EDITOR'S PREFACE. 



IN no science is the advance at any one time general. 

 Some sections of the line are pushed forward while other 

 parts may remain for years with little movement, until in 

 their turn they are enabled to progress in consequence 

 of the support afforded by the advance of the adjacent 

 sections. The increasing number of series of monographs 

 in different sciences is a recognition of this fact, as well as 

 of the concentration of interest which characterizes this 

 age of specialization. 



In the present series it is intended to set out the 

 progress of physiology in those chapters in which the 

 forward movement is the most pronounced. Each mono- 

 graph will contain an account of our knowledge of some 

 particular branch of physiology, written by one who has 

 himself contributed in greater or less degree to the attain- 

 ment of our present position. It is hoped that by securing 

 the help of men who are actively engaged in the advance 

 of the subject the outlook of each monograph will be for- 

 wards rather than backwards. An exhaustive account of 

 previous writings on the subject concerned is not aimed 

 at, but rather an appreciation of what is worth retaining 

 in past work, so far as this is suggestive of the paths along 

 which future research may be fruitful of results. The 

 more valuable the monographs in inspiring the work of 

 others, the greater will be the success of the series. 



THE death of Dr. W. H. Gaskell, just after he had com- 

 pleted the MS. of the present volume, deprived us of a 



