Popular Science Monthly/Volume 65/August 1904/Dextrality and Sinistrality
|DEXTRALITY AND SINISTRALITY.|
THE theories that have been advanced as to the origin of dextrality and sinistrality are:
1. A natural provision. (Sir Charles Bell, and others.)
2. The left-sided location of the heart. (Referred to by Wilson.)
3. A greater supply of nerve force to the muscles because of an earlier and greater development of the brain upon one side. (Professor Gratiolet.)
4. Obstruction to the flow of blood in the vena cava, by the pulsation of the aorta. (Dr. Barclay.)
5. Inspiration produces, mechanically, a superior efficacy of the muscles of the right side. (Professor Buchannan.) This theory is based upon the observation of the anatomic peculiarities of the liver, lungs, etc., and their supposed influence upon the center of gravity of the body. (So far as pertains to the center of gravity, the theory has been adopted by Dr. Struthers and by Dr. Allis.)
6. The center of gravity theory. The influence of the weight of the viscera of the two sides of the body, upon the position of the center of gravity. (Dr. Struthers, accepted by Buchanan, Allis, etc.)
7. The origin of the subclavian arteries, the left before the right in the-left-handed, with superiority of blood-supply to certain structures. (Professor Hyrtl.)
8. The development of one cerebral hemisphere more than the other. (Wilson.)
9. The Topsy theory—'just growed.'
These theories merit little argument in rebuttal. No. 3 and No. 8 are essentially the same, and, of course, are mere avoidances of an explanation. No. 2, No. 4, No. 5, No. 6 and No. 7 are not based upon facts, and contain fallacies of observation, rendering them at least of insufficient reach and validity. No. 9 is almost as good as any or all of the rest, and we are left with the frank confession of Dr. Struthers, that the mystery 'has baffled satisfactory explanation.' Carlyle said it was 'a question not to be settled, not worth asking except as a kind of riddle.' It is, however, of great and practical importance in medicine and in social life.
In a large way and notwithstanding a certain number of exceptions, it is an illuminating truth of biology that 'the ontogeny repeats the phylogeny.' We can, therefore, never explain the phases of development through which an organism passes except by knowing the corresponding stages of evolution of the line of its ancestral forms. If, therefore, we ever solve the mystery of dextrality and sinistrality, it will be by the study of the conditions, habits, necessities, etc., of the ancestral types when dextrality and sinistrality arose. The infant of a few months shows no signs of preference in the use of the hands; it is not yet dextromanual, nor ambidextral; it is simply nondextrous, or ambisinistrous. Almost as soon as it exhibits any conscious effort toward skillful use of the hands it begins to show signs of dextromanuality. Before it walks, before it is one year old, dextrality is clearly pronounced. Baldwin (Pop. Sci. Mo., Vol. XLIV.) has demonstrated experimentally that it is plainly established as early as the seventh or eighth month. The period in phylogenous savage life to which this of the infant corresponds must, therefore, be that of the earliest phase of humanization. The animals, even the anthropoid apes, do not, so far as I have observed, exhibit it. Vierordt says that parrots grasp food with the left foot, by preference, and that lions strike with the left paw. Livingston is quoted as thinking 'all animals are left-handed' I suspect this is all error, because, as a rule, it would disadvantage rather than help in the animalian struggle.
Since any sort of consciousness of the facts has existed the wisdom of dextromanuality has been emphatically exhibited: (1) In the word dexterity, which is the prized and honored quality of savage and civilized man; (2) in the secondary meaning of the word sinister—unlucky, ill-omened, evil; (3) in the persistent training of all left-handed children, by parents, teachers, etc., to make them like the rest of the right-handed world. These three facts, the residue of the psychologic habits of ages, persistent in all history, crystallized and embedded in the very language itself which chronicles all mentality, help to give us the clue to the solution of the riddle.
Skillfulness, 'handiness,' expertness of sense and act, were the sole means whereby the savage could win his place in the world, domesticate animals, conquer in all sorts of conflicts, supply himself with food, clothing, house, etc. It was necessary that one hand should be chosen to do the dextrous or more skilled tasks, for the simple reason that exercise develops and perfects function, and one would learn to be more skilled and 'handy' with one hand than with both. The savage required no treatise on logic nor even any conscious reasoning to teach him this primary lesson. His food and life depended upon his learning it.
But that it was an acquirement, that the law and necessity were not exceptionless, that it was due to no absolute fatalism of anatomy or physiology, is evident from the fact that so large a proportion of left-handed children and adults exist in all races and times. The education of left-handed children, whereby their writing center, naturally dextrocerebral, is by forced training and long habit transferred to the left cerebral hemisphere, is another demonstration that no inherent neurologic or psychologic law governs the location of the cerebral center or its peripheral outworking. When the occasions arose in the humanization process, and the demand for the differentiation of cerebral mechanisms was made, the plastic brain on either side could take up the work. And pure, or untrained, left-handed persons are to-day as expert as their right-handed fellows. All that is needed to explain dextrality in 98 per cent, of children is some ancestral savage custom, habit, or necessity, widely prevalent, which inclined to the use of the right hand and eye for one or two exceptionally intellectual tasks. The inheritance of aptitude, the force of custom, and the necessities of the struggle for existence would certainly fix the persistence of dextrality.
We must not forget that the somewhat sudden and clear preference of dextrality and sinistrality of the child of to-day was in the far-away ancestral line spread out over long periods of time. A year or two of the child's life represents thousands of years of slow acquirement and habit.
Again, it should be remembered that even in our preferences and habits it is only in a few things that one hand, etc., has the greater expertness, accuracy and rapidity. It is often rather a division of functions, a differentiation of ability, than a unique one. In the dextral the left hand does many tasks of as great or greater importance, and with equal or superior skill, as the right. In eating, the fork is now more used than the knife; in gunning, the left hand is given the vastly more important, difficult and onerous task; in chopping, hoeing, shoveling, picking, lathe-work, railway locomotive engineering and other tasks the left arm and hand often execute the chief and more expert tasks. Especially noteworthy is the playing of the violin, 'cello and bass viol. The 'fingering' is done with the left hand, and forms a striking reversal of dextrality, because it is hy all odds the function requiring more manipulative skill, accuracy and rapidity. I do not know that the fact itself has ever been observed and stated, but certainly the reason of this strange contradictory practise has hitherto escaped the attention. It is, I think, due to dextrocularity. With few and easily explained exceptions dextromanuality is a result, or a concomitant, of dextrocularity. If the violin, 'cello and viol were fingered with the right hand the learner would be greatly handicapped by the foreshortening which would exist as his dextral eye glanced along the neck of the instrument straight in front or below this eye. The learner must see his fingers and gain precision in placing them by careful visual estimates. But when placed sinistrad the right eye sees the neck of these instruments and the fingers at an angle which permits more accurate observation, estimates of distances, etc., than would be possible if the instrument were fingered with the right hand. In those instruments necessarily held in the median line, some wind-instruments, the flageolet, hautboy, etc., the right hand asserts its selective and more difficult task. When the hands are not seen at all, as in the flute, fife, etc., the right again has its choice. No pupil with sinistromamiality established can learn piano-playing easily. I know of one who was a great lover of music who failed utterly after long perseverance.
There are other cautions to be emphasized relating to the acquirement of dextrality by the savage: Nearly all the actions which we now call right-handed were in primeval times to him unknown. This is especially true of three things. Knives and forks have only been used in eating for a few hundred years. He ate with his fingers, and one may suspect he used the left as much as the right in this way. The Mussulman custom and its reason are, of course, both modern. Secondly, the modern gun and revolver had not been devised. The bow and arrow, the spear, boomerang, club, etc., could be used as well with the left hand by the sinistromanual. Thirdly, writing was unknown, or relatively so, and, as we have now learned, that locates the speech center in the cerebral hemisphere opposite the writing hand. It is thus evident that dextrality in the savage, at the time when it began to become habitual, must have been at best only partial, incomplete, and for a very few acts. The left-handed arrow-chippers, basket-weavers, club-wielders, sewing-women, etc., even if more numerous relatively than in civilized life, would perhaps attract little or at least less attention than now, and would be less discouraged, surely less taught to reverse the natural inclination.
In default of systematic banding and military training, also, the left-handed spearmen, bowmen, swordsmen and clubmen might not have much attention directed to themselves and sometimes might have an advantage over their single and dextral adversaries, e. g., in tilting. The preference in heraldry for dextral quarterings, etc., is by no means uniform.
But there was one overlooked factor which was doubtless decisive in setting up the trend toward dextrality. This was the development of sign-language synchronously, and even preceding that of spoken language. The ineffaceable relics of this long and arduous period exist in present day language, plainly in many savage tribes and customs, but the most striking proof is displayed in our so-called Roman numerals. The fingers of the hand held up, or counted off, were beyond question the beginnings of arithmetic, the means of barter, the method of stating the fundamental fact of number requisite in all thinking and doing. Military and intertribal dealings, especially made the custom powerful and even sacred. One finger was the origin of our figure one, the second equaling two, etc., up to five, or V, which fork was made by the thumb stuck up opposite the first I. When the counting was more than five, the other hand was made to represent the first five, the digits being added up to ten, when two forks were used, or the crossed thumbs, which constituted X., or ten. The impressive ceremonies of warring and bartering tribes would stamp with distinctive approval the hand used in the sign-language, and henceforth it would become the honored one, the stamping and writing hand, and in time the sword-hand. The right was chosen as the sign and numbering hand because the left was naturally used for the highly important task of guarding the sinistrally-placed heart with the shield. War is the substance of all early history and of the savage aeons which preceded all history. Dr. Flint (The Sun, April 17, 1904) says that deaf-mutes may have an aphasia that prevents the use of the right hand in the sign-language.
Speech is the sole example of the higher functions, sensational or motor, which is single. Feet, legs, arms, hands, vision, hearing, all are dual in nature, requiring dual centers of coordination and innervation in the two halves of the brain. But speech, being a single function, can have but one center, and that, of course, must be located, not in any median place, because there is no such place, but in one or the other side. We also know by physiology and pathology that in the dextral it is in the third left frontal convolution, and in the sinistral it is in the corresponding position on the right side. We know, furthermore, that it is the intellectual act of writing, rather than the grosser acts and functions, which localizes the speech center. A man may be left-handed for everything but writing and the judgments issuing in the correlations of spoken words are formed and innervated from Broca's convolution. Or vice versa in the case of I;he sinistromanual writer who is dextromanual for all other acts.
The reason why dextromanuality, dextrocularity, etc., must coexist with sinistrocerebrality becomes manifest. The function of speech or writing is the method whereby judgment or volition passes into action. The initial, dominating and guiding motility to vocal organs, to hand, and even to foot, springing from closely contiguous, and hence more quickly and accurately acting, cerebral centers, will be better correlated and certain than if the centers were in opposite cerebral hemispheres. The indicator of all action, the very creator of intellect, is vision. Hence all right-handed people are also right-eyed. The centers for right vision, right motion, and for speech are thus in close relationship and upon the same side of the brain. As I have said (Science, April 8, 1904):
This furnishes the physiologic reason why all attempts at ambidexterity are failures, and unwise.
All agree that perfect ambidexterity has never existed, despite all training. It is neither possible nor desirable. Sinistrality is no defect and of no disadvantage. That said to exist in criminals, idiots, etc., like many things 'Lombrosal,' is not true, or it is post hoc, etc.
It seems that there is an 'Ambidextral Culture Society' in England which, in default of something to do of use and in accord with nature's indications, wishes to insure that every child at school shall be so drilled in both separate and simultaneous use of the two hands that he shall have the two equally strong, sensitive and skillful. The pitiable victims! The organization might better call itself the society for nullifying the law of the differentiation of function necessary to all progress, for returning to barbarism in the handicrafts, and for lifelong cruelty to the left-handed.
The essential and clarifying thought of the foregoing explanation is that as the writing act now locates the speech-center, although all other acts may be opposite-handed, so the right-hand sign-language and numbering would necessarily have had the same effect in barbarous times. That this sign-language of primitive man was dextral is not to be questioned, as about 98 per cent, of babies are now clearly righthanded before they are one year old. The protection of the heart by the shield would constitute sufficient reason for the institution of dextrality in counting and sign-making, and custom and uniformity of habit especially in early times, would result in almost a universality. But not an absolute one, for one or two per cent, are now sinistral. And the Bible story of the Benjamite tribe illustrates how the habit would not be absolute. There is in all this one noteworthy neurologic fact: In view of the long continuance and vast preponderance of dextrality it seems strange that the brain preserves all the preformed mechanisms, plastic and ready to make a sinistral child, and the outworking of sinistrality is as prompt, the result as dextrous, as if dextrality had been chosen. The wonder at this is, however, lessened when one notes that all the functions of completed dextrality are at the same time and in the same person now possible to the sinistral; there is a mere difference in the degree not in the kind of expertness. Besides this a number of left-handed acts in the dextral, e. g., those of the violinist, gunner, etc., are far more expertly and finely coordinated than those of the right, etc.
If the foregoing explanation of the origin and perpetuation of dextrality is adequate, it remains to explain the origin of sinistrality. Why are there about two out of a hundred naturally left-eyed and left-handed? Fundamentally, of course, because the speech-center is located in the right cerebral hemisphere, and the contributing and executing centers of vision and motion act in better unity if they are in close connection and contiguity than if connected by long commissural fibers to and from the opposite sides of the brain. The dextrocerebrality of two per cent, of sinistral exceptions to the usual law appears explainable, perhaps in part by persistence of original sinistral types, but more certainly they are due to accident, injury, disease, etc., of dextral organs, in the young of our ancestors. Especially in savage life would these accidents be more numerous than now. The loss of even one dextral finger might compel the education of the undeveloped speech center on the right side. Injury to the right hand and arm, even of the right foot or leg would do the same. Deafness of the right ear would compel a turning of the left ear forward and might work out complete sinistrality. But more important than all these causes combined would be the more frequent greater ametropia, amblyopia, disease, leukoma, etc., of the right eye, compelling the use of the left, and thus transferring all centers of dextrousness to the right side. I have repeatedly demonstrated the persistence of dextrocularity even with visual acuteness considerably less than that of the left. But there is a limit to this 'accommodation,' and if the amblyopia of the right is greater than double that of the left, the patient becomes left-eyed. In savage and in semicivilized life these accidents, diseases, ametropias, heterophorias and strabismuses of the right eye would again be far more numerous than in our day and civilized peoples. Our two per cent, of sinistral children seem for the greater part to be the descendants, by the laws of heredity, of ancestors who in childhood and youth have been compelled to become sinistral by the causes enumerated.
Is it possible that there are proportionally more left-handed among oriental nations who read from right to left, than in those who read from left to right? A writer in the Cornhill Magazine (1889) says that the change in writing whereby the proceeding from right to left was reversed was due to the use of ink and pigments in writing, and the avoidance of smearing the fresh-writing by tracing the letters from left to right. But individual writers would not do this, and, if they did, they could not get their writing accepted! It is difficult to see how there would be less smudge and smear by the reversal.
The arguments for upright writing are incontestably strengthened by some facts I have lately discovered as to the frequent influence of an axis of astigmatism in the dominant eye varying by about 15° from 90°, in producing a habitual canting or sideways inclination of the head. This habitual cant of the head is often followed by spinal curvature. Probably most spinal curvatures are produced in this way, and the number of cases is much greater than is supposed. Such a patient can see upright lines, which predominate over all others in civilized life, especially in those who read much, only by holding the head to one side. When the axis of astigmatism is about 75° the head must be canted to the right to see plainly. When it is 105° it must be canted to the left. Slanted handwriting is itself pathologic, or produces pathologic results of many kinds. The printed letters of the alphabet should be refashioned to avoid all lines except the vertical and horizontal. This would greatly conduce to lessening of ocular and neurologic labor, and would increase ease and celerity of reading. Almost the only letters that could not he thus remodeled and bettered are V, Z, X, which are little used. K and R also require some slanted strokes. The others could all be made up of vertical and horizontal lines. The vast majority of astigmatisms cluster about axes 90° or 180°, and those which are anomalous and unsymmetric produce disease unless corrected, as they may be by proper spectacles.
The summary of the foregoing theory of the origin of dextralitv and sinistrality is:
Modern pathology has demonstrated that the intellectual acts of writing and reading locate the speech center upon the cerebral side opposite the writing hand.
The centers for vision, audition, and motion of the hand and foot for correlated dextral or sinistral actions and sensations, are in the same side of the brain.
Coordination of sensation, perception, judgment and act are rendered more accurate, expert and quick by this close contiguity and inter-relationship than if made by commissural fibers from the other cerebral hemisphere.
The original location of the speech center in the dextral was caused by the almost universal employment of the right or spear-hand in sign-language preferred to the left or shield-hand, because this was more restricted in movement by holding the shield over the heart.
The origin of left-handedness was in large measure due to the location or education of the speech center in the right brain because of injury to dextral organs, but chiefly to disease or deficient vision of the right eye.
Ambidexterity of any general or thoroughgoing kind is neither possible nor desirable, and the attempt to bring it about results in suffering and disease.
Vertical handwriting, and printed letters made up of vertical and horizontal lines, should be encouraged.
- In the comparative absence of interest in these subjects there is a resultant dearth and awkwardness of words describing the conditions. It seems necessary to coin a few in order at least to avoid more cacophanous ones. The following, some of them already listed in the dictionaries, may be found useful in future discussions:
Dextral, pertaining to the right side of the body.
Sinistral, pertaining to the left side.
Dextrality, Sinistrality, the corresponding abstract qualities.
Dextrad, Sinistrad, toward the right, or left, respectively.
Dextromanual, Sinistromanual, dextrality and sinistrality, respectively, as relating to the hands.
Dextropedal, Sinistropedal, as relating to the feet.
Dextrocular, Sinistrocular, as relating to the eyes.
Dextraural, Sinistraural, as relating to the ears.
Dextrocardial, Sinistrocardial, as relating to the heart.
Dextrohepatal, Dextrosplenic, etc., may be formed.
Dextrocerebral, located in the right cerebral hemisphere.
Sinistrocerebral, located in the left cerebral hemisphere.
Dextrorse, turned, turning, or moving to the right.
Sinistrorse, turned, turning or moving to the left.
Sinistrous, awkward, unskilled.
Dextrous, skilled, expert.
Ambidextrous, equally skilled with both hands.
- It does not matter with which hand the first numbering, in some cases, was done; the intelligent attention must have been directer to the action with the dextral or spear side. Homer and the earliest Greek vases show the right was ὲπὶ δόρν, the spear side, and ὲπˋ ὰσπίδα the shield side.
- One of the best tests of predominant dextrality or sinistrality is the 'sighting' of a stick to see if it is straight, or the sighting of a gun or pistol. Dextrocularity is largely a dictator of general dextrality. And of dextropedality also, for the dextral is right-footed also. Errors of judgment, however, have been frequent as to the function of the feet. The 'spade-foot' is the left, naturally, because the right leg and foot are the directing ones, in the dextral, who also, as the masonic ritual directs, steps off with the left foot first. The dextral must spring from the right foot. It has been said that the oblique line of the body of the dog in trotting is due to incipient right or left-footedness. But all soft-footed animals avoid 'interfering' by this obliquity of progression. The much discussed knockout blow of the pugilist with the left is, I suspect, because of the better spring from the firmer right foot.
- See the case of Morse, reported by Wilson; especially his own, and that cited on p. 146.
- There is thus no danger and no need of a greater weight of the half brain initiating dextrality in the dextral, and all the discussion and labor of comparative weighing the two halves is relatively useless. Moreover the cerebral mechanisms must be equally perfect even if not equally exercised. Taken in the average the two sets of organs, central and peripheral, do about the same amount of work.
- This writer says that artists paint from left to right, that the spectator views the paintings of a real landscape in the same way, etc. Even corkscrews, buckles, buttons on clothing—men's, he says, not women's—are for the right-handed, and asks, why? The figures on the faces of our clocks and watches are traced to the same cause; but he forgot that the clock face is the modified sundial made round, the location of the shadow of the meridional instant line, dictating the placing of the figure 12, and of all the related ones of the day.