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In the central nervous system, we recognize commissures and decussations, the first connecting like parts of the right and left sides, and the second connecting unlike parts. More accurate knowledge of the connections of these crossing fibers has made it clear that most of the so-called commissures are either decussations or mixtures of commissural and decussating fibers. In fact, the truly commissural fibers generally branch and so connect both like and unlike structures, and the decussating fibers frequently divide, sending a branch to each side of the midplane. The functions of strictly commissural fibers can readily be understood as mediating coordinated action of the bilaterally symmetrical organs of the body. But the reasons for the extensive decussations so characteristic of most types of nervous systems have never been satisfactorily presented, though many ambitious attempts have been made.
The first third of this book is devoted to a detailed review and