The Dancing Mouse Part 4

Inasmuch as equilibration occurs similarly in darkness and in daylight, Zoth thinks that there is neither visual dizziness nor fear of heights.

But it is doubtful whether he is right concerning fear. There is no doubt in my mind, in view of the way the mice behave when placed on an elevated surface, that they are timid; but this is due probably to the uncomfortable and unusual position rather than to perception of their distance from the ground. That they lack visual dizziness seems fairly well established.

When rotated in a cyclostat[1] the dancer, unlike the common mouse, does not exhibit symptoms of dizziness. The following vivid description of the behavior of both kinds of mice when rotated is given by Alexander and Kreidl (1 p. 548). I have not verified their observations.

[Footnote 1: An apparatus consisting of a gla.s.s cylinder with a mechanism for turning it steadily and at different speeds about its vertical axis.]

The common mouse at first runs with increasing rapidity, as the speed of rotation of the cyclostat cylinder is increased, in the direction opposite to that of the cylinder itself. This continues until the speed of rotation has increased to about 60 revolutions per minute. As the rotation becomes still more rapid the mouse begins to crawl along the floor, its body stretched out and clinging to the floor. At a speed of 250 revolutions per minute it lies flat on the floor with its limbs extended obliquely to the movement of rotation, and at times with its back bent against the axis of the cylinder; in this position it makes but few and feeble efforts to crawl forward. When the rotation is suddenly stopped, the animal pulls itself together, remains for some seconds with extended limbs lying on the floor, and then suddenly falls into convulsions and trembles violently.

After several attacks of this kind, cramps appear and, despite its resistance, the animal is thrown about, even into the air at times, as if by an external force. This picture of the position a.s.sumed during rapid rotation, and of cramps after the cessation of rotation (the typical picture of rotation dizziness), is repeated with great uniformity in the case of the common mouse. Within fifteen minutes after being returned to its cage the animal recovers from the effects of its experience. This description of the symptoms of rotation dizziness in the common mouse applies equally well to the blinded and the seeing animal.

In sharp contrast with the behavior of the common mouse in the cyclostat is that of the dancer. As the cylinder begins to rotate the dancer runs about as usual in circles, zigzags, and figure-eights. As the speed becomes greater it naturally becomes increasingly difficult for the mouse to do this, but it shows neither discomfort nor fear, as does the common mouse. Finally the centrifugal force becomes so great that the animal is thrown against the wall of the cylinder, where it remains quietly without taking the oblique position. When the cyclostat is stopped suddenly, it resumes its dance movements as if nothing unusual had occurred. It exhibits no signs of dizziness, and apparently lacks the exhaustion which is manifest in the case of other kinds of mice after several repet.i.tions of the experiment. The behavior of the blinded dancer is very similar.

If these statements are true, there is no reason to believe that the dancer is capable of turning or rotation dizziness. If it were, its daily life would be rendered very uncomfortable thereby, for its whirling would constantly bring about the condition of dizziness. Apparently, then, the dancer differs radically from most mammals in that it lacks visual and rotational dizziness. In the next chapter we shall have to seek for the structural causes for these facts.

The behavior of the blinded animal is so important in its bearings upon the facts of orientation and equilibration that it must be considered in connection with them. Cyon insists that the sense of vision is of great importance to the dancer in orienting and equilibrating itself. When the eyes are covered with cotton wads fastened by collodion, this writer states (9 p. 223) that the mice behave as do pigeons and frogs whose semicircular ca.n.a.ls have been destroyed. They perform violent forced movements, turn somersaults forward and backward, run up inclines and fall over the edges, and roll over and over. In a word, they show precisely the kind of disturbances of behavior which are characteristic of animals whose semicircular ca.n.a.ls are not functioning normally. Cyon, however, observed that in certain dancers these peculiarities of behavior did not appear when they were blinded, but that, instead, the animals gave no other indication of being inconvenienced by the lack of sight than do common white mice. This matter of individual differences we shall have to consider more fully later.

No other observer agrees with Cyon in his conclusions concerning vision, or, for that matter, in his statements concerning the behavior of the blind dancer. Alexander and Kreidl (1 p. 550) contrast in the following respects the behavior of the white mouse and that of the dancer when they are blinded. The white mouse runs less securely and avoids obstacles less certainly when deprived of vision. The dancer is much disturbed at first by the shock caused by the removal of its eyes, or in case they are covered, by the presence of the unusual obstruction. It soon recovers sufficiently to become active, but it staggers, swerves often from side to side, and frequently falls over. It moves clumsily and more slowly than usual. Later these early indications of blindness may wholly disappear, and only a slightly impaired ability to avoid obstacles remains.

It was noted by Kis.h.i.+ (21 p. 484), that the dancer when first blinded trembles violently, jumps about wildly, and rolls over repeatedly, as Cyon has stated; but Kis.h.i.+ believes that these disturbances of behavior are temporary effects of the strong stimulation of certain reflex centers in the nervous system. After having been blinded for only a few minutes the dancers observed by him became fairly normal in their behavior. They moved about somewhat more slowly than usually, especially when in a position which required accurately coordinated movements. He therefore fully agrees with Alexander and Kreidl in their conclusion that vision is not so important for the guidance of the movements of the dancer as Cyon believes.

In summing up the results of his investigation of this subject Zoth well says (31 p. 168), "the orientation of the positions of the body with respect to the horizontal and vertical planes seems to take place without the a.s.sistance of the sense of sight." And, as I have already stated, this excellent observer insists that the ability of the dancer to place its body in a particular position (orientation), and its ability to maintain its normal relations to its surroundings (equilibration) are excellent in darkness and in daylight, provided only the substratum be not too smooth for it to gain a foothold.

It must be admitted that the contradictions which exist in the several accounts of the behavior of the dancer are too numerous and too serious to be explained on the basis of careless observation. Only the a.s.sumption of striking individual differences among dancers or of the existence of two or more varieties of the animal suffices to account for the discrepancies.

That there are individual or variety differences is rendered practically certain by the fact that Cyon himself worked with two groups of dancers whose peculiarities he has described in detail, both as to structure and behavior.

In the case of the first group, which consisted of three individuals, the snout was more rounded than in the four individuals of the second group, and there were present on the head three large tufts of bristly black hair which gave the mice a very comical appearance. The animals of the second group resembled more closely in appearance the common albino mouse. They possessed the same pointed snout and long body, and only the presence of black spots on the head and hips rendered them visibly different from the albino mouse.

In behavior the individuals of these two groups differed strikingly. Those of the first group danced frequently, violently, and in a variety of ways; they seldom climbed on a vertical surface and when forced to move on an incline they usually descended by sliding down backwards or sidewise instead of turning around and coming down head first; they gave no signs whatever of hearing sounds. Those of the second group, on the contrary, danced very moderately and in few ways; they climbed the vertical walls of their cage readily and willingly, and when descending from a height they usually turned around and came down head first; two of the four evidently heard certain sounds very well. No wonder that Cyon suggests the possibility of a different origin! It seems not improbable that the individuals of the second group were of mixed blood, possibly the result of crosses with common mice.

As I shall hope to make clear in a subsequent discussion of the dancer's peculiarities of behavior, in a chapter on individual differences, there is no sufficient reason for doubting the general truth of Cyon's description, although there is abundant evidence of his inaccuracy in details. If, for the present, we accept without further evidence the statement that there is more than one variety of dancer, we shall be able to account for many of the apparent inaccuracies of description which are to be found in the literature on the animal.

As a result of the examination of the facts which this chapter presents we have discovered at least six important peculiarities of behavior of the dancer which demand an explanation in terms of structure. These are: (1) the dance movements--whirling, circling, figure-eights, zigzags; (2) restlessness and the quick, jerky movements of the head; (3) lack of responsiveness to sounds; (4) more or less p.r.o.nounced deficiency in orientational and equilibrational power; (5) lack of visual dizziness; (6) lack of rotational dizziness.

Naturally enough, biologists from the first appearance of the dancing mouse in Europe have been deeply interested in what we usually speak of as the causes of these peculiarities of behavior. As a result, the structure of those portions of the body which are supposed to have to do with the control of movement, with the phenomena of dizziness, and with the ability to respond to sounds, have been studied thoroughly. In the next chapter we shall examine such facts of structure as have been discovered and attempt to correlate them with the facts of behavior.

CHAPTER V

STRUCTURAL PECULIARITIES AND BEHAVIOR

The activities of an animal are expressions of changes which occur in its structure, and they can be explained satisfactorily only when the facts of structure are known. Such peculiarities of activity as are exhibited by the dancing mouse, as contrasted with the common mouse, suggest at once that this creature has a body which differs in important respects from that of the ordinary mouse. In this chapter I shall present what is known concerning the structural bases for the whirling, the lack of equilibrational ability and of dizziness, the quick jerky head movements, the restlessness, and the partial or total deafness of the dancing mouse.

Comparative physiologists have discovered that the ability of animals to regulate the position of the body with respect to external objects and to respond to sounds is dependent in large measure upon the groups of sense organs which collectively are called the ear. Hence, with reason, investigators who sought structural facts with which to explain the forms of behavior characteristic of the dancer turned their attention first of all to the study of the ear. But the ear of the animal is not, as might be supposed on superficial examination, a perfectly satisfactory natural experiment on the functions of this group of sensory structures, for it is extremely uncertain whether any one of the usual functions of the organ is totally lacking. Dizziness may be lacking, and in the adult hearing also, but in general the functional facts lead the investigator to expect modifications of the sense organs rather than their absence.

I shall now give an account of the results of studies concerning the structure of the ear and brain of the dancer. Since the descriptions given by different anatomists contradict one another in many important points, the several investigations which have been made may best be considered chronologically.

Bernhard Rawitz (25 p. 239) was the first investigator to describe the structure of the ear of the j.a.panese or Chinese dancers, as he calls them.

The definite problem which he proposed to himself at the beginning of his study was, what is the structural basis of the whirling movement and of the deafness of the mice?

In his first paper Rawitz described the form of the ears of five dancers.

His method of work was to make microscopic preparations of the ears, and from the sections, by the use of the Born method, to reconstruct the ear in wax. These wax models were then drawn for the ill.u.s.tration of the author's papers (Figures 8, 9, 10).

The princ.i.p.al results of the early work of Rawitz are summed up in the following quotation from his paper: "The j.a.panese dancing mice have only one normal ca.n.a.l and that is the anterior vertical. The horizontal and posterior vertical ca.n.a.ls are crippled, and frequently they are grown together. The utriculus is a warped, irregular bag, whose sections have become unrecognizable. The utriculus and sacculus are in wide-open communication with each other and have almost become one. The utriculus opens broadly into the scala tympani, and the nervous elements of the cochlea are degenerate.

"The last-mentioned degeneration explains the deafness of the dancing mice; but in my opinion it is a change of secondary nature. The primary change is the broad opening between the utriculus and the scala tympani from which results the streaming of the endolymph from the semicircular ca.n.a.ls into the cochlea. When, as a consequence of the rapid whirling movements, a great part of the endolymph is hurled into the scala tympani, the organ of Corti in the scala vestibuli is fixed and its parts are rendered incapable of vibration. The condition of atrophy which is observable in the sense cells and in the nerve elements is probably due to the impossibility of functional activity; it is an atrophy caused by disuse "(25 p. 242).

Ampulla externa Ampulla anterior Ramus utriculi

Membrana basilaris

Lagena

Ca.n.a.lis utriculo-saccularis

Membrana basilaris Ampulla posterior Macula acustica sacculi

[Ill.u.s.tration: FIGURE 7.--The inner ear of the rabbit. Reproduced from Selenka after Retzius.]

To render the terms which occur in this and subsequent descriptions of the ear of the dancer somewhat more intelligible to those who are not familiar with the general anatomy of the vertebrate ear, a side view of the inner ear of the rabbit is reproduced from a drawing by Retzius (Figure 7). I have chosen the ear of the rabbit for this purpose, not in preference to that of the common mouse, but simply because I failed to find any reliable description of the latter with drawings which could be reproduced. The rabbit's ear, however, is sufficiently like that of the mouse to make it perfectly satisfactory for our present purpose.

This drawing of the rabbit's ear represents the three semicircular ca.n.a.ls, which occur in the ear of all mammals, and which are called, by reason of their positions, the anterior vertical, the posterior vertical, and the horizontal. Each of these membranous ca.n.a.ls possesses at one end, in an enlargement called the ampulla, a group of sense cells. In Figure 7 the ampullae of the three ca.n.a.ls are marked respectively, ampulla anterior, ampulla posterior, and ampulla externa. This figure shows also the cochlea, marked lagena, in which the organ of hearing of mammals (the organ of Corti) is located. The ear sac, of which the chief divisions are the utriculus and the sacculus, with which the ca.n.a.ls communicate, is not shown well in this drawing.

Within a few months after the publication of Rawitz's first paper on the structure of the dancer's ear, another European investigator, Panse (23 and 24) published a short paper in which he claimed that previous to the appearance of Rawitz's paper he had sectioned and mounted ears of the common white mouse and the dancing mouse side by side, and, as the result of careful comparison, found such slight differences in structure that he considered them unworthy of mention. Panse, therefore, directly contradicts the statements made by Rawitz. In fact, he goes so far as to say that he found even greater differences between the ears of different white mice than between them and the ears of the dancer (23 p. 140).

In a somewhat later paper Panse (24 p. 498) expresses his belief that, since there are no peculiarities in the general form, sensory structures, or nerve supply of the ear of the dancer, which serve to explain the behavior of the animal, it is probable that there are unusual structural conditions in the brain, perhaps in the cerebellum, to which are due the dance movements and the deafness. The work of Panse is not very convincing, however, for his figures are poor and his descriptions meager; nevertheless, it casts a certain amount of doubt upon the reliability of the descriptions given by Rawitz.

[Ill.u.s.tration: FIGURE 8.--The membranous labyrinth of the dancer's ear.

Type I. This figure, as well as 9 and 10, are reproduced from Rawitz's figures in the _Archiv fur Anatomie und Physiologie, Physiologische Abtheilung_, 1899. _C.s._, anterior vertical ca.n.a.l; _C.p._, posterior vertical ca.n.a.l; _C.e._, horizontal ca.n.a.l; _U._, utriculus.]

The unfavorable light in which his report was placed by Panse's statements led Rawitz to examine additional preparations of the ear of the dancer.

Again he used the reconstruction method. The mice whose ears he studied were sent to him by the physiologist Cyon.

As has been noted in Chapter IV, Cyon discovered certain differences in the structure and in the behavior of these dancers (11 p. 431), which led him to cla.s.sify them in two groups. The individuals of one group climbed readily on the vertical walls of their cages and responded vigorously to sounds; those of the other group could not climb at all and gave no evidences of hearing. After he had completed his study of their behavior, Cyon killed the mice and sent their heads to Rawitz; but unfortunately those of the two groups became mixed, and Rawitz was unable to distinguish them. When he examined the structure of the ears of these mice, Rawitz did find, according to his accounts, two structural types between which very marked differences existed. Were it not for the carelessness which is indicated by the confusion of the materials, and the influence of Cyon's suggestion that there should be different structures to account for the differences in behavior, Rawitz's statements might be accepted. As matters stand there can be no doubt of individual differences in behavior, external appearance, and the structure of the ear; but until these have been correlated on the basis of thoroughgoing, careful observation, it is scarcely worth while to discuss their relations.

[Ill.u.s.tration: FIGURE 9.--The membranous labyrinth of the dancer's ear.

Type II.]

[Ill.u.s.tration: FIGURE 10.--The membranous labyrinth of the dancer's ear.

Type III.]

To his previous description of the conditions of the ear sacs, sense organs, and nerve elements of the dancer's ear, Rawitz adds nothing of importance in his second paper (26 p. 171). He merely reiterates his previous statements concerning the form of the ca.n.a.ls, on the basis of his findings in the case of six additional dancers. Figures 8, 9, and 10 are reproduced from Rawitz to show the anatomical conditions which he claims that he found. As these figures indicate, the ca.n.a.ls were found to be extremely variable, as well as unusual in form, and the sacs distorted. In the ears of some specimens there were only two ca.n.a.ls, and in all cases they were more or less reduced in size, distorted, or grown together.

[Ill.u.s.tration: FIGURE 11.--Photograph of a wax model of the membranous labyrinth of the ear of the dancer. Reproduced from Baginsky's figure in the _Centralblatt fur Physiologie_, Bd. 16.]