PREFACE. 



IN the present edition of this book, while every part has received 

 a careful revision, the original plan of arrangement has been changed 

 only so far as was necessary for the introduction of new material. Al- 

 though the whole field of physiology has been cultivated, of late years, 

 with unusual industry and success, perhaps the most important advances 

 have been made in the two departments of Physiological Chemistry and 

 the Nervous System. The number and classification of the proximate 

 principles, more especially, and their relation to each other in the pro- 

 cess of nutrition, have become, in many respects, better understood 

 than formerly ; though it is evident that this fundamental part of 

 physiology is to receive, in the future, modifications and additions of 

 the most valuable kind. 



In nearly every division of physiological study, a prominent feature 

 of recent progress has been the increased attention paid to quantitative 

 investigation. The conviction has apparently become general that, 

 in physiology as well as in other natural sciences, the knowledge 

 gained by any method of study is essentially imperfect until its re- 

 sults can be stated in figures. The chemical characters of an ingre- 

 dient or product of the animal system are hardly more important than 

 its quantity; and for determining its physiological relation to other 

 substances, of similar or different kinds, the knowledge of its quantity 

 is absolutely indispensable. Investigations of this sort, in respect to 

 the living body, are surrounded with difficulties ; but the results ob- 

 tained are steadily increasing in precision and extent, and already foiun 

 a most important element in the study of physiology. 



In a text-book like the present, it is desirable that the reader should 

 not be misled by having all the frequent changes of opinion, or sub- 

 stitutions of theory, presented as discoveries in physiological science. 

 Any faithfully observed facts, however unexpected or peculiar, are of 

 course at once invested with a permanent value. But the theoretical 

 explanations, by which they are sometimes accompanied, are not of the 

 same importance. They often represent only a scheme of probabilities 



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