According to the author, the chief aim of this book is to “treat the human organism as a dynamic whole which functions as a unit under graded centers of physiological dominance,” and then to show the importance in speech of a center of chief dominance. He begins with the neuromuscular basis of speech in which he develops and amplifies Orton's theory of cerebral dominance. It is emphasized that through evolution the cerebral cortex has come to be the supreme factor in human behavior, working through the subcortical correlation centers and lower reflex levels to preserve a functional unity.

The peripheral physiological parts (coordinated by “midline levers”)