Comparative Physiology of the Brain and Comparative Psychology

explanation for this behaviour is that they crawl about in the dish during the night and in the morning stop in the darkest place. These animals have at their oral pole not only a brain but also comparatively well- developed eyes. I resolved to try whether a decapit- ated Planarian, in spite of the loss of brain and eyes, would still show the same reactions toward light as the normal animals. This is the case to a most sur- prising extent. In the evening, about sixty specimens of Planaria torva were cut transversely just behind the brain and eyes. All the pieces were put into a dish with vertical sides which was half covered with black paper. The next morning nearly all the pieces, posterior as well as anterior, were found in the covered portion of the aquarium, where they were scattered about pretty uniformly. In the uncovered portion of the dish I found a few pieces, anterior, however, as well as posterior ones, crowded together in a corner where the intensity of the light was a comparative minimum. Upon repeating this experiment with nor- mal Planarians, the same result was obtained. When the decapitated animals were at rest in the covered portion of the dish, their rest was soon disturbed if, without jarring the aquarium, the dark paper was removed suddenly. At first they crawled about on the side away from the light, then they collected again where the intensity of light was a relative mini- mum. This reaction occurred just as in normal ani- mals, except that the reaction-time of the brainless animals was greater than in normal animals. In the

pieces containing brain and eyes, the reaction be- gan about one minute after exposure to light ; in the pieces without brain, after about five minutes. In this experiment, only diffused daylight was employed as a stimulus. In a round dish with vertical sides, the Planarians do not collect, like strictly heliotropic ani- mals, on the window- or room-side of the dish, but on the right and left sides. Decapitated Planarians be- have in the same way. All these reactions occur the day after the operation. Fresh material should always be used for these experiments.

After what has been said, it is hardly necessary to mention that pieces of Planaria torva from which the brain has been removed right themselves as well as normal animals. According to some authors, the starfish represents a colony of as many individuals as it has arms. We have seen that these react har- moniously as long as the nervOUS FIG. 24. TWO-HEADED connection is uninterrupted. In Actinians that have been made two- headed artificially, this harmony no longer exists ; for instance, in taking food both heads struggle for the same piece of meat. At my suggest- ion, Dr. van Duyne tried to produce multi-headed Planarians artificially. He succeeded in making them with as many as six heads. Fig. 24 shows a

two-headed specimen. If the heads were far enough apart, they no longer moved synchronically in the same direction, in which case the pulling in opposite direc- tions (Fig. 25) was so strong that the animal was torn asun- a segmental arrangement of the central nervous sys- tem. This type of structure is also found in Arthro- pods and in Vertebrates. It will perhaps make our task easier if we conceive the segmented animal to be a colony of as many individuals or animals as there are segments (or ganglia) present in the body. Each segment is then comparable to an Ascidian in which the central nervous system consists of but one gan- glion. The fibres and cells of each ganglion form for the corresponding segment a protoplasmic bridge between the skin and muscles. A stimulation be- ginning, however, in one segment is not confined to that segment, for the single ganglia of the various segments are connected with each other by means of nerve-fibres, the so called longitudinal commissures. By means of these, a stimulation which originates in one segment is transmitted also to the neighbouring

ganglia and from these to those farther away, until at last it reaches the end of the animal. The central nervous sys- tem of Annelids corre- sponds to the spinal cord of Vertebrates and consists simply of a chain of ganglia. These lie entirely on the ventral side of the animal, with the exception of the most anterior (supraceso- phageal) ganglion (Fig. 26), which lies above the oesophagus on the dorsal side. This is connected with the subcesophageal ganglion by a double com- missure, which forms a loop through which the oesopha- gus passes. It may be called the brain, although the small analogy exist- ing between Vertebrates and worms makes the use FlG 26 THE BRAIN AND A SERIES

of the term purely arbi- OF SEGMENTAL GANGLIA OF AN j-rqrv ANNELID (NEREIS). mental interest to us arises ( After claPartde-> at this point : Is the brain simply a segmental gan- glion, or is it an organ of a higher order which In our analysis of the nerve-functions we will begin with the earthworm. We will consider first its pro- gressive movements, and will attempt to answer the question, Does coordinated progressive movement, in which all the segments of C. the body participate, de- pend upon the brain (<?, Figs. 27 and 28) ? The locomotion of the earth- worm is a very simple process. The setse play an important role, although they are not visible to the naked eye ; they act like locomotor appendages and give the animal a hold on FIG. 27. DORSAL VIEW OF THE CEN- the ground. The real mus-

«, suboesophageal ganglion ;.?, pharynx ; CUtanCOUS ITlUSCle - layCT. G, ganglia of the ventral cord. . . . and longitudinal fibres. When the worm begins to move, the ring-fibres contract first, causing the worm to become longer and thinner. The bristles are turned backward and, because of the resistance of the ground, prevent the animal from moving back- ward. In this way the head is pushed forward. As soon as the maximum elongation has been reached,

the longitudinal muscles contract and the worm becomes shorter. As the bristles are still turned backward, the shortening can only be accomplished by the approach of the posterior end toward the head. The entire worm is therefore compelled to move forward. What happens if Q we divide the ganglion-chain of the animal in the middle of the body, or if we remove some ganglia from that region ? Will the forward piece move inde- pendently of the posterior piece ? Benedict Friedlander has made this experiment and found that the coordination continues in spite of the division of the central nervous system (4). If the for- ward piece begins to move, the aboral piece will also move in the same direction and at the same rate. This overthrows the idea that coordination in these animals is determined by a special centre *• suPraces°phageai

of coordination which is located in ganglion; A intestine; c, the brain. But how, then, does the coordination take place ? When the forward piece elongates and attempts to shorten itself by contracting the longitudinal muscles, the skin of the aboral piece is stretched. This pulling probably acts as a stimulus which causes the longitudinal muscles of the aboral piece to contract reflexly or perhaps directly. In this way, therefore, coordina- tion between the oral and aboral piece is possible in spite of the interruption of the nervous connection. Friedlander obtained further proof of this by divid- ing worms completely and connecting the two halves by strings. Even then he found that the aboral piece moved with the anterior piece in a perfectly coordin- ated way. These facts prove that the brain has no leading role in the coordination of the progressive motions of the earthworm.

What part, then, does the central nervous system play in the coordination ? It serves only as a quick conductor for the stimuli. Friedlander has shown that the quick motions which an earthworm shows upon a sudden stimulus are no longer transmitted to the posterior part of the body if the ganglion- chain be severed. If the nervous connection be broken so that stimuli cannot be conducted through the nerves, the peripheral structures suffice to make coordinated movement possible.

One might suppose the coordination in the pro- gressive movements of higher animals to be of an entirely different nature from that of worms. An ob- servation made by Goltz, however, shows that in dogs, at least, this is not the case. When a dog with divided spinal cord is lifted up by its fore-legs, so that the back part of the body hangs down perpendicu- larly, a remarkable phenomenon may be observed. The hind-legs perform pendulum motions which

resemble locomotion. These motions are presumably produced by the passive stretching of the skin on the ventral side of the hip-joint by the weight of the legs. These motions are comparable to the reflex contrac- tion of the longitudinal muscles of the earthworm, which is due to the stretching of the skin. Because of this reflex, coordinated locomotion would be quite possible in a dog with divided spinal cord, if the dog only could remain standing on its hind-legs. The walking movement of the fore-legs would cause the stretching which is necessary in order to bring about the walking movement of the hind-legs. The differ- ence in the behaviour of a dog with divided spinal cord and of an earthworm with divided ventral nerve cord in regard to coordinated progressive movements, is not caused so much by differences in the functions of the central organs as by differences in the develop- ment of the peripheral organs of the skin and of the organs of locomotion. If the dog had short stumps instead of its long, jointed legs, we should have, after dividing the spinal cord, the same phenomenon of progressive movements that we have in the earth- worm. The irritability of various parts of the peri- pheral organs and the simple segmental arrangement of the nervous elements suffice to preserve the loco- motion when it has once been started. The correct- ness of this conclusion is confirmed by experiments on Nereis, which were made in my laboratory by S. S. Maxwell (5). In these animals, the coordination of the movements of the oral and aboral pieces is

practically destroyed by dividing the ganglion chain, for the deep incisions between the single segments prevent the entire cutaneous muscle layer from being stretched equally. The structure of the ventral nerve- cord in Nereis is so similar to that of Lumbricus that we should not be justified in seeking in it for the conditions which cause difference of behaviour. In earthworms, Maxwell succeeded in confirming Fried- lander's observation. I obtained similar, although not as marked, results on leeches (2).

If an earthworm be divided, the posterior, brainless piece continues to perform progressive movements. This fact confirms the opinion that the brain has no controlling part in progressive movements. 4. The question now arises, Are the remaining char- acteristic functions of the earthworm brain-functions or segmental functions ? If we place Lumbricus fosti- dus in a transparent closed vessel, the animals appear to be positively stereotropic. As soon as they reach an angle in the aquarium, they remain there, crawling along where the glass can touch them on two sides. They are also sensitive to the differences in the intens- ity of light, remaining in those places where the light is weakest. It seems, too, when one or more animals settle down anywhere, that the others stop more readily in that place. This is an illustration of " sociability " among lower animals. It is probably an instance of chemotropic irritability. The surface secretions emanating from the worm's body have a quieting influence on other worms of the same kind ;

for this reason they become quiet when in contact wifr.h a worm of the same species. These chemical stimuli act as a trap, just as the comparative minimum in the intensity of the light acts. It should be noted, in this connection, that when animals are sensitive to differences in the intensity of light, the less refractive rays which pass through red glass have less effect upon them than the more refractive rays which pass through blue glass. The earthworms become quiet under red glass sooner than under blue glass.

How do decapitated earthworms act ? Decapitated Lumbrici fcetidi show the same stereotropism that normal worms show. When they reach the concave angle of a vessel, they have no inclination to leave it again. They also show the same response to light. They rest in those places where the intensity of the light is relatively weakest, and they move when the intensity of the light is increased. It can also be shown that light passing through blue glass acts like light of greater intensity, while light passing through red glass has the effect of light of weaker intensity (2).

In all these experiments the decapitated pieces crawl about with either the tail or the anterior end in front. It is an interesting fact that the reaction-time when light is the stimulus is not appreciably greater in de- capitated than in normal earthworms. The animals used for the experiment were in a box in which they could be exposed to diffused daylight suddenly with- out being jarred. In from three to eighteen seconds after being exposed to the light, the decapitated worms made the first movements. The interval was about the same in normal worms.

Lnmbrici fcetidi live in the decaying compost of stables, and probably the chemical nature of certain substances contained in the compost holds them there. When one-half of the bottom of the box is covered with moist white blotting-paper, the other half with a thin layer of compost, all the normal worms that are placed on the paper soon gather on the com- post. The aboral pieces of divided worms behave in the same way. When placed upon the blotting-paper, they are not attracted directly by the odours of the compost, but as soon as they come in contact with it in moving about, they crawl on it and do not leave it again. After a short time all the brainless worms are on the compost. When placed on a heap of compost, most of them crawl into it within a short time. This is not due solely to the light, as the same reaction also takes place in the dark (2).

Thus we see that in decapitated earthworms all the reactions shown by the normal worms are retained. Hence the brain (supracesophageal ganglion) has in this case no leading role. We cannot be too careful in drawing conclusions in regard to the principal function of a ganglion. Nereis, a much more highly developed Annelid than the earthworm, burrows in the sand ; if decapitated, this function ceases. One might suspect that this was due to the loss of the brain, but such is not the case.

Harlier experiments had led me to suspect that the "spontaneous" or "instinctive" burrowing was only a reflex produced by the contact-stimuli of the sand. I then attempted to find out whether it were not pos- sible under special conditions to produce the same reflex in brainless pieces. I placed such a piece of a Nereis on the sand ; as usual it remained quiet. I then gradually covered the forward end with sand. The rest of the animal immediately began to make the typical movements which the animal makes in forcing its way into the sand. At the same time the glands began to secrete the sticky substance which cements the particles of sand together, forming the wall of the burrow-hole. This secretion-phenomenon regularly accompanies the burrowing of these animals ; it is the same secretion that in other animals leads to the formation of a case.

But why does the Nereis not burrow when deprived of its brain ? For the simple reason that it makes use of the organs of the mouth in burrowing, and these are amputated with the head. Hence it is the loss of a peripheral head-organ which keeps the decapitated Nereis from burrowing, and not the loss of the brain. The brain in this case merely performs the function of a segmental ganglion — that is, it acts as the ganglion of that segment to which the peripheral head-organ belongs.

5. We will now turn our attention to the brain- functions of Nereis. the same case for a long- time. If the supracesopha- geal ganglion (o, Fig. 26) be removed, the animal be- comes restless, as S. S. Maxwell has found. It crawls about on the sand unceasingly, making no attempt to burrow. This restlessness is marked by one feature which we find in higher animals after certain injuries to the brain- -namely, the Nereis does not withdraw from obstacles but attempts to force its way through them.

If normal Nereis are in a square aquarium the bot- tom of which is covered with sand, they will crawl about, if undisturbed, on the sides of the glass. This is the result of stereotropism. A Nereis that has lost the supracesophageal ganglion will behave in the same way, except that when it reaches a corner it does not turn out but attempts to go through the glass. If there are several animals which have been operated upon in a vessel, they will assume the po- sition represented in Fig. 29. The worms remained like this for many hours at a time, and then died in consequence of their vain attempt to go forward. Those reactions are wanting which in the normal Nereis result from the application of contact-stimuli to the oral end. The reader who is familiar with brain-physiology may already have been reminded in this connection of the dogs from which Goltz re- moved the anterior half of the cerebral hemispheres.

If glass tubes 20 cm. long, with a bore a little larger than the diameter of the worm, are placed in an aquarium without sand, the normal Nereis will crawl into them and will not leave them again. This is due to stereotropism. If there are, for instance, six such tubes in a vessel and six normal Nereis are put into it, we may be sure that after a few hours every Nereis will have established itself in a tube. It FIG. 29. A GROUP OF NEREIS WHOSE BRAINS HAVE BEEN REMOVED. THEY AT LAST COLLECT IN A CORNER OF THE AQUARIUM AND PERISH IN THEIR VAIN ATTEMPT TO Go THROUGH THE GLASS. (After Maxwell.)

frequently occurs that a Nereis goes into a tube that already has an occupant. In that case the new-comer withdraws with a start as soon as it touches the old occupant. As long as the new-comer is in pos- session of its brain, it leaves the tube under such circumstances, but if it has lost the supraoesophageal ganglion, the presence of the other worm in the tube has no inhibitory effect. It tries to force its way into the tube even if it perishes in the attempt. If both worms have lost the supraoesophageal ganglion, they rub their heads together until they are sore. If we wish to keep them alive, they must be separated by breaking the tube. If we compare the conduct of a Nereis whose brain has been amputated, with that of a normal worm, the difference seems to be of the same nature as that between an insane and a rational human being. It would be erroneous, however, to conclude that the normal, brain-endowed Nereis pos- sesses reason or intelligence. The peculiar irritability by means of which the Nereis draws its head back and moves backward out of the tube depends upon organs which are located in the forward end of the body and whose sensory nerves go to the supraoesophageal ganglion. Hence, if the supraoesophageal ganglion is extirpated, the connection between these organs and the rest of the body is interrupted, and the stimuli which affect the forward part of the body can no longer produce backward movements in the posterior portion of the animal.

This does not, however, explain the change of character, the restlessness of the Nereis which has been deprived of its brain. It is maintained that, if the spontaneous activity or the reflex irritability of an animal is increased after the loss of a part of the brain, that part is an inhibitory mechanism. Nothing is gained, however, by making such a statement. We wish to know how the supraoesophageal ganglion can inhibit movements, and how its absence can increase spontaneity.

It is not possible to offer at present more than a suggestion. We can increase and decrease the loco- motor activity of a jelly-fish at desire by changing the constitution of the sea-water. If we increase the number of Na ions in the sea-water, the rate of rhyth- mical contractions in Gonionemus increases and the animal becomes restless. If the number of Ca ions be increased, the animal becomes quiet. It is, more- over, a fact that the different parts of a Gonionemus are affected somewhat differently by the same ions, inasmuch as the margin is more immune against the effects of Ca ions than the centre. I think it possible that there is a similar difference between the segments belonging to the supraoesophageal and subcesopha- geal ganglion. It might be possible that the ions (or some other substance) of the blood influence the supraoesophageal ganglion or its segments in such a way as to cause a decrease in the locomotions, while the same constituents of the blood do not have such an effect upon the subcesophageal ganglion or its segment. But the blood is not the only agency which is to be considered in this connection. The supraoesophageal ganglion of Annelids is connected with the alimentary canal by nerves. The processes which go on in the intestine — that is, the chemical pro- cesses of secretion and digestion — can only affect the

whole animal nervously as long as the supracesopha geal ganglion is intact. If it is removed, the whole: influence of this so-called sympathetic nervous system ceases. It is possible that the stimuli which pass from the sympathetic into the central nervous system may condition the alternation of rest and activity which characterises the normal animal, and that the removal of this stimulation may remove the necessity of resting. Maxwell has found that a Nereis which has lost the suboesophageal ganglion becomes quiet. Such ani- mals make no attempt at burrowing. The reason for this is that the motor nerves of the oesophageal mus- cles originate in the suboesophageal ganglion, so that removal of this ganglion causes paresis or paralysis of these muscles. The pharynx plays a great role in bor- ing the hole. It is due to this same paralysis or paresis of the cesophageal muscles that a Nereis no longer eats after losing the suboesophageal ganglion (5).

We wish to mention here, however, that removal of the suboesophageal ganglion does not bring about disturbances in taking up food mall Annelids. Max- well found that the leech is still able to suck itself full of blood after losing the ganglion. McCaskill dis- covered, however, that in the leech the motor nerves of the sucking apparatus originate in the supra- oesophageal ganglion. The suboesophageal ganglion in the leech behaves like the first link in the ganglion- chain.

of i:he supracesophageal ganglion and the repose after the removal of the subcesophageal ganglion, we wish to emphasise the fact that they have nothing to do with the wound. Maxwell's observations were made on animals whose wounds were healed. If we make a wound like that made in removing the ganglion, only with the difference that the ganglion is left intact, none of the above-mentioned disturbances occur. Immediately after the operation the worm burrows again in spite of the wound.