Forced Movements, Tropisms, and Animal Conduct

In a fish like the shark the position and innervation of the eyes differ from that of the human being. In the shark the eyes are not in front but on the side, and the muscles whicli lift the eye on one side are associated with those which lower it on the other side of the head. A similar association exists in regard to the pectoral fins, the muscles which lift the right pectoral fin are associated with those which lower the left one, and vice versa. When a normal shark swims the two pectoral fins work equally and the fish swims without rolling over to the right or to the left.

If we destroy in a shark the left side of the medulla oblongata forced changes in the position of the two eyes and the two pectoral fins will follow.(There are in addition correlated changes in the other fins and the rest of the body which we will omit in order to simplify the presentation of the subject.) When a shark, whose left medulla is cut, is kept in a horizontal position, its left eye looks down and the right eye looks up. This change of position of both eyes indicates that the relative tension between the muscles of the eyes has changed. In the left eye the tension of the lowering muscles predominates over that of their antagonists, in the right eye the reverse is the case. The pectoral fins likewise show associated changes of position. The left fin is raised up dorsally, the right is bent down ventrally. Since we know that the destruction of the central nervous system causes a paralysis of muscles and not the reverse we must con¬ clude that the destruction of the left side of the medulla in a shark causes a weakening or partial paralysis of the muscles which lower the left fin and of those which raise the right fin. Hence the muscles which press down on the water will press harder in the right than in the left fin. When such an animal swims rapidly, it will come under the influence of a couple of forces which must produce a rolling movement around the longitudinal axis of its

body toward tlie left. These rolling motions are another well-known type of forced movements. When such an animal swims slowly, it will roll more than a normal fish, but it will not roll completely around its longitudinal axis. These are the same motions which were observed in dogs by Magendie and Flourens after an operation in the medulla or pons. We can state, therefore, that the rolling motions are caused by the weakening of one group of (associated) muscles while their antagonists are not weakened.

It is of interest to consider the nature of forced move¬ ments after injury of the cerebral hemispheres in a dog. When in a dog one of the cerebral hemispheres is injured the animal immediately after the operation no longer moves in a perfectly straight line, but deviates from the straight line toward that side where the brain is in- jured.^^® When the left hemisphere is injured circus motions toward the left ensue. If one offers a dog which was operated in the left cerebral hemisphere a piece of meat, remoAung it as fast as the dog approaches, the dog will move at first a certain distance in a straight line; it will then suddenly turn to the left and describe a com¬ plete circle, moving afterward for a little while in a straight line toward the meat and turning again through an angle of 360° to the left, and so on.®®^ The explanation is the same as for the foregoing cases. The lesion of the left cerebral hemisphere caused a weakening or partial paralysis of the muscles which turn the body to the right. Hence the animal will, when following the meat, deviate to the left, and this causes a displacement of the retina image in the same direction and an apparent motion of the object to the right. We shall see in a later chapter on

the orienting effect of moving retina images that tliisi deviation of the retina image to the left causes a forced, motion of the animal to the right which compensates its tendency to deviate to the left due to the effect of the brain lesion. Hence the animal approaches the meat in an approximately straight line. But it does so with diffi¬ culty and sooner or later tiring of this etfort it moves in the usual automatic way, whereby equal impulses reach the muscles on both sides. This results in .a complete circus movement to the left on account of the weakening’ (caused by the operation) of muscles which turn the body to the right. The retina image of the meat again induces a straight motion and the whole process described is repeated. When the injury to the brain was less severe the animal may follow the meat for long distances without turning in a circle.

When such a dog is offered simultaneously two pieces of meat, one in front of the left, the other in front of the right eye, it invariably moves toward the one on the left side. The equal flow of impulses caused by the sym¬ metrically located pieces of meat results in a stronger contraction in the muscles on the left than on the right side of the body, since as a consequence of the lesion th© tension of the former muscles is greater than that of th© latter. When two pieces of meat are simultaneously offered to the dog, but both pieces are in front of the left eye, the, dog tries to get the piece nearest to its mouth, but its effort carries it a little too far to the left and then it takes the other piece of meat which is situated farther to the left.^**

Some time after the operation these disturbances may become less and may ultimately disappear. If now the dog is operated on the other, e.g., the right hemisphere, circus motions to the right appear. We do not wish to exhaust the chapter on forced move¬ ments but may perhaps for the sake of completeness point out the following facts. We have seen that if one cerebral hemisphere is injured the dog shows a tendency to circus movements to the operated side. When both hemispheres are injured, e.g., both occipital lobes are removed, the dog can hardly be induced to move forward and it is impos¬ sible to cause it to go downstairs, while it is willing to go upstairts. Its front legs are extended and its head is raised high, giving the impression as if such a dog had a tendency to move backward rather than forward or that the forward movement was difficult. If the two anterior halves of the cerebral hemispheres are removed the re¬ verse happens. The animal runs incessantly as if driven by a mad impulse; its head is bent down and it is in every respect the converse of the animal operated in the occipi¬ tal lobes. These two types of forced movements corre¬ spond to the morphological polarity tail-head. This corresponds to the idea of a projection of the surface elements upon the brain either directly or by crossing.

These three types of forced movements: the circus motions, the tendency to go backward, and the irresistible tendency to move forward will appear in the form of the tropistic reactions to be described in this volume. Since we shall deal in this volume chiefly with inverte¬ brates, it may be of importance to show that forced rnove- ments can also be produced in this group of animals by lesion of one side of the cerebral ganglion, and that these forced movements depend also upon the fact that as a consequence of the operation the tension of sym¬ metrical muscles (which is equal under normal condi-

tions) becomes miequal. Fig. 1,J?, gives tbe change in posi¬ tion of the body and of the legs in the larva of a dragon fly {^schna) after the left half of the cerebral ganglion has been destroyed (Matula Such an animal moves in a circle to the right. The longitudinal muscles con¬ necting the segments of the body are under higher tension on the right side of the body than on the left and the body Fig. 1. — B, forced position of larva of the dragon fly (JSsehna) whose left cerebral ganglion is destroyed. The body is convex on the left side, due to a relaxation of the muscles connecting the segments on the left side. The position of the legs is such that the animal can only move in circles to the right. This asymmetry disappears again when both ganglia are destroyed, C. A, normal animal. (After Matula.)

is bent with its convex side to the left. The normally symmetrical position of the legs (Fig. 1, .4) is now changed in such a way (Fig. 1, B) that the animal is no longer able to move in a straight line, hut is forced to move in circles to its right. We shall see later that similar changes in the position of the legs are produced in a posi¬ tively heliotropic insect when the left eye is blackened and in a negatively heliotropic insect when the right eye

is blackened. Circus motions after destruction of one cephalic ganglion in an insect are a general occurrence and have been known for a long time. The importance of these forced movements caused by lesion of the brain for the explanation of the conduct of normal animals lies in the fact that the latter is essen¬ tially a series of forced movements. The main difference between the forced movements after brain lesion and the conduct of a normal animal lies in the fact that the former are more or less permanent; while in the normal animal conduct the changes in the relative tone of sym¬ metrical muscles leading to a temporary forced movement are caused by a difference in the velocity of chemical reactions in symmetrical sense organs or other elements of the surface.

When we send a galvanic current lengthwise through a nerve, at the region near the anode the irritability of tJie nerve is diminished, while it is increased near the cathode. The condition of decreased irritability near the anode is called anelectrotonus and the increased irrita¬ bility near the cathode is called catelectrotonus. When a current is sent through an animal, those nerve elements which lie in the direction of the current will have an ane- lectrotonic and a catelectrotonic region; while the nerves through which the current goes at or nearly at right angles are not affected. Ganglia or nerve tracts in the anelectro- tonic condition wUl, therefore, act as if they were tem¬ porarily injured, and hence we need not he surprised to find that the galvanic current causes forced movements which last as long as the current lasts, and which cease with the current.

Hermann reported in 1885 that when a current is sent through a trough containing tadpoles of a frog, the tadpoles orient themselves in the direction of the current curves putting their heads to the anode.® Blasius and iSchweLzer found soon afterwards that a largo number of animals when put into a trough with water through which a galvanic current passes have a tendency to go to the anpde. The explanation given by Hermann and by Blasius and Schweizer is not correct. They

a The writer has never been able to repeat tliis observation. assumed that the eurnuit, aetiug upon the central nervous system, causes sensations ot‘ pain wlien it goes in tlie direction from tail to head in tiie animal; while it has a soothing or hypnotizing eft't'ct when it goes in the opposite direction, namely from head to the tail. In the latter case the heatl is directed toward the anode. The authors assume that the animals choose the position with least pain, i.e., with their heads to the anode. This assumption is wrong, since we know tliat when a galvanic current is sent through the head of a human being automatic motions comparable to those observed in animals occur whicli arc not voluntary and whic,h are unac^companied by any pain sensation. Thus when a galvanic current is 8<*nt laterally through the head, the person falls toward the anode side but has no feeling of pain. Mach noticed the same effect of falling to the side of tlie anode when a galvanic current was sent sidewise through fishes.^®'’ These galvanotropic motions are in reality for<*ed move¬ ments, and this has boon proved by direct observations. It was shown by Loeb and Maxwell®'*^ in experiments on crustaeeanH and by Loeb and (larrey on salaman¬ ders that wiien we send a galvanic current through ani- itjuls whi(*h go to the anode, changes in the position of the legs occur comparable to the clmnges in the position of fms and eyes mentioned in the previous chapter, and that thesi* changes are of such a eharae.ter as to make it easy for the animal to move in the direction of the anode and difficult if not impossible to move in tlio opposite direction.

In all these experiments it is of importance to ehoose the f»r<»pf‘r density of the current. For the experiments on the shrimp (Pdemmeles)^"* the animals were put into a square trough, two opposite sides of which were formea of platinum electrodes. The cross section of the fresh water in the trough was 1,400 mm.^ and the intensity of the current about 1 milliampere or a little less. We found it advisable to increase the intensity very gradually by increasing slowly the resistance of a rheostat in a short circuit until the phenomenon of galvanotropism appeared most strikingly. When the current is too strong or too weak the phenomena are no longer clear. The com¬ mon shrimp {Palwmonetes) is a marine crustacean which

Fig. 2. —Forced position of shrimp (JPalmmoneiea) when galvanic current goes from head to tail. Tension of extensof* muscles of tail fin prevails over that of flexors. Animal can swim forward (to anode), but not backward. (After Loeb and Maxwell.) lives also in brackish water and which can stand fresh water long enough for the purpose of these experiments. The animal can swim forward as well as backward; in forward swimming the extensor muscles of its tail fin work more strongly than the flexors (Fig. 2); in swim¬ ming backward the flexors work energetically (Fig. 3) and thus produce a powerful stroke forward, while the ex¬ tensors contract with less energy. When we put a Pala- monetes in a trough through which a current goes and if we put the animal with its head toward the anode the tail is stretched out (Fig. 2). This means that the tension of the extensor muscles prevails over that of the flexors and since the forward swimming is due to the stroke of

the extensors, and since it is antagonized by the tension of the flexors, the animal can swim forward but not backward, or only with difficulty; if we put the animal with its head toward the cathode the tail is bent ventrally (Pig. 3), which means that the tension of the flexors is stronger than that of the extensors. As a consequence the animal can swim backward but not forward, or only Fiq. 3.—Forced position of shrimp when positive current goes from tail to head. Tension of flexors of tail fin prevails over that of extensors. Animal can swim backward (to anode), but not forward. (After Loeb and Maxwell.)

with difficulty. In both oases the result will be a swim¬ ming of the animal to the anode, in the former case by swimming forward in the latter by swimming backward. We can further show that the tension of the muscles of the legs of Palamonetes is always altered in such a sense by the galvanic current that motion toward the anode is facilitated, while that toward the cathode is rendered difficult or impossible. The animal uses the third, fourth, and fifth pair of legs for its locomotion (Pig. 2). The third pair pulls in the forward movement

and the fifth pair pushes. The fourth pair acts like the fifth and requires no special discussion. If a current be sent through the animal longitudinally from head to tail and the intensity be increased gradually, a change soon takes place in the position of the legs. In the third pair the tension of the flexors predominates (Pig. 2), in the fifth the tension of the extensors. The animal can thus move easily by pulling of the third and by pushing of the fifth pair of legs, that is to say, the current changes the tension of the muscles in such a way that the forward motion is facilitated, while the backwar<l motion is ren¬ dered difficult. Hence it can easily go toward the anode but only with difficulty toward the cathode. If a current be sent through the animal in the opposite direction, namely from tail to head, the third pair of legs is extended, the fifth pair bent (Pig. 3); i.e., the third pair can push, the fifth pair can pull backward. The animal can thus go backward with ease but forward only with difficulty. This again will lead to a gathering of such animals at the anode, this time, however, by walking backward.

The phenomena thus far described recall the forced movements mentioned in the third chapter, where certain injuries of the brain accelerate forward motion while other lesions in the opposite parts of the brain make forward motion difficult if not impossible. Palcemonetes can also walk sidewise. 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 ex¬ tensors). If a current be sent transversely, say from left to right, through the animal, the legs of the left side assume the flexor position, those of the right side the

fXtvnHor poHitioJi (Mg, 4). I’lu* truriHVtTHf current thuH makes it easy for the luiimal to move toward tlie loft— the anodo—and prevciitK it from moving toward the right • the cathode. If a gidvanic current flowH transverHcly through the animal, it croatoH ti*e analoguo of the eireua motioni prodaced by Injury of ono iido of tht; brain. Pigs. 5 and d show that the eurront produoos similar effeots in the crayfish as those prodnetKl in tlio shrimp (Figs, 2 and .1).

It is not difficult to suggest by aid of a diagram the arrangement of the elements in the central nervous system required to bring about the phenomena of galvanotropism just described for Palcemonetes. We take it for granted that the regular phenomena of anelectrotonus and cate- lectrotonus of motor nerve elements suffice for the ex¬ planation of these phenomena. We assume that if the cell body of a neuron is in the state of cateleetrotonus Figs. 5 and 6. —Show the same effects of current on the common crayfish m those on Palamonetea in Figs. 2 and 3.

its activity is increased, when it is in anelectrotonie con¬ dition activity is diminished. Neurons having the same orientation will always be affected in the same sense by the current. Fig. 7 is a diagram of the arrangement of neurons giving rise to the bending of the legs on the side of the anode and to the extension of the legs on the side of the cathode when the current goes sidewise through the ani¬ mal. This diagram assumes that the nerves innervating the extensors come from the opposite side of the central

uervous system, while those ianervating the flexors are on the same side. This diagram corresponds to reality, ac¬ cording to the histological work of Allen. When the cur¬ rent goes from right to left through the crustacean the cell bodies of the neurons on the right side are in catelectro- tonus, those on the left side in anelectrotonus. The for¬ mer are, therefore, in a state of increased “irritability,” the latter in a state of diminished “irritability.” Hence the flexors of the right leg are contracted and the exten¬ sors relaxed, while the flexors of the left leg are relaxed and the extensors contracted.

Fig. 7. —Diagram indicating the orientation of the neurons for flexor and extensor muscles of the right and left legs toesmlain changes of position of legs under influence of galvanic current. (After Loeb and Maxwell.) of the current when it goes sidewise through its body. When the thoracic ganglion from,which the nerves of the legs originate is cut longitudinally in the middle, all the legs assume permanently a bent position, confirming our assumption that the extensor nerves cross over while the flexors originate from the same side of the ganglion on which their muscles are. It, therefore, looks as if our diagram were the expression of the actual condition.

In the same way we can explain the results of a gal¬ vanic current when it goes through the animal length¬ wise. We only need to assume that the cell bO'dies which send their fibers to the flexors of the third pair of legs have the same orientation as the cell bodies which send their fibers to the extensors of the fifth pair of legs (Pig. 8). Hence when the positive current goes from head to tail through the animal (Fig. 8), the flexors of the third pair of legs and the extensors of the fifth pair must be thrown into greater activity, since the cell bodies of both these nerves are in a condition of catelectrotonus, i.e., increased activity.

Figs. 8 and 9. —Diagram indicating orientation of neurons for flexor and extensor muscles of third and fifth pairs of legs to explain galvanotropic reaction. (After Loeb and Maxvrell.) When the current goes from tail to head the cell bodies of the extensors of the third and of the flexors of the fiifth pair of legs are in catelectrotonus. This possibility is expressed in the diagram Pig. 9. In this way the theory of the galvanotropic reaction of those animals which go to the anode seems complete.

What has been demonstrated for Falemonetes holds not only for many crustaceans but for vertebrates also. Loeb and Garrey*®® have shown that when a current is sent througli a trough containing larvae of a salamander {Amhlystoma) the legs and head of the larvae assume definite positions depending upon the direction of the current. When the current goes from head to tail the legs are pushed backward and the head is bent (Fig. 10); Fig. 10..—Forced position of Amhlyatoma larva under influence of galvanic current going through animal from head to tail. Head down, body convex on dorsal side. Legs backward. (After Loeb and Garrey.)

when the current goes from tail to head the opposite position is observed (Fig. 11). The analogy with the observations on Palmmonetes is obvious. G-alvanotropic reactions are found throughout the whole animal kingdom and the following observations made by Bancroft on a jellyfish {Polyorchis penidllata) are of especial interest.^® Strips containing tentacles and the manubrium were cut out from the animal and put into a trough through which a current flowed of 25 to 0.200 m. a. for 1 square mm. of the cross section of the liquid (dilute sea water) in the trough.

If a meridional strip passing from the edge on one side through the center of the bell to the other edge be prepared and the current passed through transversely, tentacles and manubrium turn and point toward the cathode (Fig. 12). A reversal of the current initiates a turning of these organs in the opposite direction, which is usually com¬ pleted in a few seconds. This can be repeated many times and the tentacles continue to respond after hours of activity. The manubrium,