viii PREFACE 



In justice to the student, it should be pointed out that the 

 requirements of no particular examination have been taken into 

 consideration here. It is to be hoped that the day is not far 

 distant when preparation for an examination, other than periodic 

 questions by the teacher himself, will be a thing of the past. It is, 

 no doubt, difficult to devise a method of replacing examinations as 

 a test for such purposes as medical qualifications, but it is surely 

 not impossible. Many of the existing examinations can be passed 

 without much real knowledge, while the preparation of students 

 for examinations, when these are conducted by others than the 

 teacher himself, has a most depressing effect on those who have 

 to do it. However important a new discovery may be, hesitation 

 is naturally felt in taking up the students' time with what they know 

 will not help them to pass their examination. 



Some explanation is needed as to the diagrams given here. I 

 venture to think that, at all events in many cases, a picture which 

 attempts to represent what a part of a living organism actually looks 

 like is less instructive than one which frankly attempts no more 

 than to indicate what is essential to the working of the mechanism. 

 The real appearance can only be learned from actual specimens 

 and experiments. Such an excellent book as the " Practical 

 Biology" of Huxley and Martin shows that illustrations are not a 

 necessity. 



\Y. M. BAYLISS. 



UNIVERSITY COLLEGE, LONDON. 



