HUMAN PHYSIOLOGY. 



CHAPTER I. 



OF LIFE, AND ITS CONDITIONS. 



1. THE term Life has been used by different writers, Physiological and 

 Ontological, in a great variety of significations ; but these are for the most 

 part capable of reduction to three categories Life being regarded either (1) 

 as the aggregate of the phenomena exhibited by any Organized being from 

 the commencement to the conclusion of its individual existence, or (2) as 

 the mode of activity peculiar to such beings, whereby they are distinguished 

 from inanimate bodies, or (3) as the special agency supposed to be inherent 

 in every organism, and to be the efficient cause alike of its first development 

 and of its subsequent maintenance. The first is the sense in which the term 

 is understood by Philosophers of the "Positive" school, who refuse to con- 

 cern themselves with anything save phenomena that are immediately cog- 

 nizable by the senses; while the last is the meaning attached to it by such 

 as think that a great deal of trouble is saved by the assumption of a hypo- 

 thetical entity, whose agency may at once account for everything not to be 

 otherwise explained. To both these definitions it may be objected that they 

 tend to limit inquiry into the essential nature of Vital Action. For by 

 taking the former as a starting-point, we are led to fix our attention too ex- 

 clusively on the material conditions presented in the structure of the Organ- 

 ism, and to ignore the forces by which its activity is maintained ; just as if, 

 in studying thp operations of a cotton factory, we were to limit our attention 

 to the mechanism of the carding, spinning, weaving, and other machines by 

 whose instrumentality its products are elaborated, and were to neglect, as a 

 condition not directly cognizable by our senses, the Motive Power without 

 which those machines would all be inert. On the other hand, by resting in 

 the assumption of a "Vital Principle" or "Organic Agent" as affording a 

 sufficient account of all that is mysterious in the nature of Life, we really 

 remove it from the domain of scientific inquiry; just as if the visitor to a 

 cotton factory were to give up in despair any attempt to acquaint himself 

 with the meaning of the several processes that go on before his eyes, and 

 were to regard it as a sufficient account of the transformation of raw cotton 

 into woven calico, that it takes place by the agency of a " calico-making 

 principle." 



2. But if, on the other hand, the Physiologist takes as his standpoint the 

 conception of Life as a peculiar mode of activity, he at once finds himself 

 on a pathway of inquiry marked out for him by the antecedent researches 

 of the Physical philosopher. For as, in the study of that great cycle of 



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