yiii PREFACE. 



other ; and, although the conclusions here brought forward 

 have for some time back fully approved themselves to his 

 judgment, he is free to admit that his views had previously 

 undergone many and important modifications since this de- 

 partment of Physiology first became the subject of his par- 

 ticular attention. But in the present state of the Science, 

 indeed, no conclusions can well claim more than a provisional 

 acceptance. 



It only remains to acknowledge the kind assistance received, 

 in various ways, from several friends, and particularly from 

 Professors Sharpey, Simpson, and Allen Thomson, and to ex- 

 press the hope that the work may contribute something, at 

 least by placing facts in a new point of view, to the elucida- 

 tion of a much-perplexed department of Natural Science. . 



