Xll PREFACE. 



of readers ; and, avoiding all unnecessary ex- 

 tension of the field of inquiry, have wholly 

 abstained from entering into historical ac- 

 counts of the progress of discovery ; content- 

 ing myself with an exposition of the present 

 state of the science. I have also scrupulously 

 refrained from treading in the paths, which 

 have been prescribed to the other authors of 

 these treatises ; and have accordingly omitted 

 all consideration of the hand, the voice, the 

 chemical theory of digestion, the habits and 

 instincts of animals, and the structures of ante- 

 diluvian races ; the extent of the field which 

 remained, and which, with these few excep- 

 tions, embraces nearly the whole of the phy- 

 siology of the two kingdoms of nature, already 

 aftbrding ample occupation for a single la- 

 bourer. 



The catalogue of authors whose works have 

 furnished me with the principal facts detailed 

 in these volumes, is too long; for insertion in 

 this place. I have not encumbered the pages 

 of the work by continual citations of authori- 

 ties ; but have given references to them only 

 when they appeared to be particularly requi- 

 site, either as bearing testimony to facts not 

 generally known, or as pointing out sources 



