There remains to be considered the source suggested by Professor Gratiolet, when he turned from the organs in immediate contact with the arm and hand to the cerebral centre of nerve force. The statements advanced by him that the anterior convolutions of the left side of the brain are earlier developed than those of the right, when taken in connection with the well-known decussation of the nerve-roots, would account for the earlier development of the muscles and nerves of the right arm; but his opinion has been controverted by competent observers. This, however, does not dispose of the question. The localisation of the mental operations of speaking, naming, and writing in certain specific cerebral centres, and the recognised functional relations of those word-centres with other active cerebration, have given a new significance to the vital action of the brain as the seat of nerve-force. It was only in 1861 that M. Broca definitely assigned the posterior part of the third frontal convolution of the left hemisphere as the seat of articulate speech. More recently this has been followed up by observations suggestive of some possible correlation between the reflex action of the cerebral hemispheres on the limbs; but it has thus far been no more than a passing allusion, tending to beget observation of possible coincidences, such as may be found between left-handedness and either an accompanying transposition of the seat of articulate speech to the right hemisphere, or some prevailing characteristic of the degree of word-memory in the left-handed. A recent observer, Dr. J. Batty Tuke, definitely affirms that “the large proportion of cases of ataxic aphasia occur in association with right-sided hemiplegia, although others are on record in which it has appeared in connection with left-sided hemiplegia in left-handed persons” (Encyc. Britann., art. “Aphasia”). In those an intimate relation is thus established between right or left-handedness and the development of the opposite cerebral hemisphere.
The special limitation of the researches of Dr. Guiseppe of Pisa to “the writing of left-handed persons” naturally directed his attention to this element of cerebration. “Clinical observation and pathological anatomy,” he remarks, “have clearly shown that in the foot of the second frontal convolution of the left cerebral hemisphere there is located a centre for the co-ordination and the memory of the movements of writing. The destruction of this centre produces agraphia, that is to say, inability to co-ordinate and remember the movements for writing. This graphic centre is situated on the left hemisphere in right-handed persons.” At the same time he is careful to assert that both in this writing-centre, and in that of the foot of the third frontal convolution, to which is assigned the co-ordination and memory of the movements for articulate speech: in the case of lesions impairing their powers, it has been found possible to stimulate the corresponding centres of the opposite hemisphere so as in more or less degree to perform their functions; just as the dormant left hand may be educated to take the place of the paralysed or amputated right hand.
Dr. Guiseppe thus proceeds: “In left-handed persons the centres of the neuro-psychic factors of language are situate in the right hemisphere, as has been shown by well-studied cases. These persons, however, learn to write with the right hand and not with the left. And yet in their right hemisphere there is a potentiality which is very favourable for their education in the co-ordination and the memory of the movements for writing. Left-handed persons perceive that they could learn to write with greater facility with the left hand than with the right; but education succeeds in awaking and conveniently bringing into action in the left-hand sphere a latent cortical centre, which did not present so favourable a potentiality as that of the right hemisphere. The possibility of investigating what influence in left-handed persons practice with the right hand might develop in the left hemisphere for the explication of language, would certainly constitute a theme for important study” (Archivio Italiano, September 1890, pp. 306, 307).
With the attention that is now definitely given to this assignment of the preferential use of one or the other hand to greater or less development of the opposite cerebral hemisphere, renewed observation has been directed to the cerebral source of predominant right-handedness. The discussions in the columns of Science, suggested by Professor Baldwin’s study of its first manifestations, have revived the reference of it to the assumed excess in development of the left cerebral hemisphere. Dr. T. O’Connor of New York thus reasserts it as an unquestionable though little-known fact: “But it may not be generally known that the left cerebral hemisphere is larger than the right, its inner face (at the great longitudinal fissure) coming very near to the middle line, while the corresponding inner edge of the right hemisphere is well to the right of the median line. The existence, then, of greater nutrition and greater functionating ability in the left hemisphere might well be assumed. But that there is a reason for the greater size, development, etc., of the left hemisphere is evidenced by a study of the conditions of blood-supply to the two hemispheres. The left carotid artery ascends almost perpendicularly, so as to form, as it were, an elongation of the ascending aorta, while the right carotid is given off from the arteria innominata. The right vertebral artery is given off by the subclavian after the latter has described its arch and become horizontal, but the left vertebral arises from the apex of the subclavian’s curve. There is thus the distinct advantage to the left hemisphere of a better blood-supply because of the much straighter course taken by the great channels carrying it” (Science, 12th December 1890).
This idea of a greater development of the left cerebral hemisphere has been the subject of considerable diversity of opinion. A marked difference in the weight of the two hemispheres has indeed been repeatedly asserted by well-qualified observers as the result of careful investigation. M. Broca stated that in forty brains he found the left frontal lobe heavier than the right; and Dr. Boyd, when describing the results obtained by him from observations on upwards of 500 brains of patients in the St. Marylebone Hospital, says: “It is a singular fact, confirmed by the examination of nearly 200 cases at St. Marylebone, in which the hemispheres were weighed separately, that almost invariably the weight of the left exceeded that of the right by at least the eighth of an ounce.” Dr. Brown-Sequard also, as hereafter noted, makes this apparent excess in weight of the left hemisphere of the brain the basis of very comprehensive deductions. Again Dr. Bastian affirms, as a result of careful observation, that the specific gravity of the gray matter from the fronted, parietal, and occipital convolutions respectively is often slightly higher on the left than it is on the right hemisphere. The opinion is thus sustained by some of the most eminent physiologists who have given special attention to the brain and its functions. But, on the other hand, Professor Wagner and Dr. Thurnam both state that their careful independent investigations failed to confirm the results arrived at by M. Broca and Dr. Boyd. From the weighing of the two hemispheres of eighteen distinct brains, Professor Wagner found the right hemisphere the heavier in ten, and the left in six cases, while in the remaining two they were of equal weight. Dr. Thurnam, without entering into details, states that the results of his weighings did not confirm Dr. Boyd’s observations; adding that “fresh careful observations are certainly needed before we can admit the general preponderance of the left hemisphere over the right.” Though the two hemispheres of the brain are sufficiently distinct, they are united at the base; and even with the most careful experimenters, the section through the cerebral peduncles and the corpus callosum is so delicate an operation that a very slight bias of the operator’s hand may affect the results. That a difference, however, is occasionally demonstrable in the weight of the two hemispheres is unquestionable, and the whole tendency of the most recent investigation is to sustain the hypothesis which refers the cause of left-handedness to an exceptional excess of nervous force in the right cerebral hemisphere. But the results to be anticipated from the partial character of the bias in the majority of the right-handed would tend to suggest a doubt as to the full acceptance of the statement of Dr. O’Connor adduced above. It is rather in accordance with what has already been affirmed as to the very partial prevalence of any strongly defined bias in the majority for the preferential use of either hand, that many brains should come under the notice of careful observers where little or no difference can be found between the two hemispheres. But weight is not the only element of variation. Dr. Bastian, in The Brain as an Organ of Mind, draws attention to the unsymmetrical development of the two hemispheres as one of the most notable peculiarities of the human cerebrum. This is not only the case with reference to the number and arrangement of the convolutions, but it has been noted by various anatomists that the left hemisphere is very frequently slightly longer than its fellow.
But interesting light has been thrown by pathological observations on the comparative relations of the cerebral hemispheres; and in this more than in any other direction we may look for further elucidation. As already noted, Sir Charles Bell affirms a general inferiority in muscular strength and in vital properties of the left side of the body, and a greater liability to disease in the left extremities than the right. On the other hand, Dr. O’Connor refers a greater susceptibility to disease of the left hemisphere of the brain to what he assumes as the source of superior vital force. Having assigned the causes of greater development as already cited, he proceeds to affirm that “the greater directness of communication between the heart and left hemisphere explains the greater readiness with which the latter is subjected to certain forms of disease. A clot of fibrine whipped off a diseased valve is carried much more readily because of the direct route (viâ the carotid) to the left hemisphere; and in conditions of degenerative weakness of the arteries in general, those of the left hemisphere being subjected to greater pressure in their distal ramifications, will be more apt to yield than corresponding ones in the right.”
Dr. Bastian, when commenting in his Brain as an Organ of Mind on the specific location of the cerebral function of articulate speech in the third left convolution of the brain, remarks: “It has been thought that a certain more forward condition of development of the left hemisphere—as a result of hereditary right-handedness recurring through generation after generation,—might gradually become sufficient to cause the left hemisphere to take the lead in the production of speech-movements. Some little evidence exists, though at present it is very small, to show that it is left-handed people more especially who may become aphasic by a lesion of the right third frontal gyrus.” Dr. Bastian further assumes it to be indisputable that the greater preponderance of right-hand movements in ordinary individuals must tend to produce a more complex organisation of the left than of the right hemisphere; and this both in its sensory and motor regions. With the left-handed, however, so many motives are constantly at work tending to call the right hand into play, that the compensating influences must in their case tend to check any inequality in the development of the two hemispheres; so that there would seem rather to be a probability in favour of a more equable, and consequently healthful development of both cerebral hemispheres in the left-handed, but really ambidextrous manipulator. But it is to be noted that while Dr. Bastian recognises a correlation between the development of one or other cerebral hemisphere and the greater dexterity of the opposite hand, he is inclined to recognise right or left-handedness as the cause rather than the effect.
Dr. Brown-Sequard, who strongly favours the idea of superiority, both in size and weight, of the left over the right cerebral hemisphere, also ascribes the source of this to the greater frequency and energy of all right-hand movements. He reverts to an argument derived from left-handedness when discussing his theory that the two hemispheres practically constitute two distinct brains, each sufficient in itself for the full performance of nearly all mental operations; though each has also its own special functions, among which is the control over the movements and the organs of opposite sides of the body. “Every organ,” he says, “which is put in use for a certain function gets developed, and more apt or ready to perform that function. Indeed, the brain shows this in point of mere size; for the left side of the brain, which is used most, is larger than the right side. The left side of the brain also receives a great deal more blood than the right side, because its action preponderates; and every organ that acts much receives more blood.” He accordingly affirms that the growth of the brain up to forty years of age, if not indeed to a considerably later period of life, is sufficiently marked to require the continued enlargement of the hat. The evidence he adduces, based on observing that a hat laid aside for a time and then resumed, always proved to be too small, is probably deceptive. But the growth of the adult brain is no longer disputed. It was indeed affirmed by earlier physiologists, as by Sœmmering, the Wenzels, and Tiedemann, that the brain attained its greatest development not later than seven or eight years of age. But this idea is now entirely abandoned; and—without going so far as to affirm with Dr. Brown-Sequard, who claims that at the age of fifty-seven he found by the test of hat measurement that his head had enlarged within every six months,—the latest observers adduce proof of continuous increase of brain weight, if not of bulk, until the greatest average weight is reached between thirty and forty years of age.
On the assumption of Dr. Brown-Sequard that the left hemisphere of the brain exceeds that of the right both in size and weight: in the deviations from this normal condition there ought to be found the corresponding development of the organ brought into use. But, like most other right-handed reviewers of the phenomena of left-handedness, he fails to appreciate the bearings of his own argument in the case of a left-handed person conforming in many ways to the usage of the majority, yet instinctively giving the preference to the left hand. He dwells on the fact that very few left-handed persons have learned to write with the left hand, and that those who can do not write nearly so well with it as with the right hand. Even in persons who are left-handed naturally, so that the right side of the brain may be assumed to control the reasoning faculties and their expression, he argues that the left side of the brain “can be so educated that the right hand, which that side of the brain controls, produces a better handwriting than that by the left hand, though that is controlled by the better developed brain.” But the reasoning is alike partial and misleading. The left-handed person systematically submits to disabilities in his efforts to comply with the usage of the majority, not only in holding his pen in the right hand, but in the direction and slope of the writing. A left-handed race would naturally write from right to left, sloping the letters towards the left, and so would place the right-handed penman at a like disadvantage, wholly independent of any supposed change in the functions or preponderating energy of either hemisphere of the brain. But even in the absence of practice, the command of the left hand in the case of a truly left-handed person is so great that very slight effort is required to enable him to write with ease with that hand.
Reverting once more to the singularly interesting phenomena whereby in certain conditions of cerebral disease, or local injury, the correlation between articulate speech, writing, and even the unconscious expressiveness of gestures, used certain specific convolutions of the left hemisphere, Dr. Pye-Smith says: “The opinion that some difference between the two sides of the brain has to do with our preference for the right hand over the left may, perhaps, be supported by two very interesting cases of aphasia occurring in left-handed persons, recorded by Dr. Hughlings Jackson and Dr. John Ogle. In both these patients there was paralysis of the left side; so that it seems likely that in these two left-handed people the right half of the brain had the functions, if not the structure, which ordinarily belong to the left. To these cases may be added a very remarkable one published by Dr. Wadham (St. George’s Hospital Report, 1869). An ambidextrous or partially left-handed lad was attacked with left hemiplegia and loss of speech; he had partly recovered at the time of his death, twelve months later, and then the right insula and adjacent parts were found softened.”