The fat of fish comprises a smaller proportion of the compounds of solid fatty acids than does the fat of land animals. It is mainly composed of the glycerides of various unsaturated acids. The fish-liver oils commonly contain certain bile products (which give rise to characteristic reactions in colour with acids, and alkalies). A considerable proportion of unsaponifiable matter, chiefly cholesterin, is also a usual constituent thereof. Iodine is sparingly present in fish, but the significance of its occurrence is yet obscure. Salt fish is but slowly dissolved in the stomach, because its fibres have become hardened by the salt. Fish oil for medicinal purposes is obtained principally from the Cod, but also from the Pollock, Turbot, Ling, and Dorse. The milt, or soft roe, is the spermatic organ and its secretion (a sexual stimulant?) of the male fish; whilst the ovarian spawn, or hard roe, is that of the female fish. Hufeland, and others, have found the soft roe of herrings useful against tubercular consumption affecting the windpipe.

Considered widely, a diet comprising frequent fish, always fresh, and of proper quality, plainly cooked, is certainly calmative for excitable persons of vivaciously nervous temperament.

Nevertheless, Shakespeare has told of others who :-

"Making many fish meals, Fall into a kind of male green sickness".

Proteid, and fat, are the chief nutritive constituents found in fish, of which the value as a source of energy depends upon the amount of contained fat. Fish further includes a considerable quantity of waste substance in skin, bones, etc. Lean fish are better tolerated by the stomach than the fat ones, and are apparently more easily digested, as a rule, than the same quantity of lean meat. In hot weather, and for sedentary persons, white fish, plainly cooked, is better than meat. Boiled Haddock is very suitable for an invalid, but containing innumerable small bones. Finnan Haddies, cured and dried at Findhorn, near Aberdeen, were originated through a fire in one of the fish-curing houses at Port Lethen, on the North Sea, which fire partly burnt a pile of lightly-salted, freshly-caught Haddock lying on beds of dry kelp. After the flames were extinguished these smoked fish were found to be so delicious to the taste, that from then until now no one at Port Lethen, or the larger fishing village a mile away (Findhorn), has ever cured a Haddock except by smoking it over seaweed.

With respect to fish as specially stimulating the sexual functions, this opinion is open to question, and Dr. Pereira has pointed out the significant fact that maritime populations are not especially prolific. In the time of Elizabeth, on great occasions the stewards of noblemen provided dinner for their lord's guests; beef, and venison for the rich, but salted fish, then known as "Poor John," also apple pies, for the humbler visitors. Beating the rolling-pin on the dresser served as a dinner bell. In the middle ages fish was a luxury obtainable only by the rich, and, except near the coast, it could never have been served in anything like a fresh condition, the consequence being that smaller folk had to subsist on fish imperfectly salted, (particularly during the Lenten Fast), and disastrous effects on the skin followed. Pepys complained: "Notwithstanding my resolution, yet for want of fish, and other victuals, I did eat flesh this Lent." Sir Henry Thompson has advised that as a rule fish should be roasted (in a Dutch, or American oven), that is, cooked by radiated heat, so that none of its juices may be dissolved away, and lost. Mattbieu Williams commends equally for this purpose the side oven of a kitchen range, or a gas oven, these being practically roasters.

He directs that as a matter of course the roasted fish shall be served in the dish wherein it is cooked. Here is a way of dressing a fish to make it taste excellent, if you are camping out far afield: "Take some nice clean clay, and work it up a little; then, without either scaling, or dressing, plaster your fish (fresh from the water) all over with the clay, about an inch thick, and put him right into the hot ashes. When 'tis done, the clay, and scales will all peel off, and you'll have a dish that would bring to life any starved man if he hadn't been dead more than a week! That's the ordinary way, but if you want an extra touch, cut a hole in the fish, and stick in a piece of salt pork, and a few beech-nuts, or meat of walnuts, or butternuts, and you'll think you are eating a water-angel." Many sorts of fish will break if suddenly immersed, for cooking, in water under agitation by boiling, which misfortune may be prevented by not allowing the water to actually boil at all from beginning to end of the cooking. Otherwise, not only does the breaking disfigure the fish, but it further opens outlets by which the juices escape, and thereby depreciates the flavour, besides sacrificing some of the nutritious albumin.

Izaak Walton advised that. "lying long in water, and washing the blood out of any fish after they be gutted, abates much of their sweetness. You will find, for example, the Chub being dressed in the blood, and quickly, to be such meat as will recompense your labour, and disabuse your opinion of him; yet the French esteem him so mean as to call him 'Un villain.' "

Respecting the Pike, it is observed by Gesner that "the jawbones, and hearts, and galls of Pikes are very medicinable for several diseases, or to stop blood, to abate fevers, to cure agues, and to be many ways medicinable, and useful for the good of mankind." The practice obtains generally with doctors to advise convalescent patients that they should first resume animal diet after a severe illness by taking a Sole, lightly and plainly cooked. This fish has a very delicate flavour, and is easily digested by an invalid. To stew the same in milk, carefully lift the fillets from a very fresh Sole, then roll each piece of fish, and fasten with white tape; lay the fillets in a perfectly clean stewpan, and cover them with new milk; season with a little salt, and simmer very gently until tender. The salts of potash, and phosphate of lime thus supplied, are highly nutritious mineral constituents, whilst the comparatively small quantity of proteids is an advantage. An easy way to test the freshness of such fish is to press a finger on the flesh, when, if fresh, it will be firm, and elastic, but if it be stale, then an indented impression is made in the soft flesh. Again, Whiting may be similarly allowed when baked in milk.