An author who attempts the popular exposition of a science must stand sufficiently far away from his subject to lose sight of its details, whilst keeping its outlines clearly in view. The difficulty of finding such a position is probably greater in the case of physiology than in that of any other science. Few of its conclusions are indisputable—even those which seem to be most in accord with the balance of evidence. If my treatment of any vexed questions is unjustifiably dogmatic, this will, I trust, be attributed to the desire to present a definite picture, and not to forgetfulness of considerations which seem to call for qualified statements. All physiologists will agree that a book which recorded every piece of evidence which is difficult to reconcile with the views generally adopted would not only extend to an inordinate length, but would leave a very indefinite impression on the mind of the reader. In many cases the value of a conclusion depends upon the reputation for insight and accuracy of the physiologist who recorded the observations upon which it is based. It is no want of appreciation of the genius of the workers who have contributed most largely to the advance of the science which has led me to omit, save in a few classical instances, the names of all authorities. It is solely due to a desire to lighten this book of all details not essential to the comprehension of the propositions which it sets forth. The illustrations are reproductions of blackboard drawings. A few of them have already appeared in my _Physiologist’s Notebook_ and _Primer of Physiology_; but the large majority are now printed for the first time. ALEX HILL. _November, 1908._ CONTENTS CHAPTER PAGE I. PROLEGOMENA 1 II. THE BASIS OF LIFE 6 III. THE UNIT OF STRUCTURE 26 IV. THE FLUIDS OF THE BODY 37 V. INTERNAL SECRETIONS 84 VI. DIGESTION 96 VII. RESPIRATION 164 VIII. EXCRETION 194 IX. THE CIRCULATION 217 X. MUSCLE 248 XI. THE NERVOUS SYSTEM 293 XII. SMELL AND TASTE 364 XIII. VISION 372 XIV. HEARING 404 XV. SKIN-SENSATIONS 423 XVI. VOICE AND SPEECH 431 INDEX 441 LIST OF ILLUSTRATIONS