During the four years since the appearance of the second edition of MacCallum's textbook, particular attention has been paid to rickets, diabetes, epidemic encephalitis and a few other conditions. In preparing a revision, the author has not found much that demanded rewriting, but has carefully attempted to add important contributions to the literature and to express any changes in opinion resulting therefrom. His textbook is the outgrowth of an extensive teaching experience, and has found a welcome reception in many institutions of teaching. The illustrations, many of them in colors, are most valuable. Unfortunately, no standard rule has been adopted for the references to periodical literature, so that many of them appear without titles and many others are even more incomplete. A section on light and other radiant energy presents a brief outline of our present knowledge, based largely on the excellent reviews that have appeared, in Physiological Reviews.