2 PREFACE. 



.number of the works to which he has referred. As far as 

 such an end could be accomplished by patient labor, the ref- 

 erences are accurate, and given in such a way that they can 

 be easily verified. The author has not hesitated, however, 

 to give his own opinions upon every subject considered, even 

 when they have been opposed to high authority. 



In the preparation of this work, the author has formed 

 his opinions, to a great extent, from the results of direct 

 observation and experiment, as the true basis of what is 

 positively known in physiology ; and, while the earlier vol- 

 umes might be modified by the addition of new facts, they 

 contain comparatively little that has been disproved by re- 

 cent investigations. Experimental observations have been 

 studied and criticised from a practical point of view ; and, in 

 this, the author's training as an experimentalist and a public 

 teacher for more than fifteen years has given him a certain 

 degree of confidence. It is the practical physiologist who is 

 best qualified to judge of the correctness of the results of 

 physiological experiments and of the accuracy of methods of 

 investigation ; and the author has learned, from his own at- 

 tempts at original observation, to estimate the difficulties of 

 direct research and to appreciate the inaccuracies into which 

 careless, inexperienced, or over-enthusiastic workers are liable 

 to fall. 



As regards certain new views enunciated in the first four 

 volumes, the author has found no reason to modify his opin- 

 ions. In the first volume, under the head of respiration, he 

 stated, as the result of numerous experiments, that the besoin 

 de respirer, or sense of w^ant of air, the impression which 

 gives rise to the first inspiratory effort in the newly-born and 

 which excites the reflex acts in normal respiration, is refer- 



