Comparative Physiology of the Brain and Comparative Psychology

2. It may seem as though no conclusions could be drawn from these cases in regard to the " reflex centres " of higher animals. It is frequently stated that in higher animals the ganglia have assumed func- tions which in lower animals can be performed by the peripheral organs. It is similarly stated that the higher the animal ranks in the natural system, the more the functions "migrate" toward the cerebral hemispheres. But how such an upward migration of functions is conceivable, none of these authors attempt to explain. It can easily be shown, however, that conditions are the same in higher and lower animals. We must only be careful to homologise a lower form with a single organ or segment of a higher animal. When the in- tensity of the light is suddenly increased, the pupil of our eye becomes narrower. The sphincter of the iris contracts, and the rays of light are excluded just as foreign bodies are shut out by the contraction of the sphincters in the Ascidians. In the eye, just as in the Ascidian, we have to deal with a typical reflex act. The increased intensity of the light stimulates the retina. The stimulation passes through the optic nerve to its centres, and is carried from there by means of the oculomotorius nerve to the sphincter of the iris, which contracts. It would nevertheless be wrong to assume that the centre for the pupillary reflex plays any other part in this process than that of a protoplasmic connection between the retina and the iris. It has been shown by Arnold, and later by Brown-Sequard and Budge, that even in the excised iris the pupil still contracts when the light strikes the former. I myself have often observed in sharks,

whose brain I had removed, that light caused the pupil to contract several hours after death, when signs of decomposition had already begun to appear. Steinach has proved that in this case the muscle- elements in the iris are stimulated directly by the light (3). This reflex is therefore determined by the mus- cles of the iris, and the nervous connections serve only as quicker and more sensitive conductors. Thus we see that the eyeball behaves toward light just as the Ascidian behaves toward mechanical stimuli.

Some physiologists seem to doubt that the muscles can be stimulated directly by light without the inter- vention of the ganglion-cells. But we know that phenomena of contraction are also produced by the light in the unicellular swarmspores of algae, which certainly contain no ganglia. Furthermore, no one doubts that muscles without ganglion-cells can also be stimulated chemically or mechanically. Why should there not also be muscle-fibres that can be stimulated directly by light ? There is no reason for assuming that all muscles must behave exactly like the muscles of the frog's leg, simply because the experiments on it have by chance furnished the prevailing views con- cerning muscles.

The reader may believe that the pupillary reflex is an exceptional case, but this is not true. Defsecation and urination in higher animals may be considered as reflex phenomena of the spinal cord. The pressure of the faeces or of the urine acts as a stimulus, which affects the centres for the activity of the muscles of these organs, and this stimulation is said to cause the contracted sphincters to relax. Goltz and Ewald have found, however, that after extirpation of the en- tire spinal cord up to the cervical part, defalcation and urination still occur normally (4). Only for a time after the operation the sphincters are relaxed. Later on everything again becomes normal. These phenomena probably belong to the same class as the one already described in the Ascidian. The processes in the normal evacuations of the bladder and rectum are not determined by the morphological structure of the so- called reflex centre, but by the muscles of the bladder and of the rectum themselves. The spinal cord serves only as a more sensitive and quicker conductor for the stimulus. Goltz and Ewald are inclined, it is true, to assume that, after all, ganglion-cells or un- known nervous structures determine these results. But the fact that the muscles of the skeleton can be caused to contract rhythmically when put in the right solution, makes this assumption unnecessary ; more- over, the facts of comparative physiology must also be taken into consideration. The Actinia mesembryan- themitm of the East Sea and the Mediterranean per- haps show fewer differences morphologically than the sphincter ani and the gastrocnemius, and yet the Actinia mesembryanthemum of the Mediterranean shows a form of irritability which the Actinian of the same name from the East Sea does not show, namely, negative geotropism. I mention this illustration, to which many others might be added, in order to show

that forms which are morphologically alike need not necessarily be alike in all their reactions. Experiments on fermentation show that a small stereochemical dif- ference of a carbohydrate or proteid can produce an entirely different physiological effect. The possibility, of course, remains that scattered ganglion-cells exist in Ascidians under the epidermis just as in Medusae. Mr. Hunter, who has studied the nervous system of Ascidians, informs us that he has found cells in certain places under the epidermis of As- cidians which he believes to be ganglion-cells. But after all that has been said about the scattered gan- glion-cells in Hydromedusae (see page 19) and their role in rhythmical contractions, it is not necessary to consider the importance of scattered ganglion-cells for reflexes. Schaper has recently made an observation which makes it seem as though in the young larvae of Amphibians conditions similar to those in Ascidians exist. He amputated the brain of the larva of a frog during the first days of development, and saw that the animal was still able to move spontaneously seven days after the operation. When sections of the animal were made, it was found that the spinal cord had also per- ished (2). This observation should be repeated and enlarged upon. It is quite possible that during the first days of development a direct transmission of the waves of stimulation may take place from the skin to the muscles in the larva of the frog, without the in- tervention of the central nervous system, as happens in the Ascidians.

3. The objection might now be raised that the bladder and rectum are minor organs of the body. But what has been said above concerning them also holds good for larger and more important groups of organs, namely for the blood-vessels. These are able to adapt their width to external con- ditions ; the vessels of the skin become dilated when a loss of heat is desirable, and they contract in the cold when the loss of heat should be reduced. It is assumed that the mechanisms for these purposeful reflexes are contained in the central nervous system. Goltz and Ewald (4) have found, however, that dogs which have lost the spinal cord almost up to the me- dulla oblongata live for years. This alone proves that the blood-vessels can adapt themselves to the external temperature, independently of the central nervous system. Goltz had already proved that the

limb be severed, the limb being connected with the animal only by means of the blood-vessels. The same thing occurs after extirpation of the spinal cord. The temperature of the hind-paws of animals whose spinal cord has been destroyed up to the thoracic part becomes normal again after the operation — that is to say, the hind-paws have the same temperature as the fore-paws which remain connected with the central nervous system. If we hold the hand in snow for a time, we observe as a local after-effect a relaxation of the muscles of the blood-vessels and an increase in the temperature of the hand. Goltz and

Ewald were able to show that the same phenomena may also be observed when the hind-legs of dogs whose spinal cord has been destroyed are packed for a time in snow. From the standpoint of human physiology these results seem strange, but from that of comparative physiology they are readily understood. The various reactions of plants to external stimuli are just as pur- poseful as those of animals. Why should it not be possible, then, for single organs and tissues of higher animals to react purposefully to external stimuli, and is there any reason why the purposeful character of a reaction should be dependent upon the structure of the central nervous system ?

We have been able to rid ourselves of erroneous views concerning the significance of the ganglia of the central nervous system in higher animals through the help of the Ascidians ; they also help us further to determine the true role of the nervous system. Al- though the dogs experimented upon by Goltz and Ewald were able to adapt the width of their blood- vessels to the variations of temperature, it was neces- sary to shield them much more carefully from sudden changes of temperature than is necessary in the case of normal animals. The threshold of stimulation was raised and probably the rapidity of the conduction decreased. For this reason, dogs whose spinal cord is destroyed are no longer fit to live out-of-doors. As regards regulation of temperature, they are like an intoxicated person, and would perish in the cold

much sooner than a normal animal. Hence the nervous system does not contain any regulating me- chanisms, but it serves as a quicker conductor, and allows the peripheral organs to work with greater precision. 4. Bethe has recently made a difficult experiment on Carcinus mcenas, which, however, was successful in only two cases. If this experiment is correct, it proves that, in the conduction of a reflex in the cent- ral nervous system, the process of conduction does not of necessity pass through the ganglion-cell itself (5). An anatomical observation caused Bethe to perform this operation. " Almost all the ganglion- cells of Carcinus are unipolar, and often the axis-cyl- inder of the cell runs for a long distance before it gives off the first dendrites and sends out the peri- pheral fibre. It seemed very strange to me that a stimulus entering through the sensory nerves into the central organ should go through the dendrites to the far-distant motor-ganglion cells, and travel the great part of the same path before entering the peripheral motor fibre, instead of going directly to the motor fibre. It was easy to decide this question by sep- arating the ganglion-cells with their axis-cylinder process from the motor neurons without injuring the neuropiles. If the ganglion-cell were absolutely es- sential for the reflex, the muscles involved should become paralysed immediately after the operation ; if it were not essential, no paralysis should occur, at least for some time, and the stimulus could go across

directly from the dendrites to the peripheral fibre." It was possible to perform the operation in Carcinus on the ganglion-cells which innervate the muscles of the second antenna. The cutting of the peripheral nerves (Antennarius secundus) that go to these gan- glion-cells immediately causes a complete paralysis of the antennse, a proof that the fibres of these nerves are the only conductors of the stimulus which can call forth a reflex movement of these antennae. But when Bethe removed the ganglion-cells, without in- juring the neuropile of the second antenna, " the second antenna retained its tonus and its reflex irrit- ability. It does not hang down limp, but remains stiff and in the normal position. When stimulated, it is withdrawn, but is stretched out again when the stimulation ceases. From this it is evident that the ganglion-cells are not necessary for reflexes. The re- flex arc either does not pass through the ganglion- cells or does not need to pass through them. It is further apparent that the ganglion-cell has nothing to do with the tonus of the muscles, and that the per- manent influence which the central nervous system exercises upon the tension of the muscles is not pro- duced in the ganglion-cells (6)."

This experiment, even if it be correct, adds no- thing of importance to our conclusions. If the reflex arc acts only as a quick protoplasmic conductor, the question whether the stimulus has to pass through the ganglion itself or not becomes of secondary import- ance. 1. LOEB, J. Untersuehungen zur physiologischen Morphologic der Thiere. II. Wiirzburg, 1892, S. 37. 2. SCHAPER, A. Experimcntdle Studien an Amphibienlarven, Archiv fiir Entwieklungsmechamk, Bd. vi., 1898.

3. STEINACH, E. Untersuchungen zur vergleichenden Physiolo- gic der Iris, Pflugers Archiv, Bd. Hi., 1892. 4. GOLTZ und EWALD. Der Hund mit verkiirztem Riicken- mark. Pfliigers Arch., Bd. Ixiii., 1896. I. Theil, II. Mittheilung. Archiv f. mikroskop. Anatomie und Entwicklungsgeschichte, Bd. 1., 1897. II. Theil. Arch. f. mikroskop. Anatomie und Entwicklungs- geschichte, Bd. li., 1898. i. The two preceding chapters have furnished proof of the fact that the phenomena of purposeful reflex action, of spontaneity, and of coordination are determined, not by specific characters of the ganglion- cells, but by general peculiarities common to all pro- toplasm. These peculiarities are irritability and the power of conducting stimuli, both of which will find their explanation in the physics of colloidal sub- stances.

In this chapter we wish to put the foregoing con- clusions to a test by showing that a group of animals without any true central nervous system are able to show reactions complex as those in higher animals. Without such a parallel we should be more than ready, in the case of higher animals, to attribute such reactions to the specific structure of the ganglia or the ganglion-cells. We cannot speak of a central nervous system in Actinians in the same sense as in Ascidians. Under the ectoderm there are elements which are interpreted by some authors as ganglion-cells and nerve-fibres.

The unreliability of this interpretation is apparent, however, from the fact that Claus considers it uncer- tain. He mentions the possibility of a conduction of stimuli as one of the conditions that speak for the ex- istence of a nervous system in Actinians. But a COD- The tentacles press the meat a into the mouth, while they drop the water-soaked duction of stimuli also occurs in plants. During the year 1888 in Kiel, and 1889-90 in Naples, I made in- vestigations on the reactions of Actinians, which show how little reason we have for concluding that compli- cated reactions need depend upon similarly compli- cated reflex centres (i). It is very obvious from these experiments that the structure and irritability of the peripheral organs determine the reactions. We will begin with the description of experiments on the Ac- tinia cquina (mesembryanthemuvi) of the East Sea.

If a wad of paper soaked in sea-water be placed on the mouth of one of these Actinians it is refused, while a piece of crab-meat, which to us does not differ in taste from the wad of paper, is usually accepted with- out delay. I tied one end of a short thread around a paper wad and the other end around a piece of meat, and threw both on the outstretched tentacles of a starved Actinian. The tentacles that came in contact with the meat (a, Fig. 10) reacted at once by bend- ing in such a way as to bring the meat to the mouth, while the tentacles that were in contact with the pa- per did not react. I withdrew the thread and placed it on the oral disc in such a way that the paper rested on the tentacles where the meat had rested before, and vice versa. The meat was then drawn into the mouth and the string with it, but the paper remained outside the oral opening (Fig. 11). During the next twenty-four hours no change took place ; later on, the thread was ejected without the meat. The latter was

probably digested. I have often repeated the experi- ment, always obtaining the same result, except that occasionally the string was ejected sooner, in which case the meat remained on the string, partially or en- tirely undigested. These phenomena have the same explanation as the behaviour of insect-eating plants. The chemical substances diffusing from the meat, to- gather with the tactile stimuli exerted by it, cause a bending of the tentacles that are touched in such a way that they become concave and carry the meat to- ward the oral opening. The contact of the meat with the mouth causes the sphincter of the oral open- ing to relax ; the pressure of the tentacles, together with the activity of the oral disc, then pushes the meat into the interior of the digestive tract. But if these

specific chemical stimuli are wanting, if we give the animal, for instance, water-soaked filter-paper, the contractions of those muscles which carry the tenta- cles to the mouth are not produced. The tentacles remain relaxed or relax still more under the stimulus, and this fact, together with the ciliary movement, causes the paper wad to fall off. 2. It is said that the nerve-elements are much more numerous in the vicinity of the mouth than in any other part of the animal. One might think that this con- centration of nerve-elements determined the reflex mechanism for these reactions. For this reason, I have made use of results obtained while carrying on investigations concerning heteromorphosis. I had found that in an Actinian of the Mediterranean,

Cerianthus membranaceus, new tentacles could be pro- duced by a lateral incision in the body of the animal. But in some of these cases no mouth is formed. Fig. 12 shows such an ani- mal ; a is the normal, b the new head. If the incision was very small, only single tentacles were formed, without the oral disc. These new tentacles behave toward food exactly like the tentacles of the old mouth. If we offer FIG. 12. ACTINIAN (CERIANTHUS) WITH such a new head a

acles seize and press it against the centre of the oral disc, where the mouth should be. After pressing in vain for some min- utes the tentacles relax and the meat falls off. This experiment could be repeated for months, in fact as long as I observed the animal (2). In other cases the second head was so near the old one that it was easy to stimulate the tentacles of both simultaneously with the same piece of meat. In this case a fight arose between the two tentacle systems, each attempting to draw the meat toward its own oral disc. Parker has lately shown that even a single tentacle, after being severed from the animal, grasps a piece of meat and

Although the latter has no oral opening the ten- tacles carry the meat to the place where the mouth ought to be. bends with it toward the place where, in relation to itself, the mouth ought to be (3). If we look at these facts without prejudice, we must conclude that the reaction of the tentacles is determined only by the irritability of the tentacle- elements themselves, and by the arrangement of their contractile elements. The following observations may also be considered in support of this conclusion.

3. If an Actinia equina be divided transversely, the oral piece, which we will call the head-piece, has the normal head, with mouth and tentacles on its oral end ; on its aboral end the body-cavity is open to the exterior, and food may pass through the opening in either direction. The old mouth of a head-piece was as particular as usual in regard to the selection of its food, while the aboral end readily swallowed pieces of paper. The old mouth often refused meat, but the aboral mouth was almost always ready to accept it, even when it would refuse paper.

I laid a piece of an Actinian that took food in at both ends on its side, and tried to find out whether both mouths would take food simultaneously. I first placed a piece of meat on the aboral mouth, in order to cause it to open. As soon as this happened and the meat was being taken into the mouth I offered the oral mouth also a piece, and this was likewise accepted. The act of swallowing in the other mouth was interrupted at once by the contraction of the ring- muscles. After a few moments, however, when the meat in the oral mouth had been swallowed, the

muscles of the aboral end relaxed and the meat taken in before by this mouth fell out. When I fed the mouths in succession, the mouth that was fed first ejected the food as soon as the other began to eat. It is obvious from this that a peristaltic wave is started from the end which takes up food. Thus far we have considered only the head-piece. If we turn our attention now to the foot-piece, we find that on the oral end a new oral disc with tentacles soon begins to form. Before this has occurred, how-

ever, the mouth takes pieces of meat and swallows them. It seemed to me as though this new mouth, even before the regeneration of the oral disc, re- sembled the normal mouth more than the aboral mouth in the head-piece, for it did not accept paper wads and grains of sand, while it swallowed meat well. 4. In the foot of the Actinians the contact-irrita- bility is of special interest. The foot of a normal Actinia equina attaches itself to the surface of solid bodies. The character of the surface is of great im- portance for producing these processes of attachment. If it finds no other body, the Actinian attaches itself to the glass of the aquarium, and glides about on it. If, however, the shell of a Mytilus is placed in the aquarium and the animal comes in contact with it while moving about, it immediately attaches itself to the shell, and remains there, whether the shell is empty or inhabited. The surface of an ulva leaf has the same effect. While the animal upon contact with

the ulva leaf will at any time leave the glass and attach itself to the leaf, the reverse is not liable to happen. This contact-irritability of the foot does not change if the head or the greater oral part of the ani- mal be amputated. The mechanisms for the discharge of these reactions must, therefore, be located in the foot itself and not in the ganglion-cells of the oral disc. 5. I n higher animals we recognise a tendency to give the body a certain orientation in space. We usually call such an orientation in a higher animal its position of equilibrium. Certain Actinians also show such phe- nomena. If we put a Cerianthus into a test-tube filled with sea-water, and place the glass so that the head of the animal is down, the foot up, and the long- itudinal axis vertical, the tip of the foot will begin after a few moments to bend downward vertically. In Fig. 13 the course of such an experiment is given from life. Some minutes before 12 o'clock the ani- mal was placed in the test-tube in the manner de- scribed above. At 1 2 o'clock the foot had begun to bend downward (Fig. 13, a) ; in the next thirteen minutes the bending toward the head had progressed (fr) ; five minutes later the foot had reached the bot- tom of the tube (c). The bending progressed steadily to new elements lying near the head ; and since the foot now stood upon the bottom of the tube, the farther advance of the curvature toward the head resulted in the lifting of the latter (Fig. 13, d and e), whereupon the animal raised itself bodily, and at i o'clock had the position f. The process of righting

required one hour. The animal remained in this position for two days, and then crawled out of the glass. In analysing the conditions that determine the righting of the Cerianthus in this case, two circum- It was placed on the net horizontally, and within half an hour had regained its normal vertical position, by pushing itself through the meshes of the net. stances must be taken into consideration, namely, gravitation and the contact-stimuli. It can be easily shown that gravitation alone is able to produce the above-mentioned reaction of the Cerianthus. A wire net, whose meshes are so fine that the body of a Ceri- anthus can only be drawn through them by force, is laid horizontally upon a glass standing in the

aquarium. A Cerianthus is laid on the wire net. After a few minutes the foot of the animal begins to bend downward and to work its way through one of the meshes of the net. In the oral pole no change takes place except that the tentacles lay themselves close to- gether, so that they look like a brush whose handle is formed by the body of the animal. The animal forces its body farther and farther through the meshes until it is at last able to keep itself in a vertical posi- tion as represented in Fig. 14. This orientation can be reached in half an hour. If we turn the wire net over as soon as the animal has reached the position represented in Fig. 14, so that the foot is up, it does not pull itself out of the net again, but the foot near the tip begins to bend downward vertically. The bending then progresses from element to element of the body, from the foot toward the head, until the tip of the foot reaches the wire net, when it again pushes itself through as far as possible. If the wire net be turned over again, the process is repeated. Thus the animal can be forced, simply with the aid of gravit- ation, to weave itself in and out of the net. Fig. 15 shows a Cerianthus that has been forced to push itself through three times in this manner. The drawing was made from life. In these experiments we have an example of a geotropic irritability, — in other words, of positive geotropism. As this kind of irritability is very common in the roots of plants, it follows that for the mechanism of these reactions no specific qualities of the ganglion-cells are necessary. If a transverse

incision be made in the middle of a Cerianthus which almost but not quite separates the two halves, and the animal be placed immediately after the operation on a wire net, the foot works itself into one of the meshes up as far as the incision and assumes a vertical position. The oral piece, on the con- trary, from the place of incision to the head, usu- ally remains lying hori- FIG. 15. ACTINIAN THAT HAS BEEN zontally on the net. This shows that the foot pos- sesses geotropic irritabil- ity. But if the Actinian be divided transversely we see that the head-piece as well as the tail-piece pushes itself through the meshes, although not so frequently. While an Actinian that is suspended vertically in a test-tube or in a mesh of a wire net seldom retains this position longer than two days, it remains indefinitely in the sand after burrowing. In addition to gravita- tion, some other stimulus must hold it there. I be- lieve that it is the contact-stimulus of the sand. I called this kind of irritability stereotropism, and have shown that in a series of animals it determines their habits. Positive geotropism and positive stereotrop- ism cause the Cerianthi to burrow in the sand vert- ically, and the positive stereotropism keeps them permanently in the burrow.

We see from this that quite complicated reactions occur in these animals although they do not possess a central nervous system like that in higher animals. Were we to come across these same reactions in higher animals we should be inclined a priori to as- cribe them to the complicated structure of the central nervous system. The experiments on Actinians will perhaps prevent us from drawing such a conclusion before we have forcible reasons for so doing. A high degree of complication in the reactions of animals can be reached where no central nervous system exists, or where it serves only as a sensitive and quick proto- plasmic conductor. The cause of complicated reac- tions lies, therefore, in the irritabilities and structures of the peripheral organs.