0 LESSONS IN ELEMENTARY PHYSIOLOGY BY THOMAS H. HUXLEY, LL.D., F.R.S. London MACMILLAN AND CO. AND NEW YORK 1892 The Right of Translation and Reproduction is Reserved HARVARD RICHARD CLAY AND SONS, LIMITED, LONDON AND BUNGAY. First Edition printed 1866. New Edition, 1868. Reprinted 1869, 1870, 1871, March and May, 1872. New Edition, October, 1872. Reprinted 1873, 1874, 1875, 1876, 1878, 1879, 1881, 1883, January, February, May, September and November, 1884. New Edition, 1885. Reprinted 1886, 1888, 1890, 1892. PREFACE TO REVISED EDITION, (1885). It is now more than twenty years since I planned and began to write these "Lessons," the object of which is defined in the preface to the first edition. During this period physiological investigations have displayed an activity unprecedented in history. Here, as in all such branches of natural knowledge, the method of experiment has shown itself to be the one path which leads to truth, and has not only revealed multitudes of novel physiological facts, but has suggested wholly new lines of inquiry. As successive editions of the "Lessons have been demanded, it has been my effort to incorporate with them such solid acquisitions of the ever advancing science of physiology as fall within their scope, while rigorously excluding all debatable matter or, at any rate, carefully indicating it as such. At the same time, long experience as a Teacher and Examiner having forcibly brought home to my mind the difficulty of making any statement that cannot be misunderstood, an amount of attention has been devoted to questions of mere exposition, which really deserves, but probably has not attained, the reward of abolishing such misunderstandings. The present edition has been more extensively revised than any of its predecessors. The chapter on Histology, in particular, has been entirely reconstructed and provided with new illustrations, several of which have been taken from Ranvier and from Quain. In the preface to former editions of this work, I have had the pleasure of expressing my obligations to Dr. Foster, Secretary to the Royal Society, for his very valuable assistance. That aid has been still more freely rendered to the present edition, which, in fact, could not have appeared unless Dr. Foster had taken upon himself the whole burden of seeing the work through the press. My friend has indeed done so much during my enforced idleness, that I should have been better satisfied if he would have permitted me to associate his name with mine on the title-page. ROME, February, 1885. T. H. HUXLEY. CONTENTS. § 1. Modes of studying the action of man's body. 3. Experimental proof that a living active man gives out heat, exerts mechanical force, and loses sub- stance in the form of carbonic acid, water, and 4, 5. These losses made good by the taking in of air, drink, 6. Balance of bodily income and expenditure. 7. Work and Waste; the body compared to a steam- 8. General build of the body-head, trunk, and limbs. 9. The vertebræ and spinal cord. The cavities of the |