INTRODUCTION. 



THE object of the science of Physiology is to bring together, in a systematic 

 form, the phenomena which normally present themselves during the existence 

 of living beings; and to classify and compare these in such a manner, as to 

 deduce from them those general Laws or Principles which express the conditions 

 of their occurrence, and to determine the Causes to which they are attributable. 



The term " Law" having been frequently applied to physical and physiological 

 phenomena, in a manner very different from that which sound philosophy sanc- 

 tions, it is desirable to explain the acceptation (believed by the author to be the 

 only legitimate one) in which it is here employed. Viewed in their scientific 

 aspect, the so-called Laws of Nature are nothing else than general expressions 

 of the conditions under which certain assemblages of phenomena occur, so far 

 as those conditions are known to us. Thus the law of Gravitation in General 

 Physics (the most comprehensive of any with which we are acquainted), is 

 nothing else than a simple expression of the fact, that, under all circumstances, 

 two masses of matter will attract each other, with forces directly proportional 

 to their respective bulks, and inversely as the square of their distances. So, 

 again, the law of Cell-growth, which seems to hold nearly the same rank in 

 Physiology with that of Gravitation in Physics, embodies these two general 

 facts (1), that all organized beings originate in cells, and (2), that the various 

 functions of life are carried on, even in the adult condition, by the continued 

 growth and development of cells. 



It is a very common fallacy, however, to assume that we have sufficiently 

 accounted for a phenomenon, when we have referred it to a "general law." 

 Thus if a man be asked why a stone falls to the earth, he commonly considers 

 that he gives a satisfactory explanation when he replies, " Because of the Law of 

 gravitation." But it must be remembered that this " law of gravitation" is a 

 law, that is, a generalized expression of facts, only because all stones fall to the 

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