O'SHEA A8PECTS OF MENTAL ECONOMY. 77 



Tier as to preserve right proportions between them in ministering 

 to the needs of the organism, no matter whether we depend upon 

 appetite, chemical analysis, or custom as our guide; a point 

 which will receive fuller discussion in another place. The prin- 

 cipal constituents of foods are: fats, carbohydrates, albumen (or 

 protein), and salts ;* and the uses to which each of these are put 

 in building up, repairing, and sustaining mental and motor ac- 

 tivity are satisfactorily stated by Church, 2 whose treatment of 

 the subject may here be adopted. 



Class I. — Nutrients. 



Division 1. — Incombustible Compounds. 



Group i. Water — The carrier of nutritive materials and waste prod- 

 ucts; forms an essential part of all tissues; is present in large 

 proportion where change is most active. 



Group ii. Salts or Mineral Matter, such as common salt and phosphate 

 of lime, which serve to effect changes and build up certain tissues. 



Division 2. — Combustible Compounds. 



Group iii. Carbon Compounds, such as starch, dextrin, sugar, and fat, 

 which serve to keep up the heat and movements of the body by 

 the discharge of their potential energy during oxidation in the 

 organism. The fat of the body is formed in part from fat or oil 

 in the food. The members of this group are often called "heat- 

 givers," a term which is equivalent to "force-producers." 



Appendix to Group iii. Gum, mucilage, and pectose, approach starch in 

 chemical composition, and probably serve the same end. Cellulose 

 may be named here, but its value as a nutrient is doubtful. 



Group iv. Nitrogen Compounds, such as albumen, myosin, and casein, 

 the chief formative and reparative compounds of food; they also 

 may yield fat, and by their oxidation set free heat and motion. 

 They are often named "flesh-formers," while the group is known 

 as albuminoids. 



1 Martin, Human Body, chap. XXI, employs a more detailed classification, but 

 it is common now among writers on dietetics to speak only of the four great 

 classes of nutritive elements, and this will be deemed sufficiently detailed for our 

 purposes. 



i Food, p. 9, (London, 1889, Chapman and Hall). 



