54 INTRODUCTION. 



the latter can never be in the least degree inferred from those of the 

 former, but must be studied ly themselves. 



Thus Chemistry is essential to Anatomy, because certain sub- 

 stances, as we have already shown, belonging to Chemistry, such 

 as chloride of sodium, occur as constituents of the animal body. 

 Chemistry teaches us the composition, reactions, mode of crystal- 

 lization, solubility, &c., of chloride of sodium ; and if we did not 

 know these, we could not extract it, or recognize it when extracted 

 from the body. But, however well we might know the chemistry 

 of this substance, we could never, on that account, infer its presence 

 in the body or otherwise, nor in what quantities nor in what situa- 

 tions it would present itself. These facts must be ascertained for 

 themselves, by direct investigation, as a part of anatomy proper. 



So, again, the structure of the body in a state of rest, or its 

 anatomy, is to be first understood ; but its active phenomena or its 

 physiology must then be ascertained by direct observation and 

 experiment. The most intimate knowledge of the minute struc- 

 ture of the muscular and nervous fibres could not teach us any- 

 thing of their physiology. It is only by experiment that we 

 ascertain one of them to be contractile, the other sensitive. 



Many of the phenomena of life are chemical in their character, 

 and it is requisite, therefore, that the physiologist know the ordi- 

 nary chemical properties of the substances composing the animal 

 frame. But no amount of previous chemical knowledge will 

 enable him to foretell the reactions of any chemical substance in 

 the interior of the body; because the peculiar conditions under 

 which it is there placed modify these reactions, as an elevation or 

 depression of temperature, or other external circumstance, might 

 modify them outside the body. 



We must not, therefore, attempt to deduce the chemical phe- 

 nomena of physiology from any previously established facts, since 

 these are no safe guide ; but must study them by themselves, and 

 depend for our knowledge of them upon direct observation alone. 



Y. By the term Vital phenomena, we mean those phenomena 

 which are manifested in the living body, and which are character- 

 istic of its functions. 



Some of these phenomena are physical or mechanical in their 

 character; as, for example, the play of the articulating surfaces 

 upon each other, the balancing of the spinal column with its ap- 

 pendages, the action of the elastic ligaments. Nevertheless, these 



