[Footnote 2: Marshall, _Microbiology_.] CHAPTER XI FISH PRODUCTS The industrial value and importance of fishes is by no means limited to their use as food. They yield large quantities of valuable oil. The fish waste, or offal, chiefly heads, skins, bones and viscera—that is discarded by the fish curer, is worked up to yield fish glue, fertilizers and cattle food. The skins of certain large fishes, for example the shark, are tanned and manufactured into a valuable leather. The story of the fishing industry would not be complete without a brief description of the methods by which these products are manufactured. =Fish Oils.= The various kinds of oil that are obtained from different species of fish and other marine animals, such as whales and seals, may be divided into three classes, according to the part of the fish from which they are extracted. (1) Fish oils proper are disseminated throughout the flesh of the fish in the form of fine globules. They are extracted from the entire fish, e.g. herring, sardine, sprat, menhaden. (2) Liver oils are located in the fish liver, e.g. cod, shark. (3) Blubber oils constitute a thick layer of adipose tissue just under the skin of the marine mammalia, e.g. whale, seal, dolphin, porpoise. In oily fish, such as herrings and sprats, each minute globule of oil is enclosed within a thin skin. It is practically impossible to rupture this skin and liberate the oil simply by the application of pressure. When, however, these globules are heated the skin shrivels, the oil globules expand and burst the skin, and the liquid oil is liberated and can then be extracted from the flesh by pressure. To obtain the oil, therefore, the fish are boiled or steam heated in large vats until the oil is set free. The hot mass is then placed in a press and the oil squeezed out. The residue is made into cattle food and fertilizer. In obtaining the best sorts of liver oils, e.g. codliver oil, the livers are taken from the fish as soon as they are caught, and are heated in steam-jacketed vessels until the cell membranes burst and the oil exudes. The oil is then separated by pressure. Inferior qualities of oil are obtained by treating putrid livers in the same way at the end of the voyage. These tainted liver oils are unfit for medicinal purposes, but are used in large quantities in the leather industry.