Page:American Journal of Psychology Volume 21.djvu/554

542 as movement becomes easier. The decrease of associations and the growing absence of irrelevant imagery as the subject goes on from the copying of simple English words up to the upsidedown and reflected writing (in which they do not occur at all) shows that as attention becomes more necessary and more concentrated the range of consciousness narrows; and the converse is also true, as attention becomes less concentrated, the range of consciousness becomes wider, and more or less extraneous imagery appears. It is therefore clear that in case of easy movements attention responds more readily to other stimuli and as a consequence becomes a mixed process, including, so far as the movement is concerned, both relevant and irrelevant elements. It would therefore be easy to fail to see the woods because of the trees and report no imagery when in reality it may have been present. As a matter of fact there are here two things to be kept distinctly in mind; the first is that in highly voluntary movement, but not too difficult to permit Zerstreutheit, attention is intense and consciousness narrow, and therefore the imagery or sense data are strictly limited at any given pulse of attention. The second fact is that in well learned and facile reactions the attention to the activity is minimal and the range of consciousness correspondingly wide. Strictly speaking it is therefore not true, at least from the point of view of the objective movement which it produces, that consciousness is always a unitary process, for clearly the most important elements in the configuration of consciousness may not always participate in the production of a specific movement.

It is the purpose of this section to review, as succinctly as the complexity of the subject will permit, the leading anatomical facts and physiological experiments, together with a number of pathological cases of anaesthesia, that have a bearing upon voluntary movement.

1. The Anatomical Facts as reflected in Recent Neurological Literature.

Recent neurological studies on the relative number and distribution of sensory and motor fibres in the peripheral nerves throw a certain light upon the relation of sensory impulses to voluntary movement. Donaldson has pointed out that, "Taking the central system segment by segment, the sensory nerves are more numerous and have a greater area than the motor. In man, as we pass cephalad, the superiority of the sensory nerves becomes most marked,–"(Growth of the Brain, p. 196.) Ingbert counted the nerve fibres of the left dorsal roots of a man weighing 180 Ibs. and found the total number for the left side to be 653,627 which would give a total, for both sides, of