On account of the great importance of the fundamental energy expenditure both for the study of normal nutrition, and as a basis for comparison in the investigation of disease, the experiments above described have been followed by others designed to establish with even greater exactness the "basal metabolism" which goes on when the direct effect of food is excluded and when muscular activity is suppressed as completely as possible. Experiments of this latter type must necessarily be carried out in shorter periods than were used in the Atwater investigations described above. Being shorter, they can be more frequently repeated and more readily extended to cover a larger number of individuals.

Data obtained in such studies of "basal metabolism" will be cited later in connection with the study of the various conditions which influence the energy metabolism and total food requirement.

A systematic analysis of the maintenance requirement of the body with reference to its principal functions has not yet been made, but results obtained by Armsby, Atwater, Benedict, Lusk, Magnus-Levy, Rubner, Zuntz, and others indicate that in the healthy adult the expenditure of energy when at rest and no longer influenced by the direct effect of food ("basal energy metabolism") may be attributed in part, perhaps up to one tenth, to the work of the heart in maintaining the circulation ; from one tenth to two tenths to the muscular work of respiration; from one third to one half, or perhaps even more, to the maintenance of muscular tonus (tone, tension, elasticity); and an unknown fraction to other forms of internal work.