DEXTRAL PREEMINENCE 857 



Reasoning from the facts just stated, Ogle has assumed that dextral 

 preeminence depends on a natural predominance of the left side of the 

 brain, the reverse obtaining in the left-handed. Ordinarily it is true that 

 the members of the right side are stronger than the left, particularly the 

 arm ; but this is not always the case, even in the right-handed, although 

 the right hand is more conveniently and easily used than the left. In 

 many feats of strength, the left arm appears less powerful than the right, 

 because there is less command over the muscles. As regards the cause 

 of the superior development of the left side of the brain, it must be 

 admitted that the anatomical explanation is not entirely satisfactory. 

 It is a fact, however, that the two sides of the brain usually are not 

 exactly equal in their development, the left side being superior to the 

 right, and that the muscles of the right side of the body are used habitu- 

 ally in preference to those of the left side. 



While it is not yet possible to explain why the left side of the brain 

 has peculiar psychic functions not possessed by the right side, it is never- 

 theless true that intellectual processes take their origin mainly and in 

 some instances entirely in the left half of the cerebrum. In man, sight, 

 hearing and speech are closely connected with mental operations, at 

 least in so far as they give rise to or express ideas. The two eyes are 

 necessary to perfect vision ; but the psychic visual centre, which receives 

 ideas or meaning conveyed by objects seen, is on the left side of the 

 cerebrum, except in the left-handed. The same may be said of the 

 sense of hearing, the psychic auditory centre being on the left 

 side, except in the left-handed. The location of the speech-centre on 

 the left side was made in 1836 (Marc Dax); and a case of aphasia with 

 right -hemiplegia was fully reported by Pourfour du Petit, in 1766. 

 Agraphia, or inability to express ideas in written language, is due to 

 lesion of the left side of the brain. All these conditions are reversed, 

 however, in the left-handed. When one eye is used as a means of form- 

 ing a judgment or opinion, it usually is the right eye for the right-handed 

 and the left eye for the left-handed. Curiously enough, it has lately 

 been observed that deaf-mutes may have an aphasia that prevents the 

 use of the right hand in the sign-language. It seems, indeed, that 

 movements, more or less automatic, may be executed by the muscles of 

 either side, remembering, always, that muscles of the left as well as 

 of the right side may be educated ; but in movements that involve men- 

 tal operations and attention at the time they are made, the right side 

 usually predominates. Apart from the question of education of muscles, 

 it appears that the more automatic acts are performed indifferently by 

 either the right or the left side ; but movements more closely connected 

 with direct mental operations are made preferably by right muscles in 



