4 PREFACE. 



of reasoning are not difficult to understand. The obstacle that 

 is most frequently encountered by the student lies in the com- 

 plexity of the subject, the large number of more or less dis- 

 connected facts and theories which must be considered in a dis- 

 cussion of the structure, physics, and chemistry of such an intri- 

 cate organism as the human body. But once a selection has been 

 made of those facts and principles which it is most desirable that 

 the student should know, there is no intrinsic difficulty to prevent 

 them from being stated so clearly that they may be comprehended 

 by anyone who possesses an elementary knowledge of anatomy, 

 physics, and chemistry. It- is doubtless the art of presentation 

 that makes a text-book successful or unsuccessful. It must be 

 admitted, however, that certain parts of physiology, at this par- 

 ticular period in its development, offer peculiar difficulties to the 

 writers of text-books. During recent years chemical work in the 

 fields of digestion and nutrition has been very full, and as a result 

 theories hitherto generally accepted have been subjected to crit- 

 icism and alteration, particularly as the important advances 

 in theoretical chemistry and physics have greatly modified the 

 attitude and point of view of the investigators in physiology. 

 Some former views have been unsettled and much information 

 has been collected which at present it is difficult to formulate and 

 apply to the explanation of the normal processes of the animal 

 body. It would seem that -in some of the fundamental problems 

 of metabolism physiological investigation has pushed its experi- 

 mental results to a point at which, for further progress, a deeper 

 knowledge of the chemistry of the body is especially needed. Cer- 

 tainly the amount of work of a chemical character that bears di- 

 rectly or indirectly on the problems of physiology has shown a re- 

 markable increase within the last decade. Amid the conflicting 

 results of this literature it is difficult or impossible to follow always 

 the true trend of development. The best that the text-book can 

 hope to accomplish in such cases is to give as clear a picture 

 as possible of the tendencies of the time. 



Some critics have contended that only those facts or conclu- 

 sions about which there is no difference of opinion should be pre- 

 sented to medical students. Those who are acquainted with 

 the subject, however, understand that books written from this 

 standpoint contain much that represents the uncertain compromises 

 of past generations, and that the need of revision is felt as fre- 

 quently for such books as for those constructed on more liberal 

 principles. There does not seem to be any sound reason why a 

 text-book for medical students should aim to present only those 

 conclusions that have crystallized out of the controversies of other 

 times, and ignore entirely the live issues of the day which are 



