PREFACE. 



THE author has become convinced, by several 

 years' intimate intercourse with students of medi- 

 cine, that their acquaintance with the subjects he 

 has endeavored to include in this little volume 

 would be facilitated, if the views, which are now 

 taught and scattered throughout the often expen- 

 sive works of their authors, were collected in a con- 

 venient form for study and reference. Taking it 

 for granted that a knowledge of this subject is of 

 fundamental importance in its bearing upon the 

 study of physiology and pathology, and stimulated 

 by the frequent inquiries of students for an appro- 

 priate source of information, he has prepared what 

 he now submits to them. 



He has sought to obtain a continuous history of 

 the evolution of the " cell doctrine " up to its present 

 state, without embarrassing his pages with a large 

 number of isolated facts. He has attempted, how- 



