BILATERAL ASYMMETRY OF FUNCTION. 103 squarely into the other, the optical images would not fuse, while in ordinary stereoscopy we have only to exchange the impressions of the two eyes to invert the perspective. Again, how is it that, despite the independence of the two halves of the body which, e.g., inclines many to make a face on the side of the muscles most strained in great effort or of the pain most keenly felt, yet the homologous points on the two hands are related, not like the identical points of the eye, or like the upper and lower note of the octave which blend into each other, but by a peculiar psychic affinity, so much closer in a sense than that between two adjacent, but just clearly distinguishable, local signs on the same side? n. After bringing together from original sources, and carefully considering the above facts and opinions, not all of which can be regarded as final, we are led to think that other methods than those hitherto applied might bring us into closer quarters with the comprehensive bilateral problem. Of these two were con- sidered. 1. Operations on the corpus callosum of animals, and perhaps the pyramid, to partially isolate the action of the hemi- spheres, and also to reduce the action of one of them. 2. More accurate measurements of the bilateral asymmetry of function. This section is devoted to the general results of the latter method, so far as applied to the arms. In the first series of experiments, a ruler about six feet in length was fastened on edge transversely on a table. The person whose movements are to be observed sits with carefully measured squareness before the middle of the table, and places his index fingers on each side of a pin that marks the middle of the edge of the ruler. He intends and then executes a sudden movement along the edge of the ruler to the right with the right, and to the left with the left hand simultaneously, endeavouring to make the excursions of both hands alike. His eyes must of course be closed, for they detect and correct for far slighter asymmetries of movement than the hands. The other side of the ruler is marked off each way from the central pin by millimetre scales, and a second person sitting at the other side of the table reads off and records in the protocol-book the distance of each excursion, an ink-line being drawn down the middle of the index finger-nails to aid the accuracy of the readings. Fifty or more such records can easily be made in half- an-hour, and many thousands altogether have been made and arranged. Observations were made, for control, upon many people, but were chiefly confined to two right-handed and two left-handed young men. From tables far too extensive to print here, we have formulated the following conclusions for such movements as the above. A. I. The preferred hand makes the greater excursion. This
