The best cod-liver oil is known as "cold-drawn oil," and is prepared from the raw fresh livers of the codfish by subjecting them to heavy pressure, by which the oil is squeezed out.

Chemical And Physical Properties

Cod-liver oil, according to analyses made by De Jongh, contains glycerin, olein (70 per cent), myristic, butyric, acetic, margaric, and other acids, biliary elements such as cholesterin, also iodine, bromine, chlorine, and phosphorus in small amount, and a peculiar material which is called "gaduin." Gautier and Mourges obtained besides a half dozen substances resembling alkaloids and other materials, a basic substance which they call morrhuine, about one half milligramme of which occurs in each gramme of the oil, and has, it is claimed, all of its active medicinal properties.

The refined oil may contain no iodine, and it is seldom present in greater amount than 0.5 per cent; hence the value of the oil does not depend upon this substance, as at one time suggested.

When pure, its odour resembles shoe leather. On adding strong acid, the biliary odour is intensified if the oil is unadulterated. If the odour is absent, the oil is spurious.

Cod-liver oil is one of the most easily absorbed of all the oils, owing to the quantity of its fatty acids. It is quickly oxidised in the body. It is highly nutritious and alterative, and is slightly laxative, especially for infants.

Some of the oil supplied in market is derived from the livers of other fishes than the cod, but the cod-liver oil is undoubtedly the best. Pains should be taken to secure a pure preparation thoroughly clarified from the impurities which oil may contain, such as traces of iodine, phosphorus, and sometimes bile salts. None of these substances add to its food value, and, if present in excess, the oil is much more likely to disagree.

The crude "straits" oil contains products of decomposition, is strong, malodorous, and of a dark-brown or even black colour. It is so disagreeable and indigestible that it possesses no advantage over the " shore " oil. The clarified Norwegian oil is the best variety to prescribe.

Cod-liver oil is sometimes adulterated with the cheaper cottonseed oil, to which a fishy flavour is artificially given, and it should therefore only be purchased from a well-known and responsible firm.

An advantage of cod-liver oil is that when pure it can be taken longer than any other fat excepting butter and cream, and in larger quantities than those substances without disordering the stomach. This is no doubt owing to the fact that the oil is already in a condition in which it has once served in the animal body for nutrition or at least for storing up energy. This form of oil is readily emulsified and saponified, and is absorbed by the intestinal villi with probably greater facility than any other fat.

Dosage

Many patients who dislike the taste of the oil at first soon acquire tolerance for it if given in small and gradually increasing dosage, commencing sometimes with not over 10 to 30 drops twice a day, and children not rarely become so fond of it as to object to its discontinuance. Infants may be given from 5 to 20 drops two or three times a day. It is so valuable a food that where its use is clearly indicated every effort should be made to secure its toleration, and even though it may at first disagree, exciting nausea and vomiting, perseverance and care in its administration may enable it to be very well borne. If it is not digested at first, but one dose of two or three teaspoonfuls a day need be given, and this should be at night.

Those who like it can sometimes take as much as an ounce three times a day with advantage, although half an ounce is all that can usually be tolerated without disturbing digestion.

The oil must be given under proper supervision if its successful use is to be continued; otherwise it defeats its own object by upsetting digestion. Patients who dose themselves with it often use too much, and almost invariably take it at the wrong time in relation to meals.

It is best to prescribe it about two hours after dinner. If given earlier, it remains in the stomach, interfering with the digestion of other food, whereas if digestion is already well under way, it passes with the chyme into the intestine, where it is absorbed. While the oil is being taken the stools should be occasionally examined, as the absence of fat globules in them is a proof of its complete absorption.