I purpose to express myself candidly and earnestly on all topics relating to health, and appeal to the common sense of the reader for justification. Although it is my aim to simplify the work, and render it a practical common-sense guide to the farmer, mechanic, mariner, and day-laborer, yet I trust that it may not prove less acceptable to the scholar, in its discussion of the problems of Life. Not only does the method adopted in this volume of treating of the Functions of the Brain and Nervous System present many new suggestions, in its application to hygiene, the management of disease, generation and the development and improvement of man, but the conclusions correspond with the results of the latest investigations of the world's most distinguished _savants_. My object is to inculcate the facts of science rather than the theories of philosophy. Unto us are committed important health trusts, which we hold, not merely in our own behalf, but for the benefit of others. If we discharge the obligations of our trusteeship, we shall enjoy present strength, usefulness, and length of days; but if we fail in their performance, then inefficiency, incapacity, and sickness, will follow, the sequel of which is pain and death. Let us, then, prove worthy of this generous commission, that we may enjoy the sweetest of all pleasures, the delicious fruitage of honest toil and faithful obedience. * * * * * PART I. PHYSIOLOGY. CHAPTER I. BIOLOGY. In this chapter we propose to consider Life in its primitive manifestations. _Biology_ is the science of living bodies, or the science of life. Every organ of a living body has a function to perform, and _Physiology_ treats of these functions. _Function_ means the peculiar action of some particular organ or part. There can be no vital action without change, and no change without organs. Every living thing has a structure, and _Anatomy_ treats of the structures of organized bodies. Several chapters of this work are devoted to _Physiological Anatomy_, which treats of the human organism and its functions. The beginning of life is called _generation_; its perpetuation, _reproduction_. By the former function, individual life is insured; by the latter, it is maintained. Since nutrition sustains life, it has been pertinently termed _perpetual reproduction_. LATENT LIFE is contained in a small globule, a mere atom of matter, in the sperm-cell. This element is something which, under certain conditions, develops into a living organism. The entire realm of nature teems with these interesting phenomena, thus manifesting that admirable adjustment of internal to external relations, which claims our profound attention. We are simply humble scholars, waiting on the threshold of nature's glorious sanctuary, to receive the interpretation of her divine mysteries.