This book is a reprint of the first edition of 1922 and carries a preface that establishes the value of the publication. Krogh's original observations are ageless, and in this era of rapid scientific development, it is gratifying to find that his careful observations, made without the benefit of our newer scientific instruments, remain unchallenged by the modern "doubting Thomas." It is important that such classics in medical literature be made available to our young investigators for ready reference, stimulation, and evaluation. It is hoped that those who refer to this book will take time to read portions of it for the sheer beauty of the prose and clarity of expression. The power to observe may be enhanced by our present-day developments; the ability to record those observations seems less apparent in our scientific publications.