PREFACE 
Yale University did me the great honour of inviting 
me to deliver the Silliman Lectures for 1915. Owing 
to the war I was unable to give the lectures in the ap- 
pointed year; and I must first of all thank the Univer- 
sity for permitting me to postpone them till the present 
time. 
The subject of the full lectures, as they will, I hope, 
before long appear in book form under the imprint of 
the Yale University Press, is the Physiology of Breath- 
ing. Much of the material contained in them is, how- 
ever, of a technical character, hardly suited for public 
lectures. With the approval of the President, I have 
therefore delivered the following four public lectures 
confined to points of more general interest, the nature 
of which is indicated by the title. 
Joun Scott HaLpane. 
New Haven, October, 1916. 
