┌────────────────┬─────────────┬─────────────┐ │ │ HUMAN MILK. │ COW’S MILK. │ ├────────────────┼─────────────┼─────────────┤ │_Casein_ │ 2.4 │ 4.0 │ │_Albumin_ │ .6 │ .9 │ │_Fat_ │ 2.9 │ 3.5 │ │_Sugar_ │ 5.9 │ 4.0 │ │_Salts_ │ .16 │ .7 │ │ │ ──── │ ──── │ │ _Total Solids_ │ 11.96 │ 13.1 │ │ _Water_ │ 88.04 │ 86.9 │ └────────────────┴─────────────┴─────────────┘ It is evident from this analysis of milk that our food must contain (at least) representatives of all the above divisions. We have, therefore:— 1. =Nitrogenous Foods.= 2. =Hydrocarbons or Fats.= 3. =Carbohydrates or Amyloids.= 4. =Salts.= 5. =Water.= Condiments and stimulants (tea, coffee, alcohol) are not foods in the strict sense of the word, and will be discussed in a later chapter. =Nitrogenous Foods= include albumin, casein, gluten, legumen, fibrin, and gelatin. They all agree in consisting of a complex molecule containing many atoms of carbon, hydrogen, oxygen, and nitrogen, with the addition of smaller quantities of sulphur, and in some cases phosphorus. The nitrogenous substances used as food may be divided into two groups, (_a_) those containing gelatin, and (_b_) numerous bodies which receive the common name of proteids or albuminoids. The percentage composition of gelatin is:— ┌───────────┬───────────┬───────────┬───────────┐ │ CARBON. │ HYDROGEN. │ NITROGEN. │ OXYGEN. │ ├───────────┼───────────┼───────────┼───────────┤ │ 50.0 │ 6.6 │ 18.3 │ 25.1 │ └───────────┴───────────┴───────────┴───────────┘ The percentage composition of all proteids lies within the following limits:- ┌───────────┬───────────┬───────────┬───────────┬───────────┐ │ CARBON. │ HYDROGEN. │ NITROGEN. │ OXYGEN. │ SULPHUR. │ ├───────────┼───────────┼───────────┼───────────┼───────────┤ │ 52.7 to │ 6.9 to │ 15.4 to │ 20.9 to │ 0.8 to │ │ 54.5 │ 7.3 │ 16.5 │ 23.5 │ 1.6 │ └───────────┴───────────┴───────────┴───────────┴───────────┘ Proteids also contain a small amount of phosphorus, chiefly as phosphate of lime, but also in minute quantity in their essential structure. Various proteids are used in food, _e.g._ serum-albumin in the blood and tissues of animals; egg-albumin in the white of eggs; myosin in flesh; casein in milk; legumin, or plant-casein, in the seeds of leguminous plants; gluten in wheat-flour, etc.