THE PROPAGATION OF PHYSIOLOGICAL 



CHANGE 



A PHYSICAL PROCESS 



By J. S. MACDONALD, B.A., L.R.C.S., L.R.C.P., 



Professor of Physiology, Sheffield. 



It is a leading characteristic of living matter that it is readily 

 excited by slight sudden changes in its environment. To these 

 stimuli it frequently responds by chemical and physical reactions 

 in which much more energy is transformed than vv^as supplied 

 by the exciting cause, in this way resembling all unstable 

 structures and compounds. It differs, however, from these 

 unstable bodies in that it rapidly returns to its original state, 

 abstracting matter and energy from its surroundings to make 

 good its loss — a process which can be repeated indefinitely. 

 Nor is the word indefinitely made use of in any limited sense, 

 as will be apparent when the processes of growth and multi- 

 plication are considered. Unstable when momentary changes 

 are dealt with, stable when time is taken in larger units, living 

 matter is usually spoken of as " labile." 



Now when an attempt is made to penetrate the meaning of 

 this state there are certain facts which always obtain a primary 

 hearing for circumstances of purely chemical rank. In the first 

 place many of these exciting causes lead obviously to chemical 

 change, as in the case of muscle, from which, when excited, 

 waste products trail away as definitely as the smoke and fumes 

 from any chemical factory. In the second place there is no 

 doubt that all the energy transformed in the body is liberated 

 as a consequence of the oxidation processes by which its food 

 material is overtaken. This statement has within recent years 

 been placed in a position of almost absolute truth. 



Explanations of both phases of this condition of lability have 

 therefore most frequently been arranged in terms of chemical 

 reactions. At one time we were invited to think of these 

 vaguely as due to processes of assimilation and dissimilation, 



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