respiratory nerves must originate in the abdomi- nal ganglion-chain. The conditions existing in Verte- o, supracesophageal ganglion ; c, commissure ; «, suboesophageal ganglion ; I-IV (V and VI A 6d.), abdominal ganglia of the respiratory segments.

brates evidently gave rise to the idea of a coordinating ganglion located in the head. In Vertebrates the diaphragm, the chief respiratory muscle, is far sepa- rated from its nervous segment, but this is due merely to a shifting during development. The Anlage is really located near the head. Such displacements during growth do not take place to this extent in Arthropods. Faivre seems to have been entirely under the influence of current views of Vertebrate physiology, especially those of Flourens, and so he was led into making incorrect statements regarding the physiology of the invertebrates.

The central nervous system of Limulus consists of the following parts (Fig. 31): A supracesophageal ganglion o, which is usually called the brain, an cesophageal ring (c, Fig. 31) which encloses the oe- sophagus and consists of fibres and ganglia, a sub- cesophageal ganglion u, and the ventral chain with six abdominal ganglia. These parts send out a series of peripheral nerves. In Limulus the situation of the nerve-centres is schematically developed : every peripheral organ has its nerve-centre in that part of the nervous system which belongs to its segment. Perhaps this can be best seen from the following ex- periment made by Miss Hyde : The whole central nervous system of a Limulus was removed with the exception of a little piece of the cesophagus-ring (c, Fig. 31) and the abdominal ganglia (I-VI Abd, Fig. 31). No connection remained between these two pieces. The piece of the cesophageal ring lay at the same height with the three mouth-appendages that are used for taking in food. These three mouth-

appendages retained their function, and eating move- ments were performed reflexly when meat was placed on the appendages. The rest of the appendages were entirely paralysed, with the exception of the gills on the ventral side of the abdomen. The animal was reduced to a mere eating and breathing machine. It was fed artificially and so kept alive. Patten has shown further that each feeding-appen- dage continues to take food normally and carries it to the mouth if the piece of the cesophagus-ring from which its nerves take their origin is preserved. These feeding-appendages discriminate the chemical and tactile nature of the food that is offered them, just as the tentacles of the Actinians do — they refuse to accept it unless the substances offered fulfil cer- tain chemical and mechanical conditions. As regards the conception of these phenomena and their mechan- ics, no difference, of course, exists between the be- haviour of the tentacles of the Actinians and the behaviour of the mouth-appendages of the Limuli ex- cept that determined by the skeletal relations.

If we remove one half (for instance, the right half) of the supracesophageal ganglion in Limulus (0, Fig. 31), the animal usually moves no longer straight ahead, but in a circle with more or less of a curvature toward the uninjured (left) side. This is an instance of the well-known circus-motions. We shall return to the mechanics of such motions in a later chapter. If the whole supracesophageal ganglion be re- moved, the animal is able to take food placed upon

the mouth-organs, but loses its spontaneity in so far as this is expressed by progressive movements. It will even retain abnormal postures in which it is placed. The operations were performed during the period of heat. Male Limuli that had lost the supra- cesophageal ganglion no longer noticed the females. On the other hand, the legs attempted to remove an irritating object from the surface of the body. De- capitated frogs act in the same way. In the cases mentioned above, the Limuli had en- dured the operation well and their wounds were entirely healed. If the cesophageal commissure (c, Fig. 31) be severed on one side, circus-movements will appear in the direction of the injured side, but these only last until the wound is healed. The circus- motions which ensue upon extirpation of one-half of the brain also disappear after a time. If ganglia be removed from the cesophagus-ring, the appendages corresponding to the extirpated ganglia are perma- nently paralysed.

2. After extirpation of the subcesophageal ganglion (u, Fig. 31) the animal remains extremely quiet, and often lies on the same spot for days. But its respira- tion continues normal, and this proves the erroneous- ness of Faivre's opposed assertion. Except for its immobility and the fact that the extensors of the joint between the thorax and abdomen are paralysed as a result of nerve-injuries, the animal appears normal. innervate the five gills which are located on the abdomen of the animal. If the whole central nervous system with the exception of these ganglia be removed, the rhythmical respiratory activity continues un- changed. Immediately after the operation, which is accompanied by a great loss of blood, the respiration may be interrupted for an hour or more. If we touch the gill-plates during this time, the stimulation occa- sions a series of rhythmical respiratory movements which, however, soon cease. After a time, the gills begin their respiratory activity again spontaneously, and are only interrupted by an occasional cramp. This interruption of the respiratory movements is also found occasionally in the normal Limulus, where, if it remains quiet, the respiratory movements may cease for an hour or more. At this place we will not go into details concerning this phenomenon.

The abdominal ganglia are thus centres for the automatic movements of the abdominal gill-plates. All the gills move in the same phase. It is probable that the inspiration begins with the first gill and ex- tends to the following gills in succession, but rapidly enough to make the whole appear simultaneous. Ac- cording to the prevailing opinions, we should be obliged to assume from this either that only one, for instance the first, of the four abdominal ganglia, is automatically active, and that the rest are stimulated from this, or that, if each of the four ganglia is rhythmically active, a common centre of coordination exists somewhere in the four ganglia. If we sever

the ventral cord between two ganglia, for instance between the second and third, we find that in spite of the division all the gills continue to breathe. Any ganglion may be entirely isolated — that is, the commis- sure before and behind it may be severed, and the corresponding gill continue to make respiratory motions. This proves that every ganglion is the seat of an automatic periodic activity. But how does it happen that all the gills move simultaneously as long as their ganglia are connected ? The number of the respirations produced is the same even when the abdominal ganglia are isolated. This is probably due to the fact that the number of respirations is de- termined by the temperature and the chemical nature of the blood. The amount of carbon dioxide and certain other substances, especially those formed in the muscles (Zuntz and Geppert), controls the number of respirations. The phase of the movement, on the other hand, is not the same in the various segments where the ganglia are isolated. The gills that are situated anterior to the place of incision may be in- spiring while those behind the incision are expiring. These phenomena lead me to believe that in the nor- mal animal coordination is regulated in the same way that it is regulated in the activity of the heart and in the movements of Medusae. The ganglion that acts first, that is to say, the ganglion that acts quickest, stimulates those connected with it nervously and so determines the correspondence of phase. This view is supported by the fact that no matter how the ganglia may be

separated from each other, those that are connected nervously always keep their gills in the same phase of activity. Were there a centre of coordination in any ganglion, a group of ganglia separated from this centre would be active in an uncoordinated manner, but such is never the case. 3. In higher animals, the conditions controlling re- spiration scarcely differ from those in Limulus. There is a series of segmental ganglia in the thoracic portion of the spinal cord which sends nerves to the thoracic respiratory muscles of the respective segments. These ganglia extend into the cervical portion of the spinal cord, and the fourth, third, and fifth pairs of spinal nerves give rise to the fibres of the phrenic nerve which goes to the diaphragm. The diaphragm in reality belongs to the corresponding segments of the neck portion, and has attained its present position only through a shifting of position during growth. One would expect in text-books of physiology to find the phenomena of respiration explained as follows : Chemical changes which are continually going on in the body, or in these segmental ganglia, under the influence of heat (the temperature of the body), pro- duce a periodic activity in these ganglia and conse- quently in the respiratory muscles. The segmental connection existing between the ganglia and the muscles would bring about coordination just as it does in Limulus. But in the majority of text-books we find statements of the following character : The automatic activity of the respiratory muscles is pro-

duced much higher up, at a certain point of the medulla oblongata near the place where the vagus enters, which Flourens called the nceud vital. This place is supposed to be the respiratory centre. This view is justified by two facts : first, the destruction of the noeud vital causes a cessation of respiration ; and, second, severing the spinal cord between the nceud vital and the origin of the phrenic nerve like- wise causes respiration to cease. These facts do not justify the conclusion that Le Gallois, Flourens, and with him the majority of modern physiologists, have drawn -- namely, that the automatic activity of respiration is located not in the segmental ganglia, but higher up in the nceud vital. We should have just as much right to assume that in Limulus the rhythmical respiratory activity was produced higher up, in the subcesophageal ganglion for instance, for in this animal, too, respiration ceases for a time immediately after the removal of the subcesophageal ganglion. We have seen in this case, however, that the cessation is only temporary, and is due to the shock, for respiratory activity can go on again even when the whole central nervous system, with the exception of the abdominal ganglia, has been removed. Neither is the cessation of respiration in Vertebrates permanent after removal of the nceud vital or division of the spinal cord be- tween the nceud vital and the third cervical vertebra. Langendorff has made the important discovery that decapitated Vertebrates which have lost the nceud vital are still able to perform independent respiratory

movements (2). It was necessary to make these exper- iments on young or new-born Vertebrates, as on them the effect of the shock does not last so long. If one succeeds in keeping these animals alive by introducing artificial respiration until the effect of the shock result- ing from the operation has passed off, spontaneous respiration begins again. I consider it possible that, if we could keep an adult Vertebrate alive without the nceud vital for some time, the respiratory motions would be resumed again. But why does respiration stop temporarily after the isolation of the segmental re- spiratory ganglia from the higher parts of the central nervous system ? An answer to this question would be in part an explanation of the mystery of the shock- effects. It might be possible that something has to be supplied constantly by certain nerve-elements in the subcesophageal ganglion or the medulla to the seg- mental respiratory ganglia, which enables the latter to be active automatically. In destroying the nceud vital we perhaps destroy the pathway along which these constant impulses are carried to the segmental re- spiratory ganglia in the spinal cord. But where do these impulses come from and what is their character ? In watching the respiratory motions of a Limulus, I received the impression that the operculum always moves first, and that the respiratory motions of the lower segments follow successively. In the lower Vertebrates, e. g. the frog, we have a mouth respir- ation, whose segmental ganglia are situated in the medulla. Likewise the segmental ganglia for the

respiratory activity of the gills in fish are situated in the medulla. Could it not be possible that in Mam- malians the segmental ganglia for the gill-respiration continue to be active, although the gills or the oral respiration have disappeared ? If this were so, we can understand that the segmental ganglia for gill-respi- ration in the medulla begin to be active first. Their activity is the stimulus for the activity of the next lower segmental ganglion, and so on.

If we cut the cord between medulla and phrenic nerve, respiration must stop. But if we could keep such an animal alive lonof enough, the lower seamen- tal ganglia would be altered in such a way as to breathe automatically again. That the shock-effect after such an operation can- not be due to an exhaustion of the phrenic ganglia is made obvious by the following experiment : W. T. Porter made hemisections of the spinal cord between the medulla oblongata and the origin of the phrenic nerves (3). If one half, for instance the left half, of the medulla be cut, the left half of the diaphragm no longer partakes in the respiratory movements, while the respiratory motions of the right half continue. But if the right phrenic be cut, the left half of the diaphragm begins its rhythmical motion again, while the right half of the diaphragm stops breathing. It is of course, at present, just as impossible to explain why the cutting of the right phrenic nerve causes the left half of the diaphragm to breathe again, as it is to explain why a frog that had lost its spontaneity after

an operation in the thalamus opticus begins to move spontaneously again if the optic lobes and the pars commissuralis of the medulla are removed. In Limulus an anterior and a posterior nerve originate from every ganglion of the ventral chain. It was interesting to determine whether these nerves have functional differences like those of the anterior and posterior roots of the spinal cord of Vertebrates. It has been maintained that Arthropods are Verte- brates that walk on their backs. Faivre has stated that there is not only a separation of the motor and sensory roots in Arthropods, corresponding to Bell's law, but that also in Arthropods, in contrast with Ver- tebrates, the ventral side of the ganglia is sensory, the dorsal motor. Now this is not true of the nerve- roots which start from the ganglia in Limulus. If the posterior nerve be severed and its peripheral stump stimulated, we get inspiratory movements of the half of the gills to which this nerve goes. All the other gills are unaffected. Hence this nerve contains motor fibres. If the ventral stump be stimulated, the whole animal becomes much excited. From this we see that the posterior nerve also contains sensory fibres. If the anterior nerve be severed, stimulation of the peripheral stump has no effect. Stimulation of the central stump excites the entire animal. Hence the anterior nerve is purely sensory. Limulus is better adapted for deciding this question than the smaller Arthropods. The conditions in the latter are probably the same as in the former, for Vulpian (4)

and latterly Bethe (5) energetically reject the idea that dorso-ventrally the ganglion-chain of Arthropods is the reverse of the spinal cord of Vertebrates. 4. We will now turn our attention to the crayfish as the next representative of the Arthropods whose brain-physiology has been carefully investigated. Fig. 32 gives a diagram of the central nervous system of the lobster, which is almost identical with that of the crayfish, o is the supraoesophageal ganglion with the nerves for the eyes and antennae. In addition it gives off the sympathetic nervous system which goes to the intestine. Both cesophageal commissures, c, go backwards to the subcesophageal ganglion, u. The latter is seemingly one ganglion, but it supplies six pairs of segmental organs, namely, the mouth-ap- pendages. The microscopical examination shows that this subcesophageal ganglion in reality consists of six separate ganglia. We often meet with a fusion of ganglia, and consequently an apparent lack of clear- ness in the segmental arrangement. It is due to this fact that in the brain-physiology of Vertebrates the segmental arrangement of the central nervous system has been left entirely out of consideration. Next after the subcesophageal ganglion come the five thoracic ganglia (I-V 7", Fig. 32) belonging to the segments of the forceps and the four pairs of loco- motor appendages. In addition to these, there are the five ganglia of the abdomen (I-V Abd., Fig. 32) that innervate the swimmerets, and the tail, which serves as a swimming-organ. The best experiments

3, supracesophageal ganglion (brain) ; c, commissure ; u, subcesophageal ganglion; /-K7", five thoracic ganglia ; I-V A bd.^ first five abdominal ganglia. on the central nervous system of these animals have unquestionably been made by Bethe, and we shall in the main follow his presentation. Many of the facts which Bethe describes from the animals used in his experiments are familiar to me from personal observ- ation, and I am convinced that the picture he gives is correct.

If in a crayfish both the commissures (c, Fig. 32) which connect the supracesophageal ganglion o with the rest of the brain be severed, the behaviour of the animal is no longer controlled by the brain o. It does not make spontaneous progressive movements. When stimulated it begins to move, but after having gone about 20 cm. it stops. This lack of spontaneous pro- gressive movements agrees with the description given by Flourens of the Vertebrate from which the cere- bral hemispheres had been removed. Flourens's repre- sentation was wrong, however, for a dog operated upon in this way shows increased spontaneity in its progressive movements.

Annelids and Arthropods are closely related as re- gards the central nervous system. However, Nereis shows an excess of progressive movements after removal of the supracesophageal ganglion, while As- tacus no longer moves spontaneously. I believe that the difference depends only upon circumstances of minor importance. Ward has already found — and Bethe has confirmed the fact — that in brainless cray- fish the legs are unceasingly active, either cleaning each other or performing pendulum -movements.

They, however, make no progressive movements. I believe this is due possibly to a secondary effect of the extirpation of the supracesophageal ganglion. The legs of such an animal have an abnormal posi- tion, being more strongly flexed at the joints nearest the body than they are normally. The tension of the extensors probably suffered severely from the operation. Such mechanical disturbances might easily cause difficulty in locomotion, while simple pendulum- movements of the legs, which require practically no labor, could still be performed. The fact that after re- moval of the brain of crayfish the tension of the flex- ors predominates in certain joints is of interest, as we meet with the same phenomenon in dogs that have lost the anterior region of the cerebral hemispheres, and as it also comes to our attention in man after apoplexies which result in the paralysis of an arm.