Fats are not dissolved--that is, in the sense in which meats and other foods are dissolved--in the process of digestion; the only change which they undergo is a minute subdivision caused principally by the action of the pancreatic juice. In this condition of fine emulsion they are taken up by the lacteals; they may also be absorbed by the blood-vessels. It has been found that fat emulsions pass more easily through membranes which have been moistened with bile, and it is probable that the function of bile is partly to facilitate the absorption of fat. That the pancreatic juice is the chief agent in forming fats into emulsion was discovered in 1848. Bile is, however, essential to their perfect digestion, and we may therefore say that they are digested by the united action of the pancreatic juice and the bile.[27] Fat forms in the body fatty tissues, and serves for muscular force and heat; it is also necessary to nourish nerves and other tissues,--in fact, without it healthy tissues cannot be formed. A proper amount of fat is also a sort of albumen sparer. It is probable that the fat which is used in the body either to be stored away or for energy, is derived from other sources than directly from the fat eaten. From experiments made by Lawes and Gilbert on pigs, it is evident that the excess of fat stored in their bodies must be derived from some other source than the fat contained in their food, and must be produced partly from nitrogenous matter and partly from carbohydrates, or, at least, that the latter play a part in its formation. It would appear from this that life might be maintained on starch, water, salts, and meat free from fat; but although the theory seems a good one, practically it is found in actual experiment[28] that nutrition is impaired by a lack of fat in the diet. The ill effects were soon seen, and immediate relief was given when fat was added to the food. Besides, in the food of all nations starch is constantly associated with some form of fat; bread with butter; potatoes with butter, cream, or gravy; macaroni and polenta with oil, and so forth. A man may live for a time and be healthy with a diet of albuminoids, fats, salts, and water, but it has not yet been proved that a similar result will be produced by a diet of albuminoids, carbohydrates, salts, and water without fat. Fat is necessary to perfect nutrition. Health cannot be maintained on albuminoids, salts, and water alone; but, on the other hand, cannot be maintained without them. Probably the value of fats, as such, is dependent upon the ease with which they are digested. The fats eaten are not stored in the body directly, but the body constructs its fats from those eaten, and from other substances in food,--according to some authorities from the carbohydrates and proteids, and according to others from proteids alone.