498 1873

PREFACE TO THE SIXTH EDITION.

A CONSIDERABLE number of illustrations have been added to this edition; and several of them have been taken, not from the Human subject, but from the Rabbit, the Sheep, the Dog, and the Frog, in order to aid those, who, in accordance with the recommendation contained in the Preface to the Second Edition, attempt to make their knowledge real, by acquiring some practical acquaintance with the facts of Anatomy and Physiology.

My thanks are again due to my friend Dr. Foster, F.R.S., for many valuable suggestions, and, more especially, for the trouble he has taken in superintending the execution of the new woodcuts.

LONDON, September 1872.

PREFACE TO THE SECOND EDITION.

THE present edition of the "Lessons in Elementary Physiology," has been very carefully revised. A few woodcuts have been added; others have been replaced by better ones, as in the case of the figures of the retina, which embody the results of Schultze's latest researches.

Some additions (but as few as possible, lest the book should insensibly lose its elementary character) have been made; among the most important I count the very useful "Table of Anatomical and Physiological Constants" drawn up for me by Dr. Michael Foster, for whose friendly aid I am again glad to express my thanks.

It will be well for those who attempt to study Elementary Physiology, to bear in mind the important truth that the knowledge of science which is attainable by mere reading, though infinitely better than ignorance, is knowledge of a very different kind from that which arises from direct contact with fact;

and that the worth of the pursuit of science as an intellectual discipline is almost lost by those who seek it only in books.

As the majority of the readers of these Lessons will assuredly have no opportunity of studying anatomy or physiology upon the human subject, these remarks may seem discouraging. But they are not so in reality. For the purpose of acquiring a practical, though elementary, acquaintance with physiological anatomy and histology, the organs and tissues of the commonest domestic animals afford ample materials. The principal points in the structure and mechanism of the heart, the lungs, the kidneys, or the eye, of man, may be perfectly illustrated by the corresponding parts of a sheep; while the phenomena of the circulation, and many of the most important properties of living tissues, are better shown by the common frog than by any of the higher animals.

Under these circumstances there really is no reason why the teaching of elementary physiology should not be made perfectly sound and thorough. But it should be remembered that, unless the learner has previously acquired a knowledge of the elements of Physics and of Chemistry, his path will be beset with difficulties and delays.

LONDON, July 1868.

T. H. H.

PREFACE TO THE FIRST EDITION.

THE following "Lessons in Elementary Physiology " are primarily intended to serve the purpose of a text-book for teachers and learners in boys' and girls' schools.

My object in writing them has been to set down, in plain and concise language, that which any person who desires to become acquainted with the principles of Human Physiology may learn, with a fair prospect of having but little to unlearn as our knowledge widens.

It is only by inadvertence, or from an error in judgment, therefore, that the book contains any statement, or doctrine, which cannot be regarded as the common property of all physiologists. I have endeavoured simply to play the part of a sieve, and to separate the well-established and essential from the doubtful and the unimportant portions of the vast mass of knowledge and opinion we call Human Physiology.