What has determined the predominance of the left hemisphere in speech?  I can find no adequate anatomical explanation.  There is no difference in weight of the two hemispheres in normal brains.  Moreover, I am unable to subscribe to the opinion that there is any evidence to show that the left hemisphere receives a larger supply of blood than the right.  Another theory advanced to explain localisation of speech and right-handedness in the left hemisphere is that the heavier organs, lung and liver, being on the right side have determined a mechanical advantage which has led to right-handedness in the great majority of people.  This theory has, however, been disposed of by the fact that cases in which there has been a complete transposition of the viscera have not been left-handed in a larger proportion of cases.  The great majority of people, modern and ancient, civilised and uncivilised, use the right hand by preference.  Even graphic representations on the sun-baked clay records of Assyria, and the drawings on rocks, tusks, and horns of animals of the flint-weapon men of prehistoric times show that man was then right-handed.  There is a difference of opinion whether anthropoid apes use the right hand in preference to the left.  Professor Cunningham, who made a special study of this subject, asserts that they use either hand indifferently; so also does the infant at first, and the idiot in a considerable number of cases.  Then why should man, even primitive, have chosen the right hand as the instrument of the mind?  Seeing that there is no apparent anatomical reason, we may ask ourselves the question:  Is it the result of an acquired useful habit to which anatomical conditions may subsequently have contributed as a co-efficient?  Primitive man depended largely upon gesture language, and the placing of the hand over the heart is universally understood to signify love and fidelity.  Uneducated deaf mutes, whose only means of communicating with their fellow-men is by gestures, not only use this sign, but imply hatred also by holding the hand over the heart accompanied by the sign of negation.  Moreover, pointing to the heart accompanied by a cry of pain or joy would indicate respectively death of an enemy or friend.  Again, primitive man protected himself from the weapons of his enemies by holding the shield in his left hand, thus covering the heart and leaving the right hand free to wield his spear.  The question whether it would have been to his advantage to use either hand indifferently for spear and shield has been, to my mind, solved by the fact that in the long procession of ages evolution has determined right-handed specialisation as being more advantageous to the progress of mankind than ambidexterity.  Right-handedness is an inherited character in the same sense as the potential power of speech.

LOCALISATION OF SPEECH CENTRES IN THE BRAIN