vi PREFACE 



existent), it is due to the kindness of my friends and 

 colleagues, Professors C. S. Sherrington, F.R.S., 

 W. A. Herdman, F.R.S., and Dr. J. Reynolds Green, 

 F.R.S., in reading and criticising the MS. or proofs 

 of this work, and to them I tender my heartiest 

 thanks. 



R. J. HARVEY GIBSON. 



LIVERPOOL, 1908. 



