AUTHOR'S PREFACE 



FIFTH AMERICAN AND FOURTH ENGLISH EDITION. 



THE Author feels it necessary to apologize to those, whose kind ap- 

 preciation of the following work occasioned a call for a New Edition 

 as much as two years since, for the delay which has attended its ap- 

 pearance. Having been at that time engaged in rewriting his " Prin- 

 ciples of General and Comparative Physiology," he felt that he could 

 not do justice to that work, if he were not to bring it to a conclusion 

 before taking another in hand ; and when, on the completion of that 

 task, he applied himself to the preparation of his " Principles of 

 Human Physiology," for the press, he found that nothing short of an 

 entire remodelling of the preceding Edition would in any degree satisfy 

 his notions of what such a treatise ought to be. For although no fun- 

 damental change had taken place during the interval in the fabric of 

 Physiological Science, yet a large number of less important modifica- 

 tions had been effected, which had combined to produce a very con- 

 siderable alteration in its aspect. Moreover, the progressive maturation 

 of his own views, and his increased experience as a Teacher, had not 

 only rendered him more keenly alive to the imperfections which were 

 inherent in its original plan, but had caused him to look upon many 

 topics in a light very different from that under which he had previously 

 regarded them ; and, in particular, he felt a strong desire to give to his 

 work as practical a character as possible, without foregoing the position 

 which (he trusts he may say without presumption) he had succeeded in 

 gaining for it, as a philosophical exposition of one important department 

 of Physiological Science. He was led, therefore, to the determination 

 of, in reality, producing a new treatise, in which only those parts of the 

 old should be retained, which might express the existing state of know- 

 ledge, and of his own opinions, on the points to which they relate ; and 



