II. The functional relations of the motor roots of the brachial and lumbo-sacral plexuses

The functions subserved by the plexiform arrangement of the nerves of the limbs, and the mode of distribution of the several roots of the brachial and lumbo-sacral plexuses, have been the subject of frequent speculation and of occasional experimental research; and the question is one of considerable physiological and pathological interest. A mere naked-eye examination of the mode in which the roots unite to form the larger trunks allows of an approximate deter­mination of the possible roots of each trunk; and by more minute dissection and maceration in dissociating liquids, as has been done by W. Krause in the case of the brachial plexus (“Beiträge zur Neurologie der Oberen Extremität,” 1865), the constituent fibres of the nerve-trunks may be determined with greater precision. But it is obvious that anatomical dissection, however minute, is unable to discriminate between the sensory and motor constituents of the nerve-trunks, or to indicate their functional relations and dis­tribution. The only possible methods by which this can be arrived at are by determ ining the effects of excitation or destruction of the individual roots of the plexus.