18 INTRODUCTION TO NEUROLOGY 



subsequently more simply expressed by the late C. L. Herrick 

 in the proposition. "Life is the correlation of physical forces 

 for the conservation of the individual"; and this, in turn, may 

 be cast in the more general form, Life is a system of forces 

 maintained by a continuous interchange of energy between the 

 system and its environment, these forces being so correlated 

 as to conserve the identity of the system as an individual and 

 to propagate it. A certain measure of modifiability in the char- 

 acter of the system, without loss of its individuality, is not ex- 

 cluded. 



No one of these definitions, or any other which has been sug- 

 gested, is fully satisfactory; but biologists generally agree that 

 the common characteristics of living beings can best be ex- 

 pressed in the present state of our knowledge in terms of their 

 actions, their behavior. The properties commonly ascribed to 

 any object are in last analysis names for its behavior, and the 

 so-called vital properties are very special forms of energy trans- 

 formation. 



Now, the chief difference between a corpse and a living body 

 consists in the fact that the forces of surrounding nature tend 

 to the disintegration of the dead body, while in the living body 

 these forces are utilized for its upbuilding. If, then, the vital 

 process is essentially a special type of mutual interaction be- 

 tween the bodily mechanism and the forces of the surrounding 

 world, of the correspondence between the organism and the 

 environment, to use the Spencerian phrase, it follows that the 

 living body cannot be studied by itself alone. Quite the con- 

 trary, the analysis of the environmental forces upon which the 

 life of the body depends and of the parts of the body itself 

 in their relations to these external forces is the very kernel of 

 the problem of life. 



The measure of the fulness of life in any organism is two- 

 fold. In the first place, the life is measured by the amount of 

 energy which the organism can assimilate from surrounding 

 nature and incorporate into its own organization. This enters 

 the body chiefly in the form of chemical potential energy in food 

 eaten, air breathed, and so on, and can be quantitatively de- 

 termined and stated in the form of standard units of energy, 

 such as calories or foot-pounds of work. This measures the 



