PREFACE. 



There are two extremes open in writing a brief treatise 

 on any natural science. One is to state briefly and explic- 

 itly those facts which are seriously questioned by no one. 

 It is to enumerate and tabulate what is definitely known 

 about the subject. In the field of animal physiology there 

 is much which is "settled" information, and which it is be- 

 lieved, will not be materially changed by the developments 

 of the future. Most of the gross anatomy is finished, by no 

 means all, but many points in histology are determined, 

 while in pure physiology there are fewer things that are con- 

 sidered explained. Thus the phenomena of respiration and 

 the dynamics of the blood flow are to a very large extent 

 known in terms of chemical and physical laws. To limit a 

 book to this would be to make it stereotyped, dead, and leave 

 the reader with the impression that physiology like Sanskrit 

 was finished, and like every finished problem had no longer 

 a living interest of its own. Most of the ordinary text- 

 books err on this side. They state ex cathedra, fact after 

 fact, they seldom give the reasons which have led physi- 

 ologists to adopt the views in question, and they seldom 

 leave the idea that many things are still being studied with 

 the hope that more study will give new light. Most text- 

 books leave the mind of the student with the belief that all 

 has been told, that there is nothing more to add, and that 

 therefore there is no need for him to try to improve on the 

 text-book, by making his own observation. The feeling of 

 the authority of the text-book in physiology has robbed 

 many a student of the desire to investigate the subject 

 further. L,ike the Scholastics of the Middle Ages they turn 

 to the book as to Aristotle, fully convinced that all know- 

 ledge is contained in it, and that what is not contained is 

 impossible of access. 



The other extreme is to give in detail all the scientific 

 controversies of the past and present. It is to state indis- 

 criminately pros and cons until the conviction settles over 

 one that nothing is definite and all is confusion. Especially 

 true is this when these conflicting views are at once pre- 

 sented to the beginner in the subject. 



(iii) 



