52 INTRODUCTION". 



interior of the body, which result in the formation of new sub- 

 stances ; all these active phenomena belonging necessarily to the 

 domain of Physiology. 



The description of the proximate principles of animals and vege- 

 tables will therefore be introduced into the following pages. 



The description of the minute structures of the body, or Micro- 

 scopic Anatomy, is also so closely connected with some parts of Phy- 

 siology as to make it convenient to speak of them together ; and 

 this will accordingly be done, whenever the nature of the subject 

 may make it desirable. 



III. The study of Physiology, like that of all the other natural 

 sciences, is a study of phenomena, and of phenomena alone. The 

 essential nature of the vital processes, and their ultimate causes, 

 are questions which are beyond the reach of the physiologist, and 

 cannot be determined by the means of investigation which are at 

 his disposal. 



Consequently, all efforts to solve them will only serve to mislead 

 the investigator, and to distract his attention from the real subject 

 of examination. Much time has been lost, for example, in discuss- 

 ing the probable reason why menstruation returns, in the human 

 female, at the end of every four weeks. But the observation of 

 nature, which is our only means of scientific investigation, cannot 

 throw any light on this point, but only shows us the fact that men- 

 struation does really occur at the above periods, together with the 

 phenomena which accompany it, and the conditions under which it 

 is hastened or retarded, and increased or diminished, in intensity, 

 duration, &c. If we employ ourselves, consequently, in the discus- 

 sion of the reason above mentioned, we shall only become involved 

 in a network of hypothetical surmises, which can never lead to any 

 definite result. Our time, therefore, will be much more profitably 

 devoted to the study of the above phenomena, which can be learned 

 from nature, and which constitute afterward, a permanent acquisi- 

 tion. 



The physiologist, accordingly, confines himself strictly to the 

 study of the vital phenomena, their characters, their frequency, 

 their regularity or irregularity, and the conditions under which 

 they originate. 



When he has discovered that a certain phenomenon always takes 

 place in the presence of certain conditions, he has established what 

 is called a general principle, or a LAW of Physiology. 



