Sweetbread, which is thymus gland of the calf, is a delicate and agreeable article of diet, particularly for invalids. Tripe, heart, liver and kidneys are other forms of animal viscera used as food--valuable chiefly as affording variety. FISH. The chief difference in fish is the coarseness of fibre and the quantity of fat present. Fish which are highly flavored and fat, while they may be nutritious, are much less easy of digestion than flounder, sole, whitefish, and the lighter varieties. The following fish contain the largest percentage of albuminoids:--Red snapper, whitefish, brook trout, salmon, bluefish, shad, eels, mackerel, halibut, haddock, lake trout, bass, cod and flounder. The old theory that fish constituted "brain food," on account of the phosphorus it contained, has proved to be entirely without foundation, as in reality many fish contain less of this element than meat. The tribes which live largely on fish are not noted for intellectuality. Fish having white meat when broiled or boiled--not fried--are excellent food for invalids or people of weak digestion. Fish should be well cooked. OYSTERS. Oysters are a nutritious food, and may be eaten either raw or cooked. Lobsters, crabs and shrimps are called "sea scavengers," and unless absolutely fresh are not a desirable food. MILK. Milk contains all the elements which are necessary to maintain life; and constitutes a complete diet for infants. It will sustain life in an adult for several months. Although milk furnishes a useful food, it is not essential to a diet required for active bodily exercise. It is seldom given to athletes while in active training. Adults who are able to eat any kind of food are kept in better health by abstaining from milk, except as used for cooking purposes. An occasional glass of hot milk taken as a stimulant for tired brain and nerves is sometimes beneficial. Milk is composed of water, salts, fat, milk sugar or lactose, albumen and casein. Average milk has from 8 to 10 per cent. of cream. Good milk should form a layer of cream about 2-1/2 in. thick as it stands in a quart bottle. Lactose (milk sugar) is an important ingredient in milk. It is less liable to ferment in the stomach than cane sugar. In the presence of fermenting nitrogenous material it is converted into lactic acid, making the milk sour. Casein is present in milk chiefly in its alkaline form, and in conjunction with calcium phosphate. Milk absorbs germs from the air and from unclean vessels very readily. Good, clean, uncontaminated milk ought to keep fresh, exposed in a clean room at a temperature of 68 deg. F., for 48 hours without souring. If the milk is tainted in any way it will sour in a few hours. Boiled milk will keep fresh half as long again as fresh milk. Milk absorbs odors very quickly, therefore should never be left in a refrigerator with stale cheese, ham, vegetables, etc., unless in an air-tight jar. It should never be left exposed in a sick room or near waste pipes.