PREFACE. 5 



of so much interest and importance not only to physiology, but 

 to all branches of medicine. With this idea in mind the author 

 has endeavored to make the student realize that physiology is a 

 growing subject, continually widening its knowledge and read- 

 justing its theories. It is important that the student should 

 grasp this conception, because, in the first place, it is true; and, 

 in the second place, it may save him later from disappointment 

 and distrust in science if he recognizes that many of our conclu- 

 sions are not the final truth, but provisional only, representing 

 the best that can be done with the knowledge at our command. 

 To emphasize this fact as well as to add somewhat to the interest 

 of the reader short historical resumes have been introduced from 

 time to time, although the question of space alone, not to men- 

 tion other considerations, has prevented any extensive use of such 

 material. It is a feature, however, that a teacher might develop 

 with profit. Some knowledge of the gradual evolution of our 

 present beliefs is useful in demonstrating the enduring value of 

 experimental work as compared with mere theorizing, and also in 

 engendering a certain appreciation and respect for knowledge 

 that has been gained so slowly by the exertions of successive 

 generations of able investigators. 



A word may be said regarding the references to literature 

 inserted in the book. It is perfectly obvious that a complete 

 or approximately complete bibliography is neither appropriate 

 nor useful, however agreeable it may be to give every worker full 

 recognition of the results of his labors. But for the sake of those 

 who may for any reason wish to follow any particular subject 

 more in detail some references have been given, and these have 

 been selected usually with the idea of citing those works which 

 themselves contain a more or less extensive discussion and litera- 

 ture. Occasionally also references have been made to works of 

 historical importance or to separate papers that contain the experi- 

 mental evidence for some special view. 



