Right-handedness is no doubt due to the lack of perfect symmetry in the human body. If the latter could be folded over from a medial line so that each organ of the one side fell exactly upon a corresponding organ of the other, we should have a structure highly favourable, mechanically, to the equal use of each limb, and ambidextral individuals would be the rule, not the exception. If a vertical line be drawn dividing the body it will be found that the centre of gravity is a little to the right of this medial line. This makes the right side heavier. From a series of experiments the greater weight has been estimated at about 15 ounces. Upon this fact is founded the mechanical theory of right-handedness, or the predominance of the right hand over the left; or, more generally, of the limbs of the right side over those of the left, as expounded by Professor Buchanan of Glasgow in a pamphlet published in 1862. The three-lobed right lung is more capacious and receives more air during an inspiration than the two-lobed left. The liver during inspiration swings toward the right side, shifting the centre of gravity farther to that side. In violent muscular exertion there is more air proportionally inhaled by the lung of the side which sustains the exertion. Normally about 230 cubic inches of air are contained by the lungs, of which the right holds 20 inches more than the left. Under exertion of the right side the larger lung is better filled than the smaller, and the centre of gravity is removed until it is found in a line passing through the right foot; so that the right leg and foot afford a steadier basis of support than the left would do under similar circumstances. Whichever leg we stand on we use the arm of that side to greater advantage, and thus, through the greater use of the right lower limb, the right upper limb comes to be preferred.
Professor Buchanan's theory also explains the almost universal habit of carrying burdens on the left shoulder. In the case of a light weight, slung on the arm, the equilibrium of the body is better maintained by carrying it on the left side. If the weight be a heavy one, borne on the left shoulder, the burden is really being supported very much by the right limb, owing to the natural curve of the body towards the right side, while sustaining the pressure.
But, it may be argued, if this theory covers the case, then left-handedness, which is certainly inherited, cannot be accounted for except on the extraordinary supposition of transference of the viscera. In a very few cases left-handedness has been found to accompany such transposition. Strange to say the liver has been found on the left side, and the heart, stomach, and spleen on the right without any derangement to the health of the subject, even from the point of view of a life-insurance company. Nevertheless, the number of cases of genuine left-handedness far exceeds such instances of transposition. An explanation of left-handedness in normal structures has been sought by falling back on the fact that the cerebral hemispheres of the brain work the muscles cross-wise. Ferrier's researches have proved that when we see with the right eye we see with the left side of the brain. Another curious and instructive fact is that, although an animal be rendered blind of an eye by the destruction of a convolution on one side of the brain, the blindness is temporary. Soon the other hemisphere can take up the function, and then vision is possible with both eyes as before.
Viewed in this light, hereditary left-handedness may be due to the greater development of the right side of the brain. Suppose accident, or the cruelty of the conqueror, had deprived a comparatively young archer of his right hand or right eye (and the latter cruel custom is referred to in the Bible), then the left hand, governed by the right hemisphere, being called into work would react on that hemisphere, whose blood-vessels would be oftener replenished and whose strength and sensitiveness would grow. It is not too much to assume that in some cases this improved power of the right hemisphere might be transmitted to a descendant. 'It is practically certain,' says Dr Bastian, 'that the great preponderance of right-handed movements in ordinary individuals must tend to produce a more complex organisation of the left than the right hemisphere.' M. Broca states that in forty brains he examined he found the left frontal lobe heavier than the right. These investigations have not yet been thoroughly carried out; but possibly the explanation of obstinate left-handedness lies in that direction.
In connection with the evolution of the species right-handedness, in all probability, has been a late acquisition. The body is more symmetrical in early youth, and is more symmetrical in the female than in the male. A very young child betrays no disposition to use the right hand more than the left. The habit of using the right hand gradually increases with boyhood, and boys have to the last a wider range with the right hand than girls, who are proverbially bad stone-throwers. Hitherto, however, the oldest records of the human race, even when man sketched with a flint point on the bones of extinct animals, prove him to have been a right-handed being. His profiles are then sketched with faces toward the left, just as a street arab chalks them on a door at the present day. Nevertheless, the primeval left-handed artist often betrays himself. Bronze weapons are the weapons of right-handed individuals; witness the curious yew-tree handle of a bronze sickle fished up from the lake of Brienne, Switzerland, quite intact and ingeniously carved, 'as incapable,' says Sir Daniel Wilson, 'of being used by a left-handed shearer as our present mower's scythe.'
A few observers of the habits of savages have remarked that left-handed individuals were proportionally more numerous among them. This is what we should expect from the enormous additional demands made by civilisation, its manners, and its tools upon the activity of the right hand. It only requires to spend an hour at a carpenter's bench to see how planes, screws, &c. are fashioned to suit that member. In drawing a pattern of small repeating character we begin, as we begin to write a page, at the top left-hand side, so as to avoid placing fingers on the still undried pigments. Military drill, associated labour, and, as much as any, the tyrant fashion, all urge on the right hand in the path of greater dexterity, leaving the left as the inept drudge whose duty it is merely to assist. No wonder the right has acquired strength, size, greater tactile sensibility, and greater patience of the extremes of heat and cold. Naturalists, who observe that adult monkeys catch nuts more with the right hand, that the African elephant digs more with the right tusk, or that the Carolina parrot has a preferential claw for grasping, tell us that these habits are subject to more numerous exceptions than the exception of left-handedness with human beings. Egyptian and Assyrian painting and sculpture and Etruscan bronzes also elucidate the general law that burdens have been assigned to the left shoulder; so that the position of the shepherd's plaid is nowise a whim, but has its roots in the far past. The primitive blanket-like toga of the Etruscans covers the left shoulder, and is wrapped under the right arm-pit to allow of the freer motion of that arm.
Use and disuse, by strengthening or weakening organs, would warrant us in believing that where there is inequality of vision between one's eyes the balance would be thrown in favour of the right. As Sir John Herschell has remarked, cases can be adduced of persons who were unaware of one of their eyes being weak almost to blindness until far advanced in life. The writer has a near-sighted left eye, inherited from a grandfather short-sighted in both, the defect of which he did not discover until far advanced in manhood. It is well to know that such disparity in power of vision can be greatly mitigated by the lenses of the oculist. A good deal of folklore is to be found connected with the right and left hands. The rule of the road and the interpretation of omens tell, by the contrary rule obtaining in nations of different race, no small part of their story as to whether they belong to the conquerors or the conquered. See Sir Daniel Wilson's The Right Hand: Left-handedness (1891); Professor Dwight in Scribner's Magazine (April 1891); also the article BRAIN, Vol. II. pp. 391, 392.