The present volume is essentially a continuation of the work first published about two years ago, under the title of “The Principles of Physiology applied to the Preservation of Health and to the Improvement of Physical and Mental Education;” and its object is the same--namely, to lay before the public a plain and intelligible description of the structure and uses of some of the more important organs of the human body, and to shew how information of this kind may be usefully applied, not only in the prevention of suffering, but in improving the physical, moral, and intellectual condition of man. In “The Principles of Physiology,” the structure and functions of the skin, muscles, bones, lungs, and nervous system, the laws or conditions of their healthy action, and the unsuspected origin of many of their diseases in infringements of these laws, were explained in succession at considerable length; and the means by which their health and efficiency might best be secured were pointed out. It was stated that, in selecting these organs as subjects for discussion, I had been guided by the desire to notice in preference those functions which are most influential in their operation on the general system, and at the same time least familiarly known; and that, if the attempt to convey the requisite information in a manner suited to the general reader should prove successful, I would afterwards prepare a similar account of others, in the right understanding and management of which our interest is not less deeply involved, but in regard to which much ignorance continues nevertheless to prevail, even among the most liberally educated classes of society. The numerous proofs which I received of the utility of my former work, not only from professional and literary journals, but also from individuals previously unknown to me,--many of them guardians and instructors of youth, speaking from personal experience,--together with the rapid sale of four large editions in little more than two years, soon completely satisfied me that I had neither been deceived as to the real importance of physiological knowledge to the general public, nor been altogether unsuccessful in the method of conveying it. Thus encouraged, accordingly, I cheerfully resumed my labours, and, from materials which had been long accumulating, began the preparation of the treatise now submitted to the indulgent consideration of the reader.