It has been suggested by a professional critic for whose judgment I feel the utmost deference, that “the work would really have been more useful if the physiological or introductory part had been more condensed;” as much of it will, he thinks, be neither readily comprehended, nor usefully retained by the _general_ reader.[1] My only reason for not acting on this suggestion is, that I regard the exposition of the laws of digestion of which that part consists, as the foundation on which all the dietetic rules contained in the second part must necessarily rest,--and am therefore extremely anxious that their nature and mode of operation should be thoroughly understood by the ordinary reader, even at the risk of too great minuteness. I am quite aware that the detail into which I have entered must appear tedious to every well educated practitioner; but as the book was intended more for the general than for the medical reader, the latter is evidently a less competent judge in this particular matter than the former. On referring, accordingly, to an _unprofessional_ critic of no small ability and reputation, we find him of an entirely opposite opinion. For--“Of the two divisions of the book,” he thinks, “the FIRST is the most satisfactory and interesting, from the nature of its subject and the popular novelty of much of the information it imparts, or the force and freshness with which obvious truths are presented.”[2] And as other non-medical reviewers concur in this decision, I feel bound to attach more weight to them in what more especially concerns the class of readers to which they belong, and to retain the whole of the part objected to. In a purely medical question, on the other hand, I would as unhesitatingly have yielded to the judgment of the professional critic. EDINBURGH, 8 ALVA STREET, _November 1, 1836_. CONTENTS PREFACE, xvii PART I. PHYSIOLOGY OF DIGESTION. CHAPTER I. INTRODUCTORY REMARKS. Waste or loss of substance always attendant on action--In the vegetable and animal kingdoms waste is greater than in the physical--Living bodies are distinguished by possessing the power of repairing waste--Vegetables, being rooted in one place, are always in connection with their food--Animals, being obliged to wander, receive their food at intervals into a stomach--Nutrition most active when growth and waste are greatest--In vegetables the same causes which increase these processes also stimulate nutrition--But animals require a monitor to warn them when food is needed--The sense of Appetite answers this purpose--The possession of a stomach implies a sense of Appetite to regulate the supplies of food, 1–10 CHAPTER II.