Popular Science Monthly/Volume 1/July 1872/Nervous Control of Animal Movements
|NERVOUS CONTROL OF ANIMAL MOVEMENTS.|
SINCE the celebrated experiments of Flourens, we know for certain that all the acts of the vegetative life in animals are completely independent of the cerebral lobes, and that an animal deprived of these continues to live as well as before, with only this difference, that it loses all will and instinct. With superior as well as inferior animals the cutting away of the cerebral lobes does not put an end to the movements which were possible before; only these movements take on particular characters. In the first place, they are more regular, and may be regarded as the true normal type, for mental influences do not modify them. The locomotive apparatus acts without restraint, and we may, therefore, say that the movements are more normal than in the normal state. In the second place, when the cerebrum is removed the movements only commence after excitations; they cannot start themselves. The frog must be put in the water to swim, and the pigeon thrown in the air to fly. In animals without a cerebrum the physiologist can determine such or such an act, limit it, arrest it; he can foresee movements, and tell in advance what will take place in such conditions, as absolutely as the chemist knows in advance the reactions he will get on mixing certain bodies.
Another peculiarity of movements that take place when the cerebral lobes are removed is, their continuation when once commenced. On the earth a frog without a cerebrum, when irritated, makes two, three, or more leaps; he rarely stops with one. Placed in the water he continues to swim till he encounters an obstacle. The pigeon continues to fly, the duck and the goose to swim. The striking thing about it is, the continuation of the state determined at first by an impulse from without; and we cannot help associating these facts, about animals deprived of their cerebral lobes, with the characteristic properties of inorganic matter. Put agoing, the animal without a cerebrum continues to move till the exhaustion of the conditions of movement, or till it encounters resistance. Put in repose, it remains inert till some exterior cause sets it in motion. It is inert living matter.
The phenomena we are about to consider are caused either by impressions from without (excito-motor stimulation), or excitements from the sensorial centres (sensori-motor stimulation). In the frog, for example, the contact of the body with the earth makes him take his normal attitude, and when it is put in the water, says Vulpian, "the liquid produces a particular stimulation of all the surface of the body in contact with it; this stimulus calls into play the mechanism of swimming and this mechanism ceases to move when the stimulus is withdrawn by taking the frog from the water."
The explanation of Vulpian is exact only within certain limits, for the frog remains motionless in the water when it encounters an obstacle, even when the stimulus of the water on its body is kept up; and, on the other hand, the surface of the pigeon's body is stimulated in the same way by the air, whether the wings are open or shut, and yet it is obliged to fly when it loses its point of support. There are,
then, other causes of stimulation besides the impression upon cutaneous nerves. These are—first, the combination or solidarity of the movements which exist among animals deprived of the cerebral lobes; and, second, the necessity of maintaining an equilibrium.
What do we mean by solidarity of movement? When a brainless frog is swimming, and we apply a solid body to one of his fore-feet, immediately the corresponding hind-foot bends and touches the body in contact with the fore-foot. It is the same if we stop the motion of the fore-foot. Reciprocally, if the frog is motionless on the surface of the water, and if we set the fore-foot in motion the hind-foot immediately begins to move, and the animal swims. In a word, to speak generally, when an animal is deprived of its cerebral lobes, if one limb begins to move the others immediately follow; if one comes to rest, the others tend to cease moving. Very seldom in these animals is the movement limited to a single member. This solidarity in the movements distinguishes animals deprived of the cerebral lobes, not only from animals with a brain, but also from those in which the spinal cord is severed at top.
In a frog with the cord cut near the cranial bulb, if we move a member, it produces no effect on the movements of the animal. If we excite one foot, only the subjacent muscles contract. If the impulse is stronger, the entire foot recedes, but the rest of the body is motionless. A lively excitation is needed to put the other feet in motion. In a word, each excitation, according to its energy, produces more or less extensive movement which may be limited to a single group of muscles. It is not so with a frog in which the spinal cord is unsevered; the movements which succeed a stimulus, whether it be strong or feeble, are always movements of the whole body.
If we put a drop of vinegar on the foot of a frog in which the cord is separated from the brain, the foot retires at first, then the other foot makes coordinated movements to get rid of the cause of irritation. The frog which has lost only its cerebral lobes commences, on the contrary, to make many leaps; afterward it moves only one or other of its feet. In the frog where the cord is severed, to each excitation succeed muscular contractions; these are always in proportion to the energy of the excitation. In the frog with the cord joined to the cerebellum alone, the excitation can take place without producing movement; but, be it feeble or strong, from the moment reflex action begins the result is the same—a movement of the body which produces a leap.
According to the excitation, to the kind of impression produced on the sensitive nerves of the skin, and on the nerves of muscular sense, there is formed among the different regions of the nervous centres a common purpose, which has for its regulator the pons Varolii (bridge of Varoli) and the cerebellum.
Another interesting effect now claims our notice. Among animals deprived of the cerebral lobes there is another curious and constant phenomenon, the forced and continual tendency to maintain an equilibrium. We have seen in the frog, the carp, the eel, the pigeon, the goose, mammals, etc., that every time we disturbed their centre of gravity, immediately there took place a series of coordinated movements which have the single aim of restoring the equilibrium. A decapitated insect remains always firmly posed on his feet and can take no other position. If a frog is motionless on a piece of board, and you slowly lower the board in the water so that he is immersed, in most cases, notwithstanding the stimulus of the water on the integuments, the frog will remain immovable. If, now, you slowly withdraw the board below the frog without disturbing his position, he will remain motionless; but, if you tip it one side, the frog at once wakes from his quietude. The loss of equilibrium acts more energetically than the stimulus of the water on the skin.
Tip a carp to the right or left, at once it recovers its normal attitude. If you place a duck on one of its sides, either on the ground or in the water, it at once corrects itself and comes upright. The cerebellum alone controls equilibrium, as is easily proved by experiment. When it is wounded or destroyed, the animals He indifferently on one side or the other, and make no movements to recover the lost equilibrium. In certain cases, even, they cannot maintain an equilibrium, but tend to fall on one side.
We may conclude that the movements of animals, whether superior or inferior, are produced by certain special mechanisms, or by locomotive centres, situated at the base of the brain. These centres are essentially passive; they have no spontaneous action, and come into activity only when excited by peripheral stimulus or by the brain.
We ought, then, no longer to admit, in the habitual movements which appear perfectly voluntary, a direct action of the brain on each muscle. We must remember that there exist at the base of the brain motor centres which serve to intermediate between the will and external acts. The will calls into action such or such centres, and these immediately determine the action of certain muscular groups. We know besides, that, according to the habits and education of the muscles, particular muscles can enter only with great difficulty upon isolated movements, and that the contraction of one muscle sometimes forces the contraction of others, even against the will.
There are, then, among the encephalic centres at the base of the brain, centres of coordination and direction of movement, which, so to speak, preside over the details of external acts (walking, swimming, flying, etc.), and which receive from the cerebrum only general orders to execute such or such bodily movements.
The motions of the body produced by the locomotive centres, under the influence of the brain, are of two orders: those of instinct or heredity, and those of habit. Both are inevitable, but they differ in this, that the first take place in all animals, whether young or old; while the second occur only in the old. To employ the usual expression, one is nature; the other, second nature. If you remove the cerebral lobes of a duck that has never been in the water, and then place it in water, it will swim regularly; but it will not, like an old duck deprived of its cerebrum, make certain habitual movements of the neck. The old pigeon, although without its cerebrum, when asleep places the head under the wing; and he often even dresses his feathers. Young pigeons have never been observed to perform these acts, while they execute other movements normally like old pigeons; their flight is very regular, even when they undergo the operation before leaving the nest.
It is, then, probable that by habit there are formed in the nervous centres certain connections between cellular groups, which give rise to bodily movements that become as imperative as those which are due to instinct.
In animals deprived of the cerebrum, then, the locomotive centres are still complete, and, as we have already said, they differ from the unmutilated only by the impossibility of spontaneously beginning movement. To act, they must receive an impulse either from without or within. Exterior excitement we can produce artificially, by acting on the peripheral nerves; interior excitation is produced by the cerebrum, and we may say that, from the point of view of physiology, the cerebrum has no other function than to put in action the different motor centres. It is a simple excitant, with this important difference—that external impressions can determine only a certain number of movements, while the brain provokes an immense variety.
Let us now consider the movements of rotation, which follow from wounds of portions of the encephalon. They are of two distinct types: the one is a rotary movement round a circle, the other a motion of rolling or spinning. In the first case, the animal remains in his normal attitude, but tends always to go to one side, and describe an orbit more or less extended. In the second case, the animal can progress but little in moving himself. When he attempts to move, he is forced into a revolving motion, and turns on himself like a ball. The former movements are chiefly produced by lesions of the encephalon, and were obtained very neatly in a frog from which were removed the cerebral lobes of the left side. The movements were made from the left side toward the right.
What fixed attention at first in the attitude of this frog was, that all the right part of the body had the carriage and aspect of a frog without a cerebrum (Fig. 2). The hind-foot of this side approached the body more, and was gathered in a heap, as shown in Fig. 3, while the fore-foot was equally drawn up, and had the position that we have
seen to be constant in frogs in which the cerebrum had been removed.
At the same time, the entire animal leaned a little to the right side. This inclination of the body is neither constant nor inevitable with animals which have only the movements in a circle; it is, on the contrary, constant and forced with those that have the turning motion.
The attitude of this frog changed, and was even reversed, when it was chloroformed; the left side, which was more drawn up, where the muscles experienced a tonic contraction predominating over those of the right side, became then more feeble. This effect became more and more pronounced in proportion as the chloroform took effect, up to the time when the two sides were completely affected; then the frog would no longer rest on his fore-feet, and the hind-feet remained extended and motionless. In proportion as the anæsthesia disappeared, the feet of the healthy side folded themselves and approached the body; those of the opposite side remained still extended. Then these folded themselves similarly. The attitude of the two sides was then identical, but, when the normal state was recovered, the limbs on the side affected by the cerebral lesion again became more drawn up, and the body leaned on that side.
In this frog we have again on one side the influence of the cerebral lobes, but, on the other side, the locomotive centres are completely independent, and they act alone on the movements which take place on the corresponding side of the body. What happens, then, when the animal attempts to move? From the healthy side the movements are made according to the desire and the will, they are limited to the end wished by the animal; but, on the other side, they are made en masse, automatically, invariably; they drag the animal from their side; at the same time, the mathematical regularity of their contractions becomes, so to say, a dynamic centre for the movements of the opposite side, which are less regular. In this way we explain the movement in a circle, the side in relation with the wounded part being in the interior of the circle made by the animal. This also is the reason why such movements, particularly when the circumference is small, occur only when the animal can change his place, and very often commence only after the first moments of locomotion.
In case the cerebral influence is abolished, the locomotive centres become absolutely independent, as is shown by the forced automatic movements. Again these centres may be excited by a tumor, or a prick, and then their dependence upon the cerebral lobes ceases at once, notwithstanding the anatomical communications which still exist. It is in these cases that the compulsion, from the instant of the lesion, becomes invincible, and forces the animal into movement. The locomotive centres become active, and, as long as the excitement lasts, the animal is completely under their influence. Neither the will, nor emotion, nor physical obstacles, can prevent the limbs from moving.A hunter gave us the following account: From a considerable distance he had shot at some wild-ducks that were on the water; one of these ducks was not able to fly, and remained on the water, turning in a circle. This duck had received a superficial wound from a grain of lead in the side of its head; it had absolutely no other wound on its body. Now, the curious fact, and what astonished the hunter, was, that this duck could not fly, and yet neither the will nor the means
When the lesion is made on the two sides, some of the same phenomena are seen; only, as the stimulus to activity on both sides is the same, the animal moves in a right line. We have injected mercury in an opening made with a trepan into the upper part of the cranium of a young cat. In a little while, the mercury, by its weight, having reached the base of the brain, the animal lifted himself up, and plunged forward against the wall, making vain efforts to go straight on; deviating to one side, he continued his course till he encountered a new obstacle, and so on. He stopped only when exhausted, and yet until his death from compression of the bulb, his limbs moved without interruption.
In a man who had all the symptoms of a cranial tumor, we have observed phenomena almost identical with these presented by this young cat. When he had a crisis, he would raise his haggard eyes, and walk straight in his chamber, being guided only by the reflex action of locomotion and by habit. After his crises, he could not remember having walked.
It is evident that, in these cases, it cannot be a question of paralysis, and that the phenomena are the result of excitation of the locomotive centres. If the influence of the cerebrum, on one side, is obliterated, and the locomotive centres are not irritated, they act only when they are solicited to activity by. movements impressed by the opposite side, and then these movements are forced and automatic, but regular and without exaggeration. The result is, a movement in a circle, which occurs when the animal can change his place. If, on the contrary, the centres are directly excited, the impulse is forced, and the animal is obliged to move in the way impressed by the centres.
Better to comprehend the rolling movements, we must explain some facts which have not been dwelt upon, and which we observe in animals in repose. In lesions of the cranial centres, when we wound the pons Varolii, the animal has no longer the same exterior carriage; he leans to one side or the other, according to the side where the lesion is made. All the muscles of this side are then in a state of permanent contraction. The frog represented in Fig. 2, from which have been removed only the cerebral lobes of the two sides, is remarkable for the regularity and the symmetry of the position of his limbs. Placed in the water, he rests on its surface, and the right side is at the same level as the left side. But, if we wound the cerebellum on one side, whether the cerebral lobes are intact or removed, at once the exterior carriage becomes that which is represented in Fig. 4. In this frog, the cerebellum has been destroyed on the left side, and instantly the entire right side experiences a permanent contraction. This attitude is so regular in these lesions that, from simple sight, we can indicate the direction of the movement of rotation. The opposite side has also a constant and typical attitude; the throwing out of the fore-leg from the thorax is always as is shown Fig. 4.
When this frog is placed in water it turns on itself, the right side serving as the axis of movement. When it is at rest in the water, it takes the attitude represented in the figure. The right half of the body sinks in the water; the limbs of this side can never be on a level with those of the opposite side. These last, on the contrary, especially when the cerebral lobes are removed from the two sides, float on the surface and counterbalance the influence of the opposite side; the hind-foot of the uninjured side remains extended, and rests always at a higher level than that of the opposite side; the fore-foot comes out of the water, as well as the anterior part of the body. The limbs of the affected side are not really paralyzed, but, nevertheless, their movements are more limited, they are not so extended nor so various, and their action no longer coincides with that of the healthy side.
If, in place of pricking or deeply wounding one side of the pons Varolii, we prick or wound these nervous centres on each side, a little above the bulb, we obtain a tonic contraction of both sides of the body, and the frog takes the attitude represented in Fig. 5, an attitude due, without doubt, to a tonic contraction of all the muscles of the body. Put in the water, this frog falls to the bottom and remains motionless.
Considering successively the attitudes taken by the frogs from Fig. 2 to Fig. 5, we see perfectly that, when we get beyond the cerebral lobes, all lesion of the other parts of the encephalon leads to a sort of tonic contraction of the muscular groups corresponding to the wounded side. The nearer the lesion is situated to the bulb, the more pronounced are the phenomena, and the more the frog leans to one side.
In geese and ducks, on pricking or cutting the peduncles of the cerebellum, we can well observe phenomena analogous to these seen in frogs. When we remove the cerebellum of the two sides, there is no movement of rotation, but the animal plunges deeply into the water. In the uninjured duck, for example, the thorax penetrates the water but a little way; the duck, deprived of its cerebellum, penetrates it twice as far. The duck, represented at Fig. 6, is wounded in the right side of the cerebellum, and the animal has a movement of rotation toward the left. side.
We think these phenomena are due to an irritation of the locomotive centres, and, with Brown-Séquard, we believe that the lesions of certain points of the encephalon engender a state of irritation, whence arises, whether directly or by reflex action, a tonic contraction of certain muscular groups, and chiefly of muscles of the thorax.
It is very easy to account for this influence of the muscles of the thorax in producing the movements of swimming. This easy experiment shows us, at the same time, that simple paralysis, or the loss of function of one side, does not occasion the movements of rotation. If we swim with one arm and one leg of the same side, we do not deviate, but advance in a right line; but, if we contract the muscles of the thorax of one side, at once we lean in the water on that side. If we increase this contraction, we draw over a little more in the same direction, until we come to lie completely on one side, and at this moment there supervenes a movement of rotation, almost instinctive, which makes us spasmodically take the normal position. It is something like this which occurs in animals that have movements of rolling.
By encephalic lesion, animals are led to lean strongly to one side, an attitude which they take even in repose. At the least movement, they are drawn over upon the back; at once, spasmodically, all the limbs concur in the effort to regain the former state; but, as soon as taken, the compulsion of the injured side is reproduced, and, as the animal has already acquired motion, this carries it beyond the normal attitude, and leads it on to its side and back; immediately it seeks to get on its feet, rises, is again drawn over on one side, and so on. When the cerebral lobes are removed, animals cannot remain lying on the back. They always seek to recover their normal attitude, and, consequently, when they have deviated from it, all their efforts go to recover equilibrium. It is at this moment that the four limbs concur to execute the gyratory movement. There are two factors in the motion of rotation: the first is the contraction of the muscles of the injured side which makes the animal lean over and bear down on that side; the second is the cooperation of all the members, as soon as the animal is reversed, to produce a half-revolution and recover the normal forced attitude. The first turn, particularly when the lesion is old, is slower and less regular; the second is rapid and spasmodic. The first is the consequence of the lesion, the second is the consequence of the regular action of all the nervous centres; and, the better to express this thought,
we may say that, in an animal from which have been taken only the cerebral lobes, we might obtain a movement of rotation if, as soon as the animal is on his fore-feet, we turn him on his back; external interference would replace in this case the action produced by lesion of the pons Varolii.
We think we have demonstrated that, in an animal deprived of the cerebral lobes, the integrity of the movements of the whole body is perfect; that it is due to the action of the locomotive centres of activity, which is inevitable and always the same after certain excitations; moreover, that in these conditions there are a complete solidarity and compulsion of the movements, and a necessity for the members all to concur in maintaining an equilibrium; finally, that the movements of rotation are due to a disturbance in the equilibrium between the different locomotive centres.
- The importance of understanding the springs of animal movement and the conditions of their control is the reason for including the present article in The Popular Science Monthly. It has been translated and abridged, from the Revue Scientifique, for the general reader, but those who wish for more detail in the presentation are referred to Dr. Hammond's Quarterly Journal of Psychological Medicine, for July, where the discussion will be given complete, and where kindred questions are elaborately discussed. Fig. 1 has been inserted to give a general notion of the parts of the brain referred to in the article.—Ed.