IV PREFACE. 



provided for in the usual school and college course of study : 

 the true use of Natural Science in general education is 

 different. It should prepare the student in another way 

 for the work of subsequent daily life, by training the ob- 

 serving and reasoning faculties. 



As a department of science, modern Physiology is con- 

 trolled mainly by two leading generalisations the doctrine 

 of the " Conservation of Energy" and that of the " Physio- 

 logical division of labor. " I have endeavored in this, as in 

 the larger book, to keep prominent these leading principles ; 

 and, so far as is possible in an elementary book, to exhibit 

 the ascertained facts of Physiology as illustrations of or de- 

 ductions from them. 



The anatomical and physiological facts which can be 

 described in books of the size of the present, must be pretty 

 much the same in all. Apart from the attempt above men- 

 tioned to make elementary Physiology a more useful edu- 

 cational instrument than it has frequently hitherto been, 

 the present volume differs from most others of its grade in 

 having, as foot-notes or as appendices to the chapters, simple 

 practical directions, assisting a teacher to demonstrate to 

 his class certain fundamental things. The demonstrations 

 and experiments described necessitate the infliction of pain 

 on no animal, and require the death of no creature higher 

 than a frog, except such superfluous kittens, puppies and 

 .rats as would be killed in any case, and usually by methods 

 much less merciful than those prescribed in the following 

 pages. The practical directions are, for the most part, 

 reprints from a series of such which I drew up some years 

 ago for a class composed of Baltimore teachers ; those ex- 

 periments which required costly apparatus have, of course, 

 been omitted. The interest which my " Teachers' Class" 

 took in its work, and the good use its members subsequently 

 made of it, have encouraged me to believe that others 

 might be glad of a few hints as to things suitable to show 

 to young students of Physiology. 



