B3 1 



TO THE READER. 



THE nature of the changes which occur in matter 

 that is alive has always excited great interest ; 

 it is a question which arrests the attention of all 

 educated persons, as well as of those who make the 

 pursuit and advancement of natural science their life 

 work. 



The enquiry is necessarily brought under the 

 attention of every student of medicine, and it is 

 natural that physicians who have time and oppor- 

 tunity, should be led to investigate deeply some of 

 its ramifications. The minute structure and action 

 of the tissues and organs of living beings, in health 

 and disease, early attracted my notice. It is a study 

 which has had a rare charm for me from boyhood, 

 and for upwards of twenty years I have been dili- 

 gently engaged in original research as well as in 

 public teaching in this particular department of 

 science. Some of the observations I have made 

 are recorded in this work. I have avoided the use of 

 technical terms, and have tried to say what I have to 

 say in the simplest manner. To save lengthy descrip- 

 tions a few drawings of some of the specimens upon 



