In the second class I would place volition and attention. In these the mind has clear ly the initiative, and is capable of inducing-, certain states of body, either to move certain organs (voluntary motion), or to concentrate one or more of the inlets of sensation upon some external objects (attention). 'Ile power 9f abstraction, imagination, and all purely in tellectual processes, are obviously associated with these.
The sytnmetrical disposition of the parts of the encephalon on each side of the median plane has been recognised by all anatomists. This symmetry is so complete that we may, with perfect correctness, speak of two brains, a right and a left bmin, which are united to each other by transverse coinmissures. The right brain corresponds exactly with the left, just as much as the right eye corresponds with the left. This doubleness of the brain, no doubt, accords curiously with the double ness of all the organs of sense, and very pro bably is rendered necessary by the existence of the double set of inlets to sensation. It is remarkable, however, that a perfect symmetry of the convolutions is not found in the higher races of mankind, and in individuals of high intellectual powers ; and that the greater the mental power, the less symmetrical are the convolutions. In the inferior races, on the other hand, as Tiedemann has well shown, the symmetry of the convolutions is exact.
Upon the proved existence of two bmins, as thus explained, Dr. Wigan, adopting the mate rialist view of mental phenomena, rests the theory that the mind is dual ; that we have two minds; that each brain performs its own mental functions, which are in perfect harrnony, if the two brains harmonise in quality, structure, and action.
It cannot be doubted that two brains, dies symmetrical in structure, must have a tanta mount symmetry of function, if I may be allowed the expression ; and that, therefore, in order to insure harmony of action between them, arxl to prevent the actions of one from interfering with or neutralising those of the other, some such organic connection between them is necessary as that which exists between the two retinm, and which converts the sepa rate and in some degree dissimilar physical impressions rrraele on each of them into one sensation.
And as any interference with the organic conditions necessary to secure single vision with two eyes produces double vision, so it is not unreasonable to expect that an analogous imperfection in the organic union between the two brains may occasion doubleness of mental impression and action. Such a conclusion, aa Dr. Wigan has ingeniously suggested, gives the clue to the explanation of such phenomena as states of double consciousness, delusions, irregular volitions, and some forms of insanity; and, if fairly worked out by physiological psy chologists, may solve other obscurities con nected with the phenomena of the mind.
While, therefore, I admit that great practical interest and value attach to Dr. Wigan's views respecting the action of two bmins, I am not prepared to infer the existence of two minds from that of two brains ; no more than I can assume a duality of our visual sense from the existence of two eyes. The two, cases, indeed, are strictly analogous. '1 h organic change in each retina developes • corresponding sensorial impression ; and from the connections which subsist between the reti nm, and still more from that between the cen tres of sensation, these impressions become fused into one. In like manner the organic changes in the two brains developing nervous force in similar modes and proportions, each being capable of affecting the mind similarly, although perhaps not identically, are yet so united in their action that the double organic affection acts on the mind as one. liut if, through default of the connecting media of the two brains, or through lesion of one, the organic changes in each do not harmonise with those in its fellow, then it is plain that two separate and distinct mental affections will result, and more or less of confusion must ensue. I can see no ground for inferring the existence of two minds from such a supposition. The confu sion results from the want of simultaneous affection of the same mind by two distinct and separate brains. If, in vision, each centre of sensation affected only its own mind, or, in other words, developed only its own mental phenomena, as Dr. Wigan's theory would compel us to assume, then each mind would perceive a different perspective projection of the object presented to the eyes, and an elaborate and complex mental process would be required ta combine the two sensorial im pressions, Ilow much .simpler is the view of this process which assigns the combination of the double impression to a physical union in the brain of each physical change in the retina; so that, in truth, but one impression, different from each of' its excitant ones, reaches the mind. So also, in the normal intellectual ac tion, the organic changes of the two brains are united by the various transverse commissures, so that but one physical stimulus affects the mind and excites but one train- of thought. Not so, however, when from any defect in the brains themselves, or in the commissures, the physical conditions necessary for the organic states of the two brains cannot be fulfilled.