PREFACE TO THE SIXTH EDITION. 



IN the preparation of the sixth edition of this text-book 

 the author, as in previous editions, has made an earnest effort 

 to keep up with the progress of physiological science. Those 

 who are familiar with the complexity of the current literature 

 dealing with physiological problems will appreciate the difficul- 

 ties involved in such an undertaking. The great volume of 

 experimental work appearing in our scientific and medical jour- 

 nals makes it well-nigh impossible for a single' worker to keep 

 in intimate touch with the shifting points of view in regard 

 to the many problems considered in a general book of this 

 character. 



The author has done as well as he could, and he must beg 

 the generous indulgence of other workers if at any point he has 

 failed to reflect accurately the existing trend of belief. Most 

 of the important alterations in the text will be found in the 

 section upon Nutrition and Internal Secretions. A large part 

 of the latter subject has been rewritten. Unfortunately, it is 

 a subject whose intricacies have increased rather than dimin- 

 ished with the progress of investigative work, and it is more 

 rather than less difficult now than it was a decade ago to pre- 

 sent a fair summary of the numerous facts, speculations, and 

 hypotheses bearing upon this topic. 



W. H. HOWELL. 



September, 1915. 



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