CONFLICT OF THEORIES
It now remains to consider the source to which the preferential use
of the right hand is to be ascribed. The dominant influence of the
one cerebral hemisphere in relation to the discharge of nerve force
to the opposite side of the body is a fact which is now familiar to
the physiologist, and the influence of the left cerebral hemisphere
on the action of the right hand has already been alluded to. But this
extremely probable source of right-handedness long eluded inquirers,
as will be seen from a _résumé_ of the various hypotheses suggested
by eminent anatomists and physiologists. A very slight consideration
of the evidence already adduced in proof of the same prevalent usage
from earliest times precludes the idea of its origin in any mere
prescribed custom, enforced and developed by education into a nearly
universal habit. This becomes the more manifest when it is traced back
to primæval races; found incorporated in ancient and modern, savage and
civilised languages, and uncontroverted by any evidence calculated to
discredit the indications that it was a characteristic of palæolithic
and neolithic man.
The inevitable conclusion forced on the inquirer is that the bias in
which this predominant law of dexterity originates must be traceable to
some specialty of organic structure. On this assumption one feature in
the anatomical arrangement of the most important vital organs of the
body presents such a diversity in their disposition as would seem to
offer a sufficient cause for greater energy in the limbs on one side
than on the other, if accompanied by exceptional deviations from the
normal condition corresponding to the occurrence of left-handedness;
and in this direction a solution has accordingly been sought. The
bilateral symmetry of structure, so general in animal life, seems at
first sight opposed to any inequality of action in symmetrical organs.
But anatomical research reveals the deviation of internal organic
structure from such seemingly balanced symmetry. Moreover, right or
left-handedness is not limited to the hand, but partially affects the
lower limbs, as may be seen in football, skating, in the training
of the opera-dancer, etc.; and eminent anatomists and physiologists
have affirmed the existence of a greater development throughout the
whole right side of the body. Sir Charles Bell says: “The left side
is not only the weaker, in regard to muscular strength, but also in
its vital or constitutional properties. The development of the organs
of action and motion is greatest upon the right side, as may at any
time be ascertained by measurement, or the testimony of the tailor or
shoe-maker.” He adds, indeed, “Certainly this superiority may be said
to result from the more frequent exertion of the right hand; but the
peculiarity extends to the constitution also, and disease attacks the
left extremities more frequently than the right.”
With the left-handed, the general vigour and immunity from disease
appear to be transferred to that side; and this has naturally suggested
the theory of a transposition of the viscera, and the consequent
increase of circulation thereby transferred from the one side to the
other. But the relative position of the heart is so easily determined
in the living subject, that it is surprising how much force has
been attached to this untenable theory by eminent anatomists and
physiologists. Another and more generally favoured idea traces to the
reverse development of the great arteries of the upper limbs a greater
flow of blood to the left side; while a third ascribes the greater
muscular vigour directly to the supply of nervous force dependent on
the early development of the brain on one side or the other.
So far as either line of argument prevails, it inevitably leads to the
result that the preference of the right hand is no mere perpetuation of
convenient usage, matured into an acquired, or possibly an hereditary
habit; but that it is, from the first, traceable to innate physical
causes. This, as Sir Charles Bell conceives, receives confirmation
from the fact already referred to, that right or left-handedness is
not restricted to the hand, but affects the corresponding lower limb,
and, as he believes, the whole side; and so he concludes thus: “On the
whole, the preference of the right hand is not the effect of habit, but
is a natural provision; and is bestowed for a very obvious purpose.”
Nevertheless, the argument of Sir Charles Bell is, as a whole, vague,
and scarcely consistent. He speaks indeed of right-handedness as “a
natural endowment of the body,” and his reasoning is based on this
assumption. But much of it would be equally explicable as the result of
adaptations following on an acquired habit. Its full force will come
under consideration at a later stage. Meanwhile it is desirable to
review the various and conflicting opinions advanced by other inquirers.
The theory of Dr. Barclay, the celebrated anatomist, is thus set forth
by Dr. Buchanan, from notes taken by him when a student: “The veins of
the left side of the trunk and of the left inferior extremity cross
the aorta to arrive at the vena cava; and some obstruction to the flow
of blood must be produced by the pulsation of that artery.” To this
Dr. Barclay traced indirectly the preferential use of the right side
of the body, and especially of the right hand and foot “All motions,”
he stated, “produce obstruction of the circulation; and obstruction
from this cause must be more frequently produced in the right side
than the left, owing to its being more frequently used. But the venous
circulation on the left side is retarded by the pulsation of the
aorta, and therefore the more frequent motions of the right side were
intended to render the circulation of the two sides uniform.” The idea,
if correctly reported, is a curious one, as it traces right-handedness
to the excess of a compensating force for an assumed inferior
circulation pertaining naturally to the right side; and incidentally
takes into consideration an abnormal modification affecting the
development or relative disposition of organs. Both points have been
the subject of more extended consideration by subsequent observers.
It is curious, indeed, to notice how physiologists and anatomists
have shifted their ground, from time to time, in their attempts at
a solution of what has been very summarily dismissed by others as a
very simple problem; until, as Dr. Struthers remarks, it “has ceased
to attract the notice of physiologists only because it has baffled
satisfactory explanation.”
The eminent anatomist, Professor Gratiolet, turned from the organs in
immediate contact with the arm and hand, and sought for the source of
right-handedness in another and truer direction, though he failed to
realise its full bearings. According to the Professor, in the early
stages of fœtal development the anterior and middle lobes of the
brain on the left side are in a more advanced condition than those on
the right side, the balance being maintained by an opposite condition
of the posterior lobes. Hence, in consequence of the well-known
decussation of the nerve roots, the right side of the body,--so far as
it is influenced by brain-force,--will, in early fœtal life, be better
supplied with nervous force than the left side; and thereby movements
of the right arm would precede and be more perfect than those of the
left. The bearings of this line of argument in its full compass will
come more fitly under review at a later stage.
Dr. Andrew Buchanan, Professor of Physiology in the University of
Glasgow, in a paper communicated by him to the Philosophical Society of
Glasgow in 1862, entitled “Mechanical Theory of the predominance of the
right hand over the left; or more generally, of the limbs of the right
side over those of the left side of the body,” aimed at a solution of
the question in a new way. According to him, “the preferential use of
the right hand is not a congenital but an acquired attribute of man.
It does not exist in the earliest periods of life.” Nevertheless, “no
training could ever render the left hand of ordinary men equal in
strength to the right;” for “it depends upon mechanical laws arising
out of the structure of the human body.” This theory is thus explained:
In infancy and early childhood there is no difference in power between
the two sides of the body; but so soon as the child becomes capable
of bringing the whole muscular force of the body into play, “he
becomes conscious of the superior power of his right side, a power not
primarily due to any superior force or development of the muscles of
that side, but to a purely mechanical cause. He cannot put forth the
full strength of his body without first making a deep inspiration; and
by making a deep inspiration, and maintaining afterwards the chest
in an expanded state, which is essential to the continuance of his
muscular effort, he so alters the mechanical relations of the two sides
of his body that the muscles of his right side act with a superior
efficacy; and, to render the inequality still greater, the muscles of
the left side act with a mechanical disadvantage.” Hence the preference
for the right side whenever unusual muscular power is required; and
with the greater exercise of the muscles of the right side their
consequent development follows, until the full predominance of the
right side is the result.
This theory is based not merely on the preponderance of the liver and
lungs on the right side, but on these further facts: that the right
lung is more capacious than the left, having three lobes, while the
left has only two; that the liver, the heaviest organ of the body, is
on the same side; and that the common centre of gravity of the body
shifts, more or less, towards the right, according to the greater or
less inspiration of the lungs, and the consequent inclination of the
liver resulting from the greater expansion of the right side of the
chest. Herein may possibly lie one predisposing cause leading to a
preferential use of the right side. But the evidence adduced fails to
account for what, on such a theory, become normal deviations from the
natural action of the body. The position of the liver and the influence
of a full inspiration combine, according to Dr. Buchanan, to bring
the centre of gravity of the body nearly over the right foot. Hence
in actively overcoming a resistance from above, as when the carter
bears up the shaft of his cart on his shoulder, the muscular action
originates mainly with the lower limb of the same side, which partakes
of the same muscular power and development as the corresponding upper
limb. On all such occasions, where the muscular action is brought
directly into play in overcoming the weight or resistance, Dr. Buchanan
affirms that the right shoulder is much more powerful than the left,
but in the passive bearing of weights it is otherwise. The very fact
that the centre of gravity lies on the right side gives a mechanical
advantage in the use of the left side in sustaining and carrying
burdens; and this assigned pre-eminence of the left side and shoulder,
as the bearer of burdens, is accordingly illustrated by means of an
engraving, representing “a burden borne on the left shoulder as the
summit of the mechanical axis passing along the right lower limb.”
In the year following the publication of Dr. Buchanan’s _Mechanical
Theory_, Dr. John Struthers, Professor of Anatomy in the University
of Aberdeen, communicated to the Edinburgh _Medical Journal_ a paper,
“On the relative weight of the viscera on the two sides of the body,
and on the consequent position of the centre of gravity to the right
side.” In this he shows that the viscera situated on the right side
of the medial line are on an average 22·75 oz. av. heavier than those
on the left side. The right lung, in the male, weighs 24 oz., the
left 21, giving a preponderance of 3 oz. in favour of the right. The
average weight of the heart, in the male, is 11 oz. But the left side
is not only the larger, but the thicker, and as the result of careful
experiments by Dr. Struthers, he assigns to the right side a full third
of the weight of the heart, or 3¹⁄₂ oz. for the right, and 7¹⁄₂ for the
left side. Other viscera are estimated in like manner, with the result
from the whole that the centre of gravity of the body, so far as it
depends on their weight and position, is nearly three-tenths of an inch
distant from the medial plane towards the right side. As a physical
agent constantly in operation in the erect posture, Dr. Struthers
states that this cannot but exert an influence on the attitudes and
movements of the body and limbs; and he accordingly indicates his
belief that this deviation of the centre of gravity furnishes the most
probable solution of the causes “of the preference of the right hand by
all nations of mankind.”
The value of Dr. Struthers’s determination of the exact weight and
relative eccentricity of the viscera on the two sides of the body
was fully recognised by Dr. Buchanan; and in a communication to the
Philosophical Society of Glasgow in 1877 he stated that he had been
led to greatly modify his earlier opinions. He had, as shown above,
ascribed the predominance of the right hand over the left to the
mechanical advantage which the right side has in consequence of the
centre of gravity inclining to it. But he says in his later treatise,
“I judged hastily when I inferred that this is the ground of preference
which prompts the great majority of mankind to use their right limbs
rather than their left. The position of the centre of gravity on the
right side is common to all men of normal conformation, and furnishes
to all of them alike an adequate motive, when they are about to put
forth their full strength in the performance of certain actions, to
use the limbs of the right side in preference to those of the left.
But such actions are of comparatively rare occurrence, and the theory
fails to explain why the right limbs, and more especially the right
hand, are preferred on so many occasions where no great muscular effort
is required; and fails still more signally to explain why some men
give a preference to the limbs of the left side, and others manifest
no predilection for either.” Dr. Buchanan accordingly proceeds to
show that there is not only the element of the position of the centre
of gravity as the pivot on which all the mechanical relations of the
two sides of the body turn; but there is, as he conceives, this other
and no less important element. “The centre of gravity situated on the
right side is variously placed upwards or downwards, according to the
original make or framework of the body.” In the great majority of cases
this lies above the transverse axis of the body, with a consequent
facility for balancing best, and turning most easily and securely on
the left foot, with the impulsive power effected by the muscles of the
right lower limb. Man is thus, as a rule, right-footed; and, according
to Dr. Buchanan, by a necessary consequence becomes right-handed. By
a series of diagrams he accordingly shows the assumed variations: (1)
the centre of gravity above the transverse axis, with its accompanying
right-handedness; (2) the centre of gravity corresponding with the
transverse axis, which he assigns to the ambidextrous; and (3) the
centre of gravity below the transverse axis begetting left-handedness.
The whole phenomena are thus ascribed to the instinctive sense of
equilibrium, which constitutes a nearly infallible guide in all the
movements of the human body. The greater development of the organs of
motion of the right side is therefore, as he conceives, not congenital,
but arises solely from the greater use that is made of them. The
relative position of the centre of gravity depends accordingly on the
original conformation of the body. Broad shoulders, muscular arms, a
large head and a long neck, all tend to elevate the centre point; while
the contrary result follows from width at the haunches and a great
development of the lower limbs.
The intermediate condition, in which the centre of gravity falls
upon the transverse axis, with no instinctive tendency to call into
action the muscles of the one side of the body in preference to those
of the other, constitutes, according to Dr. Buchanan, the most happy
conformation of the body. “It belongs,” he says, “more especially
to the female sex. It is this that so often renders a young girl a
perfect model of grace and agility. It is the same conformation that
enables the ballet-dancer to whirl round on her one foot till the
spectators are giddy with looking at her, when she completes her
triumph by revolving with the same ease and grace on her other foot
also.” He further adds: “If accurate statistics could be obtained, I
believe it would be found that while a very great majority of males
are right-handed, the proportion of females is less; and that, on the
contrary, a larger proportion of females than of males are ambidextrous
or left-handed.”
Consistently with the ideas thus set forth, both Dr. Buchanan and
Dr. Struthers regard right-handedness as an acquired habit, though
under the influence and control of the mechanical forces indicated by
them. “As the question,” says the latter, “in so far as it can bear
on the cause of the preference of the right hand, must turn on the
weight and position of the viscera in the child at the period when the
predominance of the right hand is being gradually developed, in the
second and third years, and afterwards, it is necessary to make the
calculation from the facts as presented in children.” In a letter to
myself he thus writes: “I have again and again verified the fact in
my own children, that in early childhood there is no preference for
one hand more than the other.” But this, as has been already shown,
may be partly due to modes of nursing and other temporary causes
affecting the child in its first infantile stage; and though it may
undoubtedly be affirmed of many, if not indeed of the majority, of
children at that stage: a certain number will be found to manifest a
distinct preference, at a very early age, for one or the other hand.
In the case of a niece of my own, the left-handedness showed itself
very soon; and in my grandson it was independently observed by his
mother and nurse, and brought under my notice, that so soon as he was
able to grasp an object and transfer it from one hand to the other,
he gave the preference to the left hand. A like decided preference
for the right hand, though doubtless also comparatively rare, is more
frequent; and the farther research is carried, the more manifest does
it appear that--whatever be the originating cause,--the preferential
use of what we designate the right hand is instinctive with a
sufficiently large number to determine the prevalent usage; while with
a smaller number an equally strong impulse is felt prompting to the
use of the left hand, in defiance of all restraining influences. It
is indeed always necessary to give full weight to the influences of
education, the whole tendency of which, from early childhood, operates
in one direction. The extent to which this is systematically employed
to develop the use of the one hand at the expense of the other, is
illustrated by the conventional rules for the use of the knife and
fork. It is not sufficient that the knife shall be invariably held in
the right hand. The child is taught to hold his knife in the right
hand and his fork in the left when cutting his food; but when either
the fork or spoon is used alone, it must forthwith be transferred
to the right hand. All voluntary employment of the left hand in any
independent action is discountenanced as awkwardness or gaucherie; and
thus, with a large majority, especially among the more refined and
conventional classes of society, it is rendered a comparatively useless
member, employed at best merely to supplement the other.
Reference has already been made to more or less definite allusions to
an exceptional prevalence of left-handedness in the Punjab, and among
the Sandwich Islanders, the Hottentots, and South African Bushmen; but
they rest apparently on very partial and limited observations. So far
as appears, it may be confidently assumed that left-handedness is
little more prevalent among the rude and uncultured classes of society,
or among savage than civilised races; as would certainly be the case if
right-handedness mainly depended on an acquired habit. The Rev. George
Brown, who has spent upwards of fourteen years as a missionary among
the Polynesians, informs me that left-handedness is as rare among the
natives of the Pacific Islands as with ourselves; while in all their
languages the distinction is clearly indicated. Dr. Rae, whose own
inveterate left-handedness was calculated to draw his attention to its
manifestation among the Indians and Eskimos with whom he was brought
into prolonged contact, only noticed such an amount of ambidextrous
facility as is naturally developed in the paddler, the trapper, and
lumberer, in the exigencies of forest and hunter life. A similar
opinion was expressed to me by Paul Kane, whose wanderings as an
artist among the tribes of British North America gave him exceptional
facilities for observation; and this conclusion accords with the
experience of members of the Canadian Geological Survey.
Turning next to the idea set forth by Dr. Buchanan as to the greater
preponderance of ambidexterity or left-handedness among females, the
results of my own observation by no means tend to confirm this. I have
already noted the case of a lady whose left-handedness is accompanied
by great dexterity. I have repeatedly met with cases of ladies who use
the needle skilfully with the left hand; but the results of inquiries
addressed to musicians and music teachers indicate that in the great
majority of cases the cultivation of the left, as the weaker or less
skilful hand, has to be sedulously enforced in the training of the
female organist and pianist. It is because left-handed pianists are
rare that their exceptional dexterity is noted, as in the case of a
Canadian lady referred to above: “She had great advantages from her
left-handedness. She was a very good performer on the piano, and her
bass was magnificent.”
Again as to the pirouetting of the trained ballet-dancer, I have been
assured that much practice is required to obtain equal facility on
either foot. Dr. Buchanan traces the development of the limbs in their
active use from the first effort of the child to stand erect; next,
the learning to balance itself and turn round on a single foot, and
so through a succession of stages, until at length “the child becomes
right-footed. It is not till long after that the right arm acquires
its predominance.” But the co-ordination of the right or left hand
and the corresponding foot is by no means so invariable as to justify
any such theory. Hopping, pirouetting, and standing on one foot are
comparatively exceptional actions. The two lower limbs are most
frequently employed in necessarily alternate locomotion. The use of the
lower limbs, moreover, is much more independent of direct conscious
volition than that of the hands, and the purposes to which their action
is applied are rarely of a nature to invite special attention to them.
There is, however, an instinctive tendency with many, if not indeed
with the majority, to use one foot in preference to the other, but
not necessarily the corresponding one to the dexterous hand, be it
right or left. In skating, for example, where military training has
not habituated to the use of the left foot in starting, most persons
have an instinctive preference for one foot. So also in football, it
is not with most players a matter of mere chance which foot will be
used in starting the ball. Possibly the same reason may help to account
for the invariable tendency of a blindfold walker to deviate to one
side or the other. It is scarcely possible to walk in a straight line
with the eyes shut. The one leg apparently tends to outwalk the other.
Guided mainly by my own experience, I remarked, when first writing on
this subject, that “the same influences appear to affect the whole left
side, as shown in hopping, skating, football,” etc. But this is partial
and uncertain. Dr. Brown-Sequard affirms that right-sidedness affects
the arms much more than the legs, and in proof of this he states that
“it is exceedingly rare that the leg is affected in the same degree
by paralysis as the arm.” Dr. Joseph Workman, for many years Medical
Superintendent of the Provincial Lunatic Asylum at Toronto, thus writes
to me: “When you say that left-footedness is (only) as frequent as
left-handedness, I am quite sure you are in error. I remember well,
when I was a boy, observing the fact among labouring men engaged in
what was called in Ireland ‘sodding’ potatoes, in ridges about five
feet wide, instead of planting in drills, that in any given number of
men, from four up to a dozen, right and left-footedness prevailed about
equally. Each pair carrying up the work of a ridge required to be right
and left-footed men. I am myself left-footed; and of eight brothers, I
believe about four were left and four right-footed. Sir Charles Bell,
in asserting that ‘no boy, unless he is left-handed, hops on the left
foot,’ asserts far more than the fact. I believe every boy will hop on
his _spade foot_; at least I do so, and I am not left-handed; and I
instinctively do so because I dig with this foot.”
Dr. Buchanan states that “in all adults who use the right hand in
preference to the left--that is, in the great majority of mankind,--the
muscles of the right side, as well as the bones and other organs of
motion, are more highly developed than those on the left side;” and
the predominance of the upper limb follows, as a rule, the previous
development of the lower limb on the same side. The power of overcoming
weight or resistance, and that of passively bearing weights, he assigns
to opposite sides,--both naturally resulting from the centre of gravity
lying on the right side. If such be the case, the great majority of
mankind should instinctively use the same side in bearing a burden. A
favourable opportunity occurred for testing this question. During a
voyage of some days in one of the large steamboats on the Mississippi
River, my attention was attracted by the deck-porters, who at every
landing are employed in transporting the freight to and from the
levee, and in supplying the vessel with cordwood. They constitute, as
a class, the rudest representatives of unskilled labour, including
both whites and negroes. For hours together they are to be seen going
at a run to and from the lower deck of the vessel, carrying sacks of
grain, bales, chests, or bundles of cordwood. Watching them closely,
I observed that some gave the preference to the right and some to the
left shoulder in bearing their burden; and this whether, as with bale
and sack, they had it placed on their shoulders by others, or, as with
cordwood, they took the load up themselves. Noting in separate columns
the use of the right and left shoulder, and in the case of loading
with cordwood the employment of the right and left hand, I found the
difference did not amount to much more than 60 per cent. In one case I
noted 137 carry the burden on the left shoulder to 81 on the right; in
another case 76 to 45; and in the case of loading cordwood, where the
natural action of the right hand is to place the burden on the left
shoulder, so that the use of the right shoulder necessarily implies
that of the left hand, the numbers were 65 using the left shoulder and
36 the right. Here, therefore, a practical test of a very simple yet
reliable kind fails to confirm the idea of any such mechanical cause
inherent in the constitution of the human frame, tending to a uniform
exertion of the right side and the passive employment of the left in
muscular action.
While thus questioning some of the assumptions and deductions set
forth by Dr. Buchanan, it must be acknowledged that his later
theory has this great advantage over other attempts to account for
right-handedness, that it equally meets the cases of deviation from
prevalent usage. No theory is worthy of serious consideration which
deals with left-handedness as an exceptional deviation from habitual
action; as where, in his earlier treatise, Dr. Buchanan expressed the
belief that many instances of left-handedness are “merely cases of
ambidextrousness, when the habit of using the left side, in whatever
way begun, has given to the muscles of that side such a degree of
development as enables them to compete with the muscles of the right
side, in spite of the mechanical disadvantages under which they
labour.” “There is an awkwardness,” he added, “in the muscular efforts
of such men which seems to indicate a struggle against nature.” But
for those indisputable cases of “men who unquestionably use their
left limbs with all the facility and efficiency with which other men
use their right,” he felt compelled either to resort to the gratuitous
assumption of “malformations and pathological lesions in early life,
diseases of the right lung, contraction of the chest from pleurisy,
enlargement of the spleen, distortions of the spine,” etc.; or to
assume a complete reversal of the whole internal organic structure.
More recently, Dr. Humphry of Cambridge has discussed the cause of
the preferential use of the right hand in his monograph on _The
Human Foot and Human Hand_, but with no very definite results. Many
attempts, he says, have been made to answer the question, Why is man
usually right-handed? “but it has never been done quite satisfactorily;
and I do not think that a clear and distinct explanation of the
fact can be given. There is no anatomical reason for it with which
we are acquainted. The only peculiarity that we can discern is a
slight difference in the disposition, within the chest, between the
blood-vessels which supply the right and left arms. This, however, is
quite insufficient to account for the disparity between the two limbs.
Moreover, the same disposition is observed in left-handed persons and
in some of the lower animals; and in none of the latter is there that
difference between the two limbs which is so general among men.” Dr.
Humphry accordingly inclines to the view that the superiority of the
right hand is not natural, but acquired. “All men,” he says, “are not
right-handed; some are left-handed; some are ambidextrous; and in all
persons, I believe, the left hand may be trained to as great expertness
and strength as the right. It is so in those who have been deprived of
their right hand in early life; and most persons can do certain things
with the left hand better than with the right.” So far, therefore,
Dr. Humphry’s decision would appear to be wholly in favour of the
conclusion that the superiority of the right hand is an acquired habit.
But after stating thus much, he adds: “Though I think the superiority
of the right hand is acquired, and is a result of its more frequent
use, the tendency to use it in preference to the left is so universal
that it would seem to be natural. I am driven, therefore, to the rather
nice distinction that, though the superiority is acquired, the tendency
to acquire the superiority is natural.”
This “nice distinction” amounts to something very like an evasion of
the real difficulty, unless we assume Dr. Humphry to mean only what
Dr. Buchanan states, that during the weakness of infancy and childhood
the two hands are used indiscriminately; and the preferential use of
one side rather than the other does not manifest itself until the
muscular system has acquired active development. All the processes by
which dexterity in the manipulation and use of tools is manifested,
are acquired, whether the right or the left hand be the one employed.
Men are not born, like ants, bees, spiders, martins, and beavers,
with carpentering, weaving, modelling, and architectural instincts,
requiring no apprenticeship or culture; though the aptitude in
mastering such arts is greater in some than in others. If the tendency
in their practice to use the right hand is natural, that is to say,
innate or congenital, then there need be no nice distinctions in
affirming it. But on any clearly defined physiological deductions
of right-handedness from the disposition of the organs of motion or
circulation, or any other uniform relation of the internal organs
and the great arteries of the upper limbs, left-handedness becomes
mysterious, if not inexplicable, unless on the assumption of a
corresponding reversal of organic structure; for Dr. Humphry’s
assertion that “in all persons the left hand may be trained to as
great expertness and strength as the right,” is contradicted by the
experience of left-handed persons in their efforts to apply the same
training to the right hand.
To the most superficial observer it is manifest that the anatomical
disposition of the vital organs is not symmetrical. The heart lies
obliquely, from above downwards, and from right to left; the trachea is
on the right side, and the right and left subclavian veins and arteries
are diversely arranged. There are also three lobes of the right lung,
and only two of the left; and the liver is on the right side. Here,
therefore, are sources of difference between the right and left
sides of the body, which, if subject to variation, offer a possible
explanation of the phenomenon that has so long baffled physiologists.
To the variations in the disposition of those organs attention has
accordingly been repeatedly directed; as in the occasional origin of
the left subclavian artery before the right, which, as hereafter noted,
Professor Hyrtl suggested as the cause of the transfer of dexterity to
the left limb. But instances have repeatedly occurred of the entire
transposition of the viscera. “There are men born,” says Dr. Buchanan,
“who may grow up and enjoy perfect health, in whom the position of all
the thoracic and abdominal viscera is reversed. There are three lobes
of the left lung and only two of the right; the liver is on the left
side, and the heart is on the right; and so forth.” Those and other
malformations, as well as pathological lesions, especially if they
occur in early life, may affect the relative power of the two sides;
and Dr. Buchanan at a later date reported a case that came under his
own notice, in which the entire transposition of the viscera coexisted
with left-handedness. But he had already adopted the mechanical theory,
subsequently modified, as explained above; and it is only in a closing
remark in his paper of 1862 that he makes a passing reference to this
remarkable coincidence.
Professor Hyrtl of Vienna, the eminent anatomist already referred
to, in discussing the cause of left-handedness in his _Handbuch der
Topographischen Anatomie_ (1860), affirms a correspondence between the
ratio of left-handed persons and the occurrence of certain deviations
from the normal arrangements of the blood-vessels. “It happens,” he
says, “in the proportion of about two in a hundred cases, that the
left subclavian artery has its origin _before_ the right, and in
these cases left-handedness exists, as it also often actually does in
the case of complete transposition of the internal organs; and it is
found that the proportion of left-handed to right-handed persons is
also about two to one hundred.” Professor Hyrtl thinks that ordinarily
the blood is sent into the right subclavian under a greater pressure
than into the left, on account of the relative position of these
vessels; that in consequence of the greater supply of blood, the
muscles are better nourished and stronger; and that therefore the
right extremity is more used. In cases of anomalous origin of the left
subclavian, etc., the reverse occurs, and therefore the left hand
is employed in preference. The theory of Professor Hyrtl has this
feature to recommend it, that it assigns a cause for the prevalent
habit, which, if confirmed, would equally account for the exceptional
left-handedness; and no proffered solution of the question, founded
on organic structure, is deserving of attention which fails to do so.
But the statistics of such internal organic structure are not, like
those of the transposition of the heart and immediately related organs,
accessible in the living subject, unless in very rare exceptions;
and the occurrence of one or two cases in which the deviation from
the normal arrangement of the artery, or the entire transposition of
the viscera, is found to coexist with left-handedness, may only be
misleading.
A correspondent of _Nature_ (9th June 1870) refers to a case of
transposition of the origin of the right subclavian artery, disclosed
by the occurrence of aneurism, where the person was ascertained to have
been undoubtedly right-handed. In the following year an interesting
article by Dr. Pye-Smith appeared in the _Guy’s Hospital Reports_, and
was subsequently reprinted, with additions, under the title of “The
connection of Left-handedness with transposition of Viscera and other
supposed anatomical causes.” In this the author states that he found
the deviation from the normal arrangement of the primary branches of
the aorta, in which the right subclavian arises from the third part
of the aortic arch, to occur four times in 296 dissections. As this
variation, he says, “cannot be recognised during life, its connection
with left-handedness is not easy to investigate. But in one case,
at least, Dr. Peacock ascertained for me that the subject of this
abnormality, whose heart and arteries he had examined for another
purpose, was right-handed during life.” Any one can tell on which side
his heart lies; but the disposition of the subclavian artery is wholly
beyond his cognisance; and, indeed, Professor Hyrtl, while referring to
this abnormal organisation as one probable cause of left-handedness,
does not affirm more than that the one has been ascertained in some
cases to be an accompaniment of the other. The evidence that in other
cases it has been unaccompanied by left-handedness shows that it is no
necessary source of deviation from normal action.
The other theory, that left-handedness is an inevitable accompaniment
of the transposition of the viscera, is more easily tested. It is one
that has been repeatedly suggested; and has not only received the
sanction of Professor Hyrtl, but is supported by some undoubted cases
in which the two conditions coexisted. But, as Dr. Pye-Smith remarks,
“a few such instances only prove that transposition of the viscera does
not _prevent_ the subject of the abnormality from being left-handed.
Though attention has hitherto been little drawn to this point, there
are enough cases already recorded to show that for a person with
transposed viscera to be left-handed is a mere coincidence.” In
confirmation of this, Dr. Pye-Smith refers to four cases, one of which
came under his own observation in Guy’s Hospital, where the subjects
of the abnormal disposition of the viscera had been right-handed. In
the _Rochester_ (N.Y.) _Express_ of October 1877 a notice appeared
of an autopsy on the body of George Vail, of Whitby, Ontario, who
had recently died in the Rochester Hospital. Dr. Stone, as there
stated, “noticed upon the first examination, when the patient came for
treatment, that there was what is technically called ‘juxtaposition
of the heart,’ which is a very rare condition. He was gratified at
the autopsy to have his diagnosis confirmed, the heart being found on
the right side of the body instead of the left.” I immediately wrote
to Whitby, and in reply was informed that no one had ever noticed
in Vail any indication of his being left-handed. A similar case of
the transposition of the viscera, in which, nevertheless, the person
was right-handed, recorded by M. Géry, is quoted in Cruveillier’s
_Anatomie_ (i. 65). Another is given by M. Gachet, in the _Gazette
des Hospitaux_, 31st August 1861; and a third in the _Pathological
Transactions_, Vol. XIX. p. 147 (_Nature_, 28th April 1870). This
evidence suffices to prove that there is no true relation between
the transposition of the viscera and left-handedness. Dr. Struthers
has shown that “as far as the viscera alone are concerned, the right
side is at least 22³⁄₄ ounces heavier than the left, and that this is
reduced 7³⁄₄ ounces by the influence of the contents of the stomach,
leaving a clear preponderance of at least 15 ounces in favour of the
right side.” The preponderance of the right side, he adds, is probably
considerably greater than 15 ounces, and it is rendered still more so
in the erect posture. The total weight of viscera on the right side he
states at 50³⁄₄ ounces, while that of the left side is only 28 ounces,
giving a visceral preponderance on the right side of 22³⁄₄ ounces.
But if this relative excess of weight on the right side be the true
source of right-handedness, the transposition of the viscera ought to
be invariably accompanied with a corresponding change. A single example
of the preponderant cause, unaccompanied by the assumed effect, is
sufficient to discredit the theory.