SPECIFIC DYNAMIC ENERGY 153 



instance, to incomplete oxidation evidenced by the appear- 

 ance of lactic acid in the urine. 



Factors Influencing the Expenditure of Energy 



Food.- — An important question has here to be settled. 

 Does the rate of metaboUsm rest with the initiative of the 

 cell or with the amount of food supphed? Can the cell 

 only be made more active through causing a physiological 

 need for enhanced activity, or can it also be made more 

 active by feeding it? It was noted by Rubner that when 

 a large amomit of protein was given there occurred an 

 increased hberation of heat. The same thing occurred 

 after ingestion of carbohydrates and fats, but to a much 

 less extent. The surplus energy thus hberated is called 

 the specific dynamic energy of the food. The cell on 

 being flooded with protein, which it is unable to store, 

 is forced to burn it, quite irrespective of any demands for 

 heat-production on the part of the body as a whole, and 

 without any increase in voluntary activity. On the other 

 hand, it may be that the presence of protein makes the 

 cell burn carbohydrates and fats more rapidly. 



There is clear evidence that the rate at which metabohsm 

 occurs is dependent upon certain chemical substances in 

 the blood, particularly those elaborated by the thyroid 

 gland. When this organ is hyper-active the metabohc 

 processes are quickened, and when it is deficient or absent 

 they are retarded. 



External Tem'perature. — Metabohsm is profoundly in- 

 fluenced by changes in the temperature of the atmosphere. 

 This will be discussed more fully in connection with the 

 regulation of body temperature. 



Muscular activity — Basal Metabolism. — It is clear that 

 in order to estimate the effect of activity upon metabolism 

 we must first try to find the energy hberated when no 

 work is being done. In theory this means when none of 

 the organs in the body are doing any work — that is, are 

 merely existing in a healthy state. This has been termed 



