Origin

Cod-liver oil is obtained from the livers of Gadus Morrhua, or the common cod, and several other species of the same genus, inhabiting the waters of the Atlantic, near the shores of Northern Europe and America. It is prepared either by exposing the livers in mass to the heat of the sun, and skimming off the oil as it rises; or by boiling them into a pultaceous mass with water, and straining; or by expression.*

* Besides the oil derived from different species of Gadus, which is probably identical or nearly so, that of the livers of fishes of other genera has been tried, for some of which virtues are claimed very nearly or quite equal to those of cod-liver oil. Among these fish is the dog-fish (Squalus Catulus), the product of which M. Devergie believes to be superior to the officinal in sensible properties, while therapeutically it is scarcely inferior. (See Am. J. of Med. Sci., April, 1860, p. 613.) The same is asserted of the Dugong oil, obtained from the superficial fat of two species of Ilalecore (II. Australis ami //. Dugong), inhabiting the Australian rivers and bays. (U. S. Dispensatory, 12th ed., p. 588.) - Note to the third edition.

Sensible Properties. Its consistence is that of ordinary fish-oil. In its purest form, it is of a colour varying from the slightest tint of transparent yellow to a fine golden yellow; when less pure, of a light-brown colour, but still transparent; when most impure, dark-brown and opaque in mass, though transparent in thin layers. Its odour and taste are quite peculiar, scarcely disagreeable in the finer kinds, but offensive in the most impure, which are also somewhat acrid. The oil is injured by long exposure to the air.

Composition

It contains a peculiar principle called gaduin, not known to have any medicinal virtue, various biliary principles, a little iodine, olein and margarin, and many other constituents of no special interest.

Characteristics. Its most obvious characteristic properties are its odour and taste, quite different from those of ordinary fish-oils, and strongly resembling those of shoe-leather, which owes these properties to the cod-liver oil used in its preparation. Another distinctive property, derived from its biliary constituents, is that of assuming fine changes of. colour under the action of the mineral acids. A third peculiarity is that, when heated with potassa or lime, and muriate of ammonia, it yields a smell like that of herring-pickle, owing to the formation of a peculiar volatile alkaloid called propylamin. It is frequently adulterated with other oils.

Much has been said in reference to the superiority of one or another variety of cod-liver oil. I believe they are all equally effectual if equally pure, and equally acceptable to the stomach. The darker kinds, which are at the same time very offensive to the smell and taste, often disagree with the stomach, and therefore operate less favourably than the lighter-coloured; but I always regard the possession of the peculiar odour as an essential indication of efficiency; and any specimen of the oil, not possessing it, may be regarded with suspicion.

Effects on the System

When taken in the ordinary doses, cod-liver oil, in the great majority of cases, produces, for some time, no observable effect upon the system. In two or three weeks, however, a fattening process usually commences; and there is a gradual increase of weight, with a moderate exaltation of the functions generally, and an augmentation of the red corpuscles of the blood. In some persons, the medicine produces nausea and disagreeable eructations, and. when the gastric sensibilities are extremely acute, even vomiting; but these results are most frequently dependent rather on its offensive taste, than its direct influence on the stomach; and, if the oil is taken without exciting disgust, are very rare in a healthy state of the organ. Occasionally the functions are over-excited; the medicine proving laxative, diaphoretic, or diuretic, and even augmenting the menstrual flux; and I have very often noticed, after its continued use for two or three months, a decided odour of the oil exhaling from the body, distinctly observable upon a near approach. At length the system seems to be accustomed to its use, and no further change is produced.

In cases of debility and emaciation, with an anemic state of the blood, the alteration is frequently very striking. Beginning two or three weeks from the first use of the medicine, the patient often rapidly fattens, Unhealthy colour returns, the pulse, instead of being excitable and weak, becomes full, strong, and equable, the appetite and digestion improve, and a healthy vigorous tone of mind takes the place of the previous languor, listlessness, or depression. These effects are all characteristic of a tonic operation, and, as in the case of other tonics, may be carried to excess, so as to produce a plethoric state of system, which is a result to be guarded against in the use of this remedy.

Mode of Operation. It is supposed by some that cod-liver oil acts merely as a nutrient, and differs from other articles of diet only in consequence of its more ready assimilation. But from no other nutritious substance, and no combination of such substances, can equal effects be obtained, under the same circumstances. Surely there are many articles of food, much more analogous in constitution to our own tissues, and even more readily digested, which, in the ordinary condition of the system, nourish it even better than this oil could do, but utterly fail in the morbid states in which it proves so efficacious. It has been said that it fattens by simply supplying oil, in a state in which it can readily enter the blood, and that other oils of easy assimilation will answer the same purpose. But it does not simply fatten. It improves the digestive pro-cess, increases the proportion of red corpuscles in the blood, and invigorates the whole nutritive function. The mere increase in the proportion of fat in the system is one of the least important of its results. Besides, other fats do not produce the same effects. Butter, fat pork, the fat of beef, mutton, and veal, olive oil, and various other oleaginous substances, are often largely consumed by the very individuals, who afterwards find relief, by the employment of cod-liver oil, from morbid conditions which had arisen under the use of these articles of diet. Nothing has been more common, in the hospital of which I have until recently had charge in the winter season, than to see consumptive seamen rapidly improve in their condition, under the influence of the oil, though they may have been previously consuming, on shipboard, much larger quantities of oily matter in the shape of fat pork. It is true that different practitioners have suggested different oleaginous substitutes for codliver oil; each maintaining that he has obtained satisfactory results from the one specially recommended. Thus, whale oil. and other kinds of fish-oil, olive oil, almond oil, poppy oil, etc., have been recommended; but. though most of them are much more readily obtainable than pure cod-liver oil, not one of them has held its ground, and secured the confidence of the profession generally, simply because it has failed upon a more enlarged trial. It is proper to state that Dr. Theophilus Thompson, physician of the Brompton hospital for consumptive persons, states, in his published clinical lectures on pulmonary consumption (Am. ed., p. 128), that he has found cocoa-nut oil to possess properties similar to those of cod-liver oil, and to bear comparison with that in its effects; but, if this estimate should prove correct, it would simply prove a close analogy in properties between the two oils, and not that they acted merely as nutrients. From these considerations, 1 think it must be admitted that cod-liver oil has positive medicinal properties; and the best explanation, I think, of its operation is, that it possesses the power of directly stimulating the blood-making and nutritive functions, in a manner analogous to that of other tonics, and, in certain cases, more effectively than they. Whether its virtues depend upon a peculiar principle, the co-operation of two or more principles, or its general constitution, has not been determined. Some have ascribed its powers to the iodine and bromine it contains; but these are in too small a proportion to exert much influence on the system; are incapable, when given alone, of producing the same effect; and have quite failed when they have been given in combination with other oils, as has sometimes been done in the hope that the artificially ioduretted oil might prove a sufficient substitute for that of the cod.