Cod-liver Oil is generally obtained from the livers of the common Cod (q.v.), but likewise from allied species, as ling, dorse, coal-fish, torsk, &c. In these fish the Adipose Tissue (q.v.) containing oil is almost entirely confined to the liver, in which they agree with the shark tribe, whilst in other fish, as in the herring and salmon, the oil is diffused over the entire structure of the animal. The supplies of medicinal cod-liver oil used to be derived mainly from Newfoundland, but Norway now supplies the bulk of that used in Great Britain. The most famous of the Norwegian fishing-grounds is that of the Lofoden Islands. On the outer shores of these islands immense shoals of fish begin to arrive in December, but owing to the storms to which the coasts are liable, only a comparatively small number of boats (some 800) engage in the fishing. Later on, in the beginning of January, the fish pass in between the islands and enter the calmer waters of the Westfjörd to spawn. Then the fishermen, assembling in immense numbers, catch the fish by net or baited line. A good catch for a boat, with a crew of five men and a boy, is from 600 to 1200 fish; and some idea of the importance of this industry is obtained from the fact that in a single year these fisheries alone employed 4000 vessels, with a crew of 27,000 men, the catch amounting to 25,000,000 of fish. The fishing-grounds being near the shore, the fish are at once cleaned, and the livers removed. These may be treated in different ways, the various qualities of cod-liver oil being the result. The finest oil, known as 'non-freezing pale oil,' is obtained by warming the livers by means of steam heat, when the oil separates, and is either removed by straining or by allowing it to float to the surface. It is then cooled to ( C.), and filtered, either by mechanical pressure or in the usual way. A white tallow-like substance remains in the flannel filter, and a bright cod-liver oil passes through, which is now capable of being exposed to frost without turning turbid.
In the case of those fisheries which are situated at some distance from the shore, the livers are placed in tanks, till a sufficient quantity has been collected, with the result that partial putrefaction takes place. The oil obtained in this way is of a dark-brown colour, and possesses a nauseous empyreumatic taste.
A still coarser variety is manufactured for use in leather-making. In it the livers of the cod, herring, haddock, &c. are indiscriminately used, a very fishy-tasted oil being the result.
It will be seen from the above that the different varieties of cod-liver oil may be produced from the same livers. Thus, that obtained by pressure or steam heat from fresh livers is the freest from taste and colour, while the oil becomes darker and darker as the livers become less fresh. It has been supposed by some authorities that the dark-brown oil is the more efficacious, but others find no difference in actual practice, while the pale oil can alone be tolerated by delicate stomachs. Cod-liver oil mainly consists of oleic, stearic, and palmitic acids, in combination with glycerine. In addition to these, gaduin, a substance of biliary origin, but possessing no recognised medicinal virtues, is present along with iodine and bromine. Some have ascribed the virtues of cod-liver oil to the iodine or bromine, but as the iodine is only present in the proportion of 1 part in 2000 parts of the oil, this view seems untenable. Sometimes the oil is adulterated with spurious oils, to which iodine has been added, and this may be assumed if the proportion of iodine exceeds that stated above. There are unfortunately no chemical tests that can with certainty detect the presence of other oils, although there are tests which distinguish liver-oils from those of other origin. Thus with nitric acid, cod-liver oil yields a purple colour, changing to a brown; while a drop of sulphuric acid produces a violet colour, soon changing to brownish red. In order to prevent it from becoming rancid and fishy-tasted, it is well to keep it secluded from the air.
Cod-liver oil is at present one of the most important remedies in the Materia Medica. Although possessing a high reputation on the Continent, it was not till 1841, when Professor Hughes Bennett (q.v.) of Edinburgh wrote a treatise on it, that it came into general use in Britain. It is most valued in cases of pulmonary consumption, chronic rheumatism, and gout, besides all diseases of a scrofulous nature. In all these it is, however, useless to expect good results unless the remedy has been regularly taken for several weeks. Its mode of action is uncertain. While some hold that it merely acts as a nutrient, it is certain that pork-fat, butter, and other digestible oils, do not have the same specific action that characterises cod-liver oil. It is given in doses of from half a teaspoonful to one tablespoonful three or four times a day, and it does not seem that excessive doses, such as a cupful or more, are of any real benefit, while they are a severe strain on the digestive organs.
As its peculiar flavour is very repulsive to many patients, various devices have been adopted to render it more palatable. The addition of a trace of sulphuric ether, while rendering it more easily digested, does not improve the flavour. It may, however, be made into a jelly with isinglass, or be taken floating on the surface of porter; while in the form of Emulsion, of which many varieties are in the market, the flavour is more or less skilfully disguised. Most druggists also keep gelatin capsules, containing from one to one and a half teaspoonful, which are easily swallowed.