2 PROTOPLASMIC ACTION AND NERVOUS ACTION 



built up from the non-living material which it incorpo- 

 rates from the surroundings. 



Physiology regards the living organism solely in its 

 objective aspect as a physical object in external nature; 

 many aspects and manifestations of living beings do not 

 form directly a part of its subject-matter, and the general 

 philosophical question of the essential significance of life 

 in the cosmos — the question of vitalism or anti- vitalism 

 — is not one which it makes any pretensions to answer. 

 It observes simply that certain systems, living organisms, 

 exist in the external world, presenting a remarkable 

 combination of properties not found in other natural 

 systems; and its task is the analysis of these systems in 

 the terms and by the methods of physical science. 



These special or distinguishing peculiarities of living 

 organisms may be grouped under several general heads, 

 as follows: (i) metabolism, (2) growth, automatic self- 

 maintenance, reproduction and heredity, (3) irritability, 

 (4) regulation and adaptation, (5) spontaneous activity, 

 having reference to future as well as present conditions. 

 The essential character and implications of these various 

 properties will first be briefly considered. 



I. METABOLISM 



The essential peculiarity which places organisms in 

 a class apart from most non-living objects is that their 

 properties and manifestations depend on their continued 

 chemical activity; in other words, they are metabolizing 

 systems, formed, maintained, and perpetuated by 

 processes of chemical transformation. The production 

 of new chemical compounds by transformation of other 

 compounds taken from the surroundings, and the 



