50 PROTOPLAS^nC ACTION AND NERVOUS ACTION 



a way as to offset the continual breakdown; and these 

 structures subserve or render actual many activities 

 which would be impossible in any other kind of system. 

 In general, the activities which are most characteristic 

 of living as distinguished from non-living systems 

 belong in this class. We may thus understand, on the 

 basis of the general properties of systems in stationary 

 equilibrium, the possibility of the existence of material 

 systems of such complex structure and activity as living 

 organisms. The power of regulation exhibited by 

 stationary systems, i.e., of returning to the original 

 state after disturbance, is one of the chief properties 

 which they exhibit in common with living systems. 

 So long as the constitutive processes continue in action 

 such a result is to be expected. The permanence of 

 such delicate structures as filaments, films, nerve pro- 

 cesses, and the other finer products of the formative 

 activity of protoplasm depends on this continual auto- 

 matic synthesis, which compensates the tendency to 

 physical breakdown. 



When any irritable organism or cell responds to 

 stimulation, the energy for the response is derived from 

 the chemical energy of the protoplasmic constituents, 

 usually from the oxidation of carbohydrates. It is 

 clear therefore that one of the essential effects of the 

 stimulus is to alter the rate or character of cell-metab- 

 ohsm. In many cases this effect may be indirect; e.g., 

 in the voluntary muscle cell a large part of the heat- 

 production following a single stimulus succeeds the 

 contraction^ (Hill); similarly in the turgor-motors of 



' Cf. A. V. HiU, Journal of Physiology, XLII, XLIV, XL VI, XLVII 

 (1911-13); also Ergebnisse der Physiol., XV (1916), 340; Physiological 

 Reviews, II (1922), 310. 



