Butter is a milk product in daily use, and is one of the foods most used in our diet; and as it is consumed in connection with carbohydrates, we will refer to it later, when discussing the question of carbohydrates generally; but we may mention here that butter must be taken only in a fresh condition, and it should not contain a greater proportion of salt than 2.5 grains per ounce, for reasons we have so often insisted upon in our general remarks on food when referring to common salt, and also in the chapter on the hygiene of the kidneys. When butter is in a rancid condition it produces acid fermentation in the stomach, and also disorders of the intestinal functions. Cheese is a milk product very rich in fat, consisting of the coagulated casein of the milk fats and salts. American, Canadian, and English cheese are manufactured from pure milk, while the majority of cheeses of other manufacture are made from skimmed milk. A very nutritive cheese is made in Norway from the pure milk of goats; this has a very pleasant taste and is very easy to digest. By moderately pressing fresh curds cream cheese is made; and we are of the opinion that in this form it is more hygienic than old cheese, and we therefore give the preference to cream cheese, or to cheese made from pure milk that is not old or sour. Cheese is a very valuable article of nourishment on account of the large amount of albumin and fat that it contains. Gervais and other sorts of cream cheese have a very high percentage of fat. Dr. Haig[264] recommends cheese as a valuable article of food in the dietetic treatment of uric acid diathesis. It has also the great advantage of being able to check intestinal putrefaction, owing to its milk and fatty acid contents. Footnote 264: Haig: Loc. cit. On the other hand, sometimes very old cheese may cause intestinal putrefaction, with symptoms of intoxication, and serious catarrh of the intestines. Professor Vaughan, of Ann Arbor, found toxic ptomaine bodies in cheese and old and stale milk. Many people are unable to digest cheese well; others develop skin eruptions or acne after eating it; but, for those who can take it, it is very valuable as an article of diet when a lacto-vegetarian regimen is followed, as suggested in our general remarks on diet. In addition to milk and meat, the next most important animal food is eggs, which are very rich in a most soluble animal albumin, and also in a substance which plays an important part in the structure of the nervous system—lecithin. According to König[265] chicken’s eggs have, in their natural watery condition, 13 per cent. of albumin and 0.3 per cent. of fat; and 89 per cent. of albumin and 2 per cent. of fat in the dried substance of the white part; whereas the yolk, in the natural watery state, contains 16 per cent. of albumin and 32 per cent. of fat, while, if dried, 33 per cent. of albumin and 65 per cent. of fat. Eggs also contain much lime. Footnote 265: