Vlll 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 j^re^dously 

 undergone many and imj^ortant modifications smce 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 ISTatural Science. 



