58 INTRODUCTION. 



regarded as not at all different in any respect from the ordinary 

 chemical changes taking place outside the body. This ; however, 

 is not only erroneous in theory, but conduces also to a vicious 

 mode of study. For it draws away our attention from the phe- 

 nomena themselves and their real characteristics, and leads us to 

 deduce one set of phenomena from what we know of another ; a 

 method which we have already shown to be unsafe and pernicious. 

 It has also been asserted that the phenomena of the nervous 

 system are identical with those of electricity ; for no other reason 

 than that there exist between them certain general resemblances. 

 But when we examine the phenomena in detail, we find that, beside 

 these general resemblances, there are many essential points of dis- 

 similarity, which must be suppressed and kept out of sight in order 

 to sustain the idea of the assumed identity. This assumption is 

 consequently a forced and unnatural one, and the simplicity which 

 it was intended to introduce into our physiological theories is 

 imaginary and deceptive, and is attained only by sacrificing a part 

 of those scientific truths, which are alone the real object of our 

 study. We should avoid, therefore, making any such unfounded 

 comparisons ; for the theoretical simplicity which results from them 

 does not compensate for the loss of essential scientific details. 



VI. The study of Physiology is naturally divided into three dis- 

 tinct Sections : 



The first of these includes everything which relates to the NUTRI- 

 TION of the body in its widest sense. It comprises the history of 

 the proximate principles, their source, the manner of their produc- 

 tion, the proportions in which they exist in different kinds of food 

 and drink, the processes of digestion and absorption, and the con- 

 stitution of the circulating fluids ; then the physical phenomena of 

 the circulation and the forces by which it is accomplished ; the 

 changes which the blood undergoes in different parts of the body ; 

 all the phenomena, both physical and chemical, of respiration ; those 

 of secretion and excretion, and the character and destination of the 

 secreted and excreted fluids. All these processes have reference to 

 a common object, viz., the preservation of the internal structure and 

 healthy organization of the individual. With certain modifications, 

 they take place in vegetables as well as in animals, and are conse- 

 quently known by the name of the vegetative functions. 



The Second Section, in the natural order of study, is devoted to 

 the phenomena of the NERVOUS SYSTEM. These phenomena are 



