vi PREFACE 



intrinsic interest of the science apart altogether from its 

 medical applications. 



An author who attempts the popular exposition of a science 

 must stand sufficiently far away from his subject to lose sight 

 of its details, whilst keeping its outlines clearly in view. The 

 difficulty of finding such a position is probably greater in the 

 case of physiology than in that of any other science. Few of 

 its conclusions are indisputable even those which seem to be 

 most in accord with the balance of evidence. If my treatment 

 of any vexed questions is unjustifiably dogmatic, this will, I 

 trust, be attributed to the desire to present a definite picture, 

 and not to forgetfulness of considerations which seem to call 

 for qualified statements. All physiologists will agree that a 

 book which recorded every piece of evidence which is difficult 

 to reconcile with the views generally adopted would not only 

 extend to an inordinate length, but would leave a very indefinite 

 impression on the mind of the reader. 



In many cases the value of a conclusion depends upon the 

 reputation for insight and accuracy of the physiologist who 

 recorded the observations upon which it is based. It is no 

 want of appreciation of the genius of the workers who have 

 contributed most largely to the advance of the science which 

 has led me to omit, save in a few classical instances, the names 

 of all authorities. It is solely due to a desire to lighten this book 

 of all details not essential to the comprehension of the proposi- 

 tions which it sets forth. 



The illustrations are reproductions of blackboard drawings. 

 A few of them have already appeared in my Physiologist's Note- 

 book and Primer of Physiology ; but the large majority are now 

 printed for the first time. 



ALEX HILL. 

 November, 1908. 



