Although meats vary greatly in the amount of fat which they contain and to a much less degree in their protein content, the chief difference to be noted between the cheaper and more expensive cuts is not so much in their nutritive value as in their texture and flavor. All muscle consists of tiny fibers which are tender in young animals and in those parts of older animals in which there has been little muscular strain. Under the backbone in the hind quarter is the place from which the tenderest meat comes. This is usually called the tenderloin. Sometimes in beef and also in pork it is taken out whole and sometimes it is left to be cut up with the rest of the loin. In old animals, and in those parts of the body where there has been much muscular action, the neck and the legs for example, the muscle fibers are tough and hard. But there is another point which is of even greater importance than this. The fibers of all muscle are bound together in bundles and in groups of bundles by a thin membrane which is known as connective tissue. This membrane, if heated in water or steam, is converted into gelatin. The process goes quickly if the meat is young and tender; more slowly if it is tough. Connective tissue is also soluble in acetic acid, that acid to which the sourness of vinegar is due. For this reason it is possible to make meat more tender by soaking it in vinegar or in vinegar and water, the proportions of the two depending on the strength of the vinegar. Sour beef or "sauer fleisch," as it is known to Germans, is a palatable dish of this sort. Since vinegar is a preservative this suggests a method by which a surplus of beef may be kept for several days and then converted into a palatable dish. Flavor in meat depends mainly on certain nitrogenous substances which are called extractives because they can be dissolved out or "extracted" by soaking the meat in cold water. The quality of the extractives and the resulting flavor of the meat vary with the condition of the animal and in different parts of its body. They are usually considered better developed in older than in very young animals. Many persons suppose extractives or the flavor they cause are best in the most expensive cuts of meat; in reality, cuts on the side of beef are often of better flavor than tender cuts, but owing to the difficulty of mastication this fact is frequently not detected. The extractives have little or no nutritive value in themselves, but they are of great importance in causing the secretion of digestive juices at the proper time, in the right amount, and of the right chemical character. It is this quality which justifies the taking of soup at the beginning of a meal and the giving of broths, meat extracts, and similar preparations to invalids and weak persons. These foods have little nutritive material in themselves, but they are great aids to the digestion of other foods.