PREFACE TO THE FOURTH EDITION. 



IN the present edition the whole subject has been carefully 

 revised. During the five years since the last edition was 

 published, valuable additions to our knowledge have been 

 made in practically every department, whilst in the case of 

 several diseases there have been discoveries of the highest 

 importance. Our object has been to incorporate this new 

 matter and at the same time to maintain the primary object 

 of the work as a text-book for students of medicine. Thus 

 whilst we have dealt with all the facts having a direct bearing 

 on clinical medicine we have also given considerable prominence 

 to matters at present under discussion from the scientific point 

 of view. In this way we have endeavoured to give a faithful 

 representation of the subject as it at present stands both in 

 its practical and theoretical aspects. In the case of several 

 diseases which up till recent times have been investigated by 

 purely bacteriological methods there is now considerable evidence 

 that the causal agent is of protozoal nature. Amongst such 

 conditions the most important are those in which spirochsetes 

 are concerned, syphilis and the relapsing fevers being outstand- 

 ing examples. As, however, the exact biological relationships 

 of these organisms are still matters of dispute we have kept 



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