Solid food is dissolved in the alimentary canal by the watery secretions derived from the blood. Water swallowed as food, begins to pass on into the intestine at once. The statement that free consumption of water at meals delays digestion by diluting the gastric juice is therefore not well grounded. In the blood, water serves to carry nutrient materials to all the tissues; and, at the same time being circulated all over the system, equalises the temperature, favours chemical changes, and washes all the tissues. By water again, the effete matters which have been separated by the kidneys are washed out of its tubes. The =Oxygen= of the air, in a broad sense, forms one of the foods of the system. This will be considered later. Besides the above classification, foods have also been classified as follows:— 1. _Inorganic food_—Oxygen, salts. {Animal {Nitrogenous. { {Non-nitrogenous. 2. _Organic foods_ { {Vegetable {Nitrogenous. { {Non-nitrogenous. Or, as— {Animal {Nitrogenous. { {Non-nitrogenous. 1. _Solid foods_ { {Vegetable {Nitrogenous. { {Non-nitrogenous. {Water. 2. _Liquid foods_ {Milk and its products. {Tea and similar beverages. {Alcoholic beverages. 3. _Gaseous foods_—Air. CHAPTER III. THE VARIETIES OF FOOD. NITROGENOUS ANIMAL FOODS.—These are divided into two groups, the one containing gelatin, and the other all the proteid or albuminoid substances, which are taken in the flesh of various animals, and in milk and eggs. =Gelatin= is obtainable from bones, and from connective tissue wherever found. Being easily digested, and absorbed, it has been very popular as an invalid’s food; but the fact that animals cannot sustain life on it without the addition of proteids proves that its value is limited. It is incapable of building tissues, but is a valuable _proteid-saver_, being able to save from metabolism half its weight of proteid, or twice as much as is spared by an equal weight of carbohydrate. Its utility in this direction is, however, limited, because of the dilute form in which it is taken in ordinary foods. It is useful for invalids, partly because it forms a bulk, and prevents the evil tendency to give their food in too concentrated a form; partly because it forms a source of easily metabolised material, and so prevents tissue-waste; and partly because it commonly contains phosphate of lime, derived from the bones forming the source of gelatin.