PREFACE 



To write a satisfactory text-book for those commencing the study 

 of a science is well known to be of greater difficulty than to write 

 one for advanced students. In fact, it would probably be true to 

 say that the former task is a severe test of the author's comprehen- 

 sion of the subject. I venture to hope that the thought given to 

 the present work may not be found to have been in vain. Any 

 readers who may find parts of it expressed insufficiently clearly are 

 requested to help the author with suggestions. 



It was found to require much consideration to decide what 

 should be omitted. My firm conviction is, that a thorough under- 

 standing of the main fundamental conceptions is of more value than 

 a superficial acquaintance with a large number of facts and theories, 

 even if it leads, for the moment, to the omission of what many may 

 regard as essential. The student will probably be surprised to find 

 how many of the more detailed descriptions follow naturally from 

 a knowledge of a few general principles, when these are clearly 

 grasped. For this reason, more space has been given to certain 

 elementary facts of physics and chemistry than might be supposed 

 necessary ; but it has been my experience that, although a student 

 may have attended good courses in these sciences, he does not 

 readily apply the knowledge to physiological problems. Moreover, 

 there are some things essential to physiology, but often regarded 

 as outside the scope of an elementary course in the preliminary 

 sciences. I intentionally lay myself open to the charge of inserting 

 matter that is not, strictly speaking, physiological. 



The rather " intensive " treatment sometimes adopted leads to 

 what some may regard as an excursion into advanced regions. But 

 this is unavoidable when the conception cannot be passed over 

 without risk of error. 



In another way, the manner of presentation adopted may meet 

 with objection. Students do not easily remember the names of the 



