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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 A few woodcuts have been added ; others have been replaced by better ones, as in the case of the figures of the retina, which embody the results of Schultze's latest researches.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.

T. H. H.

LONDON, July 1868.

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.

The originals of the woodcuts are, for the most part, to be found in the works of Bourgery, Gray, Henle, and Kölliker.

A few are new. I am particularly indebted to my accomplished friend, Dr. Michael Foster, for the pains and trouble he has bestowed upon the Lessons in their passage through the press.

THE ROYAL SCHOOL OF MINES, LONDON,

October 1866.

CONTENTS.

2. Purpose of these Lessons.

3. Experimental proof that a living active man gives out

heat, exerts mechanical force, and loses substance in the

form of carbonic acid, water, and other matters.

4, 5. These losses made good by the taking in of air, drink, and

food.

6. Balance of bodily income and expenditure.

7. Work and Waste ; the body compared to a steam-engine.

8. General build of the body-head, trunk, and limbs.

9. The vertebra and spinal cord. The cavities of the trunk

10. The human body a double tube.

11. The tissues. Integument.

12. Connective tissue.

13. Muscle.

14. The skeleton.

15. The maintenance of an upright position the result of many

combined actions.

16. The relation of the mind to the action of the muscles.

17. The spinal cord capable of converting impressions from

without into muscular contractions.

18. Special sensations.

19. The tissues are constantly being renewed.

20. The renewal is effected by means of the alimentary appa-

ratus, which converts food into nutriment; and by the

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