The Dancing Mouse: A Study in Animal Behavior

by Robert Mearns Yerkes

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 Japanese 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 illustration of the author's papers (Figures 8, 9, 10).

The principal results of the early work of Rawitz are summed up in the following quotation from his paper: "The Japanese dancing mice have only one normal canal and that is the anterior vertical. The horizontal and posterior vertical canals 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 canals 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

Canalis utriculo-saccularis

Membrana basilaris
Ampulla posterior
Macula acustica sacculi

[Illustration: 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 canals, 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 canals possesses at one end, in an enlargement called the ampulla, a group of sense cells. In Figure 7 the ampullae of the three canals 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 canals 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.

[Illustration: 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 für Anatomie und Physiologie, Physiologische Abtheilung, 1899. C.s., anterior vertical canal; C.p., posterior vertical canal; C.e., horizontal canal; 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 classify 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.

[Illustration: FIGURE 9.—The membranous labyrinth of the dancer's ear.
Type II.]

[Illustration: 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 canals, 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 canals 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 canals, and in all cases they were more or less reduced in size, distorted, or grown together.

[Illustration: 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 für Physiologie, Bd. 16.]

The work of Rawitz was unfavorably criticised by Alexander and Kreidl (2), Kishi (21), and Baginsky (4), as well as by Panse (23 and 24). To their criticisms Rawitz replied by insisting that the other investigators could not with right attack his statements because they had not used the reconstruction method. In order to test the value of this contention, and if possible settle the question of fact, Baginsky had a model of the ear of the dancer constructed by a skilled preparator (Herr Spitz) from sections which had been prepared by the best neurological methods. This model was made eighty times the size of the ear. It was then reduced in the process of photographic reproduction to sixteen times the natural size of the ear in the mouse. Figure 11 is a photograph of Baginsky's model. It shows beyond question the presence of three canals of the same general form and relations as those of the common mouse and of other mammals. Baginsky's paper is brief and to the point. His criticisms of the work of both Cyon and Rawitz are severe, but they are justified in all probability by the carelessness of these investigators in the fixation of their materials. Of the five skilled histologists who have examined the ear of the dancer, Rawitz alone found markedly abnormal canals. It is highly probable, therefore, that the canals in his preparations in some way became distorted before the ears were sectioned. He doubtless described accurately the conditions which he found, but the chances are that those conditions never existed in the living animals.

The conflicting statements of Rawitz and Panse stimulated interest, and as a result two other investigators, without knowledge of one another's work, began careful researches on the dancer's ear. One, Alexander (2 and 3), worked in coöperation with the physiologist Kreidl; the other, Kishi (21), worked independently. The anatomical papers of Alexander and Kishi appeared at about the same time, and since neither contains a reference to the other, it is evident that the investigations were carried on almost simultaneously. Alexander's descriptions are more detailed than those of Rawitz and Panse, and in certain respects Kishi's are even more thoroughgoing. The first paper published by Alexander and Kreidl (1) contains the results of observations on the habits and behavior of the dancers. Having examined the chief facts of function, these investigators attempted to discover the structural conditions for the peculiarities of behavior which they had observed.

As material for their anatomical work they made use of four dancers, one albino mouse, and four common gray mice. The ears of these individuals were fixed, sectioned, and examined microscopically in connection with parts of the brain. In all, eight dancer ears and six common mouse ears were studied.

Very extensive descriptions of these preparations, together with measurements of many important portions of the ear, are presented in their paper, the chief conclusions of which are the following:—

1. The semicircular canals, the ampullae, the utriculus, and the cristae acusticae of the canals are normal in their general form and relations to one another as well as in their histological conditions (2 p. 529). This is contradictory of the statements made by Rawitz.

2. There is destruction of the macula sacculi (2 p. 534).

3. There is destruction also of the papilla basilaris cochleae, with encroachment of the surrounding tissues in varying degrees.

4. There is diminution in the number of fibers of the branches and roots of the ramus superior and ramus medius of the eighth nerve, and the fiber bundles are very loosely bound together.

5. Similarly the number of fibers in the inferior branch (the cochlear nerve) of the eighth nerve is very much reduced.

6. There is moderate reduction in the size of the two vestibular ganglia as a result of the unusually small number of nerve cells.

7. The ganglion spirale is extremely degenerate.

There is therefore atrophy of the branches, ganglia, and roots of the entire eighth nerve, together with atrophy and degeneration of the pars inferior labyrinthii. The nerve endings are especially degenerate (2 p. 534).

The above structural deviations of the ear of the dancer from that of the common mouse may be considered as primary or secondary according as they are inherited or acquired. Since, according to Alexander and Kreidl, the dancers' peculiarities of behavior and deafness are directly and uniformly inherited, it is obvious that certain primary structural deviations must serve as a basis for these functional facts. But it is equally clear, in the opinion of Alexander and Kreidl (2 p. 536), that other structural peculiarities of the dancer are the result of the primary changes, and in no way the conditions for either the dancing or the deafness. These authors feel confident that the facts of behavior which are to be accounted for are almost certainly due to the pathological changes which they have discovered in the nerves, ganglia, and especially in the peripheral nerve endings of the ear of the mouse (2 p. 537).

It is further claimed by Alexander and Kreidl that there are very marked individual differences among the dancers in the structure of the ear. In some cases the otoliths and the sensory hairs are lacking; in others, they are present in the state of development in which they are found in other varieties of mouse. Sometimes the cochlea is much reduced in size; at other times it is found to be of normal size (2 p. 538). These variations in structure, if they really exist, go far toward justifying the tendency of Cyon and Alexander and Kreidl, as well as many other investigators, to regard the dancer as abnormal or even pathological.

The functions of the ear as at present known to the comparative physiologist are grouped as the acoustic and the non-acoustic. The cochlea is supposed on very good grounds to have to do with the acoustic functions, and the organs of the semicircular canals on equally good evidence are thought to have to do with such of the non-acoustic functions as equilibration and orientation. Just what the functions of the organs of the ear sacs are is not certainly known. These facts are of importance when we consider the attempts made by Alexander and Kreidl to correlate the various peculiarities of behavior shown by the dancer with the structural facts which their work has revealed. This correlation is indicated schematically below. The physiological facts to be accounted for in terms of structure are presented in the first column, and the anatomical facts which are thought to be explanatory, in the second (2 p. 539).

FUNCTION

1 Lack of sensitiveness to auditory stimuli. {Structure 1,2,3 below}

2 Defective equilibrational ability. {Structure 4,5,6 below}

3 Lack of turning dizziness. {Structure 4,5,6 below}

4 Normal reactions to galvanic stimulation. (not related in table to any Structure)

STRUCTURE

1 Destruction of the papilla basilaris cochleae, etc.

2 Diminution of the inferior branch of the eighth nerve.

3 Marked degeneration of the ganglion spirale.

4 Destruction of the macula sacculi.

5 Diminution of the branches and roots of the superior and middle branches of the eighth nerve.

6 Diminution of both ganglia vestibulii and of the nerve cells.

Alexander and Kreidl themselves believe that the partial deafness of the dancers (for they admit that the total lack of hearing has not been satisfactorily proved) is due to the defective condition of the cochlea. They account for the imperfect equilibrational ability of the animals by pointing out the structural peculiarities of the sacculus, the vestibular ganglia, and the peripheral nerves. Similarly, the lack of dizziness they suppose to be due to the diminution of the fibers of the nerves which supply the canal organs, the atrophied condition of the vestibular ganglia, and a disturbance of the peripheral sense organs. Furthermore, there are no anatomical facts which would indicate a lack of galvanic dizziness (2 p. 552).

Despite the fact that they seem to explain all the functional peculiarities of the dancer, the statements made by Alexander and Kreidl are neither satisfying nor convincing. Their statements concerning the structure of the ear have not been verified by other investigators, and their correlation of structural with functional facts lacks an experimental basis.

In this connection it may be worth while to mention that a beautiful theory of space perception which Cyon (9) had constructed, largely on the basis of the demonstration by Rawitz that the dancers have only one normal canal, is totally destroyed by Panse, Baginsky, Alexander and Kreidl, and Kishi, for all of these observers found in the dancer three canals of normal shape. Cyon had noted that the most abnormal of the voluntary as well as of the forced movements of the dancer occur in the plane of the canal which Rawitz found to be most strikingly defective. This fact he connected with his observation that the fish Petromyzon, which possesses only two canals, moves in only two spatial dimensions. The dancer with only one functional canal in each ear moves in only one plane, and neither it nor Petromyzon is able to move far in a straight line (11 p. 444). From these and similar surmises, which his eagerness to construct an ingenious theory led him to accept as facts quite uncritically, Cyon concluded that the perception of space depends upon the number and arrangement of the semicircular canals, and that the dancer behaves as it does because it possesses canals of unusual shape and relations to one another. The absurdity of Cyon's position becomes obvious when it is shown that the structural conditions of which he was making use do not exist in the dancer.

The results obtained by Kishi in his study of the ear of the dancer differ in many important respects from those of all other investigators, but especially from those of Rawitz and Alexander and Kreidl.

Kishi's work was evidently done with admirable carefulness. His methods in the preparation of his materials, so far as can be judged from his report, were safe and satisfactory, and his descriptions of results are minute and give evidence of accuracy and conscientious thoughtfulness. The material for his histological work he obtained from three different animal dealers. It consisted of fifteen adult and nineteen young dancers, and, as material for comparison, ten common gray mice. The animals were studied first biologically, that their habits and behavior might be described accurately and so far as possible accounted for in the light of whatever histological results might be obtained subsequently; then they were studied physiologically, that the functional importance of various organs which would naturally be supposed to have to do with the peculiarities of the mouse might be understood; and, finally, they were killed and their ears and portions of their brains were studied microscopically, that structural conditions for the biological and physiological facts might be discovered.

The ear, which was studied by the use of several series of sections, as well as in gross dissections, is described by Kishi under three headings:—

(1) The sound-receiving apparatus (auditory organs).

(2) The static apparatus (equilibrational organs).

(3) The sound-transmitting apparatus (ear drum, ear bones, etc.).

The chief results of his structural investigation may be stated briefly under these three headings. In the sound-receiving or auditory apparatus, Kishi failed to find the important deviations from the usual structure of the mammalian ear which had been described by Rawitz. The latter distinctly says that although the organ of Corti is present in all of the whirls of the cochlea, the auditory cells in it are noticeably degenerate. Kishi does not agree with Panse's statement (21 p. 476) that the auditory organ of the dancer differs in no important respects from that of the common mouse, for he found that in certain regions the hair cells of the organ of Corti were fewer and smaller in the dancer. He therefore concludes that the auditory organ is not entirely normal, but at the same time he emphasizes the serious discrepancy between his results and those of Rawitz. In not one of the ears of the twelve dancers which he studied did Kishi find the direct communication between the utriculus and the scala tympani which Rawitz described, and such differences as appeared in the organ of Corti were in the nature of slight deviations rather than marked degenerations.

In the outer wall of the ductus cochlearis of the dancer the stria vasculosa is almost or totally lacking, while in the gray mouse it is prominent. This condition of the stria vasculosa Kishi was the first to notice in the dancer; Alexander and Kreidl had previously described a similar condition in an albino cat. If, as has been supposed by some physiologists, the stria vasculosa is really the source of the endolymph, this state of affairs must have a marked influence on the functions of the auditory apparatus and the static apparatus, for pressure differences between the endolymph and the perilymph spaces must be present. And, as Kishi points out, should such pressure differences be proved to exist, the functional disturbance in the organ of hearing which the lack of responses to sounds seems to indicate might better be ascribed to them than to the streaming of the endolymph from the canals into the cochlea as assumed by Rawitz (21 p. 477). Kishi merely suggests that the condition of the stria may account for the deafness of the mouse; he does not feel at all confident of the truth of his explanation, and he therefore promises in his first paper a continuation of his work in an investigation of the functions of the stria. This, however, he seems not to have accomplished thus far.

[Illustration: FIGURE 12.—The inner ear of the dancer. Reproduced from Kishi's figure in the Zeitschrift für wissenschaftliche Zoölogie, Bd. 71. c.c. crus simplex; o.b. anterior vertical canal; h.b. posterior vertical canal; a.b. horizontal canal.]

The static apparatus Kishi describes as closely similar in form to that of the gray mouse. In none of his twelve preparations of the ear of the dancer did he find such abnormalities of form and connections in the semicircular canals as Rawitz's figures and descriptions represent. Rawitz states that the anterior canal is normal except in its lack of connection with the posterior and that the posterior and horizontal are much reduced in size. Kishi, on the contrary, insists that all of the three canals are normal in shape and that the usual connection between the anterior and the posterior canals, the crus simplex, exists. He justifies these statements by presenting photographs of two dancer ears which he carefully removed from the head. Comparison of these photographs (Figures 12 and 13) with Rawitz's drawings of the conditions of the canals and sacs as he found them (Figures 8, 9, and 10), and of both with the condition in the typical mammalian ear as shown by Figure 7, will at once make clear the meaning of Kishi's statements. That Rawitz's descriptions of the canals are not correct is rendered almost certain by the fact that Panse, Baginsky, Alexander and Kreidl, and Kishi all agree in describing them as normal in form.

The only important respects in which Kishi found the membranous labyrinth, that is, the canals and the ear sacs, of the dancer to differ from that of the gray mouse are the following. In the dancer the cupola of the crista acustica is not so plainly marked and not so highly developed, and the raphae of the ampullae and canals, which frequently are clearly visible in the gray mouse, are lacking (21 p. 478).

[Illustration: FIGURE 13.—The inner ear of the dancer, showing the spiral form of the cochlea. After Kishi.]

The sound-transmitting apparatus of the dancer, according to Kishi, differs only very slightly from that of the gray mouse, and there is no reason to consider the differences which appear as important (21 p. 478).

Almost as amusing as the way in which Cyon's theory of space perception disappears in the light of critical research is Panse's explanation of the deafness of the dancer. Failing to find any defects in the auditory apparatus of the inner ear which seemed adequate to account for the obvious lack of responsiveness to sounds, this investigator concluded that plugs of wax which he had noticed in the auditory meatus of the dancer excluded sounds or in some way interfered with the functioning of the tympanic membrane. Kishi reports that he found such plugs of wax in the ears of one gray mouse, but in none of the dancers which he examined did he discover them (21 p. 479). Panse's explanation of the defective hearing of the dancer neither needs nor deserves further comment.

As one result of his investigation, Kishi is convinced that the dance movements are not due to peculiarities in the semicircular canals and their sense organs, as Rawitz claimed, for the general form and finer structure of these organs in the dancer is practically the same as in the common mouse. Kishi is just as certain that the whirling is not due to defects in the canal organs, as Rawitz is that it is due to such structural conditions! It is rather surprising that any one should feel confident of the power of the microscope to reveal all those structural conditions which are important as conditions of function. Probably there are histological differences between the ear of the dancer and that of the gray mouse, which, although undetectable by scientific means at present, furnish the structural basis for the marked differences in behavior. As has been set forth already (p. 9), Kishi accounts for the dance movements by assuming the inheritance of an acquired character of behavior. This inherited tendency to dance, he thinks, has been accentuated by the confinement of the mice in narrow cages and their long-continued movement in the wheels which are placed in the cages (21 p. 481).

Rawitz, Cyon, and Alexander and Kreidl felt themselves under the necessity of finding peculiarities of behavior in the dancer which could be referred to the various abnormalities of structure which they had either seen or accepted on faith; Kishi found himself in a very different predicament, for he had on his hands the commonly accepted statement that the animals are deaf, without being able to find any structural basis for this defect. To avoid the difficulty he questions the existence of deafness! If perchance they are deaf, he thinks that it is possibly because of the defect in the stria vasculosa. This suggestion Kishi makes despite the fact that our ignorance of the function of the stria renders it impossible for us to do otherwise than guess at its relation to hearing.

We have now briefly reviewed the results of the various important investigations of the behavior and structure of the dancer.

The observations of Cyon, Zoth, and the writer establish beyond doubt the existence of important individual differences in behavior if not of distinct divisions within the species of mouse, and the general results of the several anatomical investigations make it seem highly probable that the structure of the ear, as well as the externally visible structural features of the animals, vary widely. Unfortunately, the lack of agreement in the descriptions of the ear given by the different students of the subject renders impossible any certain correlation of structural and functional facts. That the whirling and the lack of dizziness and of hearing have their structural bases no one doubts, but whether it is in the brain itself, in the sense organs, or in the labyrinth, our knowledge does not permit us to say. With this statement Rawitz, Cyon, and Alexander and Kreidl would not agree, for they believe that they have discovered structural peculiarities which fully explain the behavior of the dancer. Panse and Kishi, on the other hand, contend that the ear gives no structural signs of such peculiarities as the dancing and deafness suggest; they therefore look to the cerebellum for the seat of the disturbance. With the same possibility in mind the author of "Fancy Varieties of Mice" writes: "These quaint little creatures make amusing pets for any one who is not scientific, or very fond of knowing 'the reason why.' In their case, the reason of the peculiarity which gives them their name is rather a sad one. It is now pretty conclusively established that they are no more Japanese than they are of any other country in particular, but that the originators of the breed were common fancy mice which were suffering from a disease of the brain analogous to the 'gid' in sheep. In the latter, the complaint is caused by a parasite in the brain; in the case of the Waltzing Mouse, it is probably due to an hereditary malformation therein. Be this as it may, the breed is now a firmly established one, and the children of waltzing mice waltz like their parents" (32 p. 45). Although it is quite possible that peculiarities in the central nervous system, rather than in the peripheral nervous system, may be responsible for the forms of behavior exhibited by the dancer, it must be remembered that no such peculiarities have been revealed by the examination of the central nervous system. The old fancier has neither better nor worse grounds for his belief than have Panse and Kishi.

So far as the reliability of the anatomical work which has been discussed is in question, it would seem that Rawitz's results are rendered somewhat unsatisfactory by the carelessness of Cyon in fixing the materials; that Panse's descriptions and comparisons are neither careful nor detailed enough to be convincing; that the work of Alexander and Kreidl, as well as that of Kishi, gives evidence of accuracy and trustworthiness. The fact that the statements of Alexander and Kreidl frequently do not agree with those of Kishi proves that there are serious errors in the work of one or another of these investigators. Cyon's discussion of the anatomy of the dancer is not to be taken too seriously, for by his theory of space perception and of a sixth sense he was unduly biased in favor of the structural peculiarities described by Rawitz. Nevertheless, his discussion is not without interest, for the way in which he succeeded in making every structural fact which Rawitz suggested fit into his theories and help to account for the functional peculiarities which he had himself observed, is extremely clever and indicates a splendid scientific imagination.

To sum up: All the facts of behavior and physiology which have been established lead us to expect certain marked structural differences between the dancer and the common mouse. The bizarre movements, lack of equilibrational ability, and the nervous shaking of the head suggest the presence of peculiar conditions in the semicircular canals or their sense organs; and the lack of sensitiveness to sounds indicates defects in the cochlea. Yet, strange as it may seem to those who are not familiar with the difficulties of the study of the minute structure of these organs, no structural conditions have been discovered which account satisfactorily for the dancer's peculiarities of behavior. That the ear is unusual in form is highly probable, since three of the four investigators who have studied it carefully agree that it differs more or less markedly from that of the common mouse. But, on the other hand, the serious lack of agreement in their several descriptions of the conditions which they observed renders their results utterly inconclusive and extremely unsatisfactory. The status of our knowledge of the structure of the central nervous system is even less satisfactory, if possible, than that of our knowledge of those portions of the peripheral nervous system which would naturally be supposed to have to do with such functional peculiarities as the dancer exhibits. So far as I have been able to learn, no investigator has carefully examined the brain and spinal cord in comparison with those of the common mouse, and only those who have failed to find any structural basis for the facts of behavior in the organs of the ear have attempted to account for the dancer's whirling and deafness by assuming that the cerebellum is unusual in structure. We are, therefore, forced to conclude that our knowledge of the nervous system of the dancing mouse does not at present enable us to explain the behavior of the animal.

It seems highly probable to me, in the light of my observation of the dancer and my study of the entire literature concerning the animal, that no adequate explanation of its activities can be given in terms of the structure of the peripheral or the central nervous system, or of both, but that the structure of the entire organism will have to be taken into account. The dancer's physiological characteristics, in fact, suggest multitudinous structural peculiarities. I have confined my study to its behavior, not because the problems of structure seemed less interesting or less important, but simply because I found it necessary thus to limit the field of research in order to accomplish what I wished within a limited period.

That there are structural bases for the forms of behavior which this book describes is as certain as it could be were they definitely known; that they, or at least some of them, are discoverable by means of our present- day histological methods is almost as certain. It is, therefore, obvious that this is an excellent field for further research. It is not an agreeable task to report inconclusive and contradictory results, and I have devoted this chapter to a brief account of the work that has been done by others on the structure of the ear of the dancer rather for the sake of presenting a complete account of the animal as it is known to-day than because of the value of the facts which could be stated.