Isinglass is derived from the membrane of the swimming bladder of the sturgeon, but that of other fishes is occasionally used. It is not very soluble in the crude state, but is hygroscopic and swells very much in cold water. It dissolves in boiling water, and when the water is evaporated again hardens.

Gelatin

Gelatin is a substance the potential energy of which is calculated as being even more than that of some fats and albuminates, yet in the body it is very inferior in the production of force. It is obtained from bones, ligaments, and other connective tissues.

It is a curious and interesting property of gelatin that used alone it fails to have much nutritive power, but in proper combination with other foods it is a useful aliment. "By the addition of gelatin very large quantities of albumin can be spared in the body or devoted to increase of bulk, just as by the supply of fats and carbohydrates " (Bauer).

Gelatin itself takes no part in repair and growth of tissues - it must be regarded solely as an "albumin-sparer." It cannot, therefore, replace albumin, the loss of which still goes on to some extent even when gelatin is eaten in large quantity. It also slightly spares the consumption of non-nitrogenous materials. This is a question of considerable importance in the feeding of invalids, because, while many jellies are easily digested and are agreeable to the palate, it is useless to burden the stomach with them if they do not possess nutritive properties in proportion to other foods, and the matter must therefore be discussed somewhat in detail. The "Bone Soup Commission" of the French Academy of Sciences was constituted to determine the nutritive value of prolonged boiling of bones in order to obtain, if possible, an inexpensive form of aliment for hospital patients. The commission made elaborate researches, the result of which showed that animals which were fed exclusively upon gelatin rapidly deteriorated in strength and weight, and finally succumbed to starvation.

Subsequent experiments upon gelatin have been conducted by many physiologists with these conclusions: 1. Gelatin is innocuous, and its exclusive use will not support life. 2. Mixed with other foods, it promotes nutrition and is easily digested and absorbed. 3. To some extent it saves waste of albuminous tissues.

Since bones consist of nearly two thirds of their weight of gelatin, the latter may be advantageously used as an inexpensive means for furnishing variety in the diet by addition to meat broths and jellies, pea and bean soups, etc. M. Edwards says that the proper proportion for such mixtures should be at least one fourth of meat soup to three fourths of gelatin soup. The bones themselves may be broken and made to yield fat and gelatin for soup "stock." The amount of nourishment which they afford is extremely little, and they are mainly of service for economic reasons to save waste.

Edible birds' nests are not true gelatin, but a Chinese food product allied to mucin.

Gelatinous substance may be obtained from boiling for several consecutive hours such material as calves' feet, sheep's trotters, ox tails, etc., and after clarifying, straining, and concentrating, very palatable jellies may be made, to which chicken or mutton is added for invalid use.

Calf's-foot jelly and calf's-head jelly, if not made too rich by added ingredients, make suitable invalid dishes, especially when flavoured with sherry or Rhine wine. Ox-tail soup is too rich for the sick.

Pure white gelatin is insipid, and is almost impossible to eat in considerable quantity unless it is well seasoned. If free from all gluey taste and odour, and prepared with coffee or lemon juice, or other fruit flavours, it makes an easily digested invalid food. Or it may be combined with eggs or milk as blancmange, or with soup. The addition of meat extracts to it improves the taste, and the admixture of wine, like good sherry, alters the taste rather by the introduction of its aromatic principles than by the alcohol itself, which is largely evaporated from the jelly.

"Well-prepared jellies, not containing too much acid or pungent spices, are very useful foods for invalids, and may be administered with advantage in febrile states" (Bauer).

Dry gelatin contains 17.3 per cent of nitrogen, which is even a larger proportion than is contained in albumin; consequently urea excretion is decidedly increased by gelatin feeding. Diuresis is also produced, and the desire for liquid is intensified, so that a large proportion of gelatin in the diet causes decided physiological effects.

Fish

Fish vary both in digestibility and nutritive qualities. The chief differences are in regard to coarseness of fibre and the quantity of fat present. Fish meat is less stimulating, sustaining, and satisfying than that of birds or mammals.

Eels contain the largest proportion of fat, which amounts to 28 per cent. Herring have 7, salmon about 6.5, while sole has but 0.25 per cent (Konig). Mackerel, trout, and shad have considerable fat.

Fish which, like the salmon, are rich in flavour and in fat, while they may be very nutritious, are much less easy of digestion than are the simpler varieties, such as sole or flounders and codfish. Dried codfish can be eaten on long sea voyages day after day without the repulsion which is soon excited by the continuous diet of the more highly flavoured fatty fish.

The flesh of many fish contains a large percentage of water, besides gelatin.

The following fish, in the order named by Walker, have the largest percentage of albuminoids: Red snapper, whitefish, brook trout, salmon, bluefish, shad, eels, mackerel, halibut, haddock, lake trout, striped bass, cod, flounder.

All fish are best in their proper season, for out of season they deteriorate from change in food or other causes, and are less nutritious, besides possessing inferior flavour, and sometimes disagreeable odour. They should be eaten as fresh as possible, for there are few alimentary substances capable of exciting so violent gastro-intestinal disturbance as decomposing fish. The practice of preserving fish frozen or packed in ice is open to the objection that the cold prevents malodours from revealing commencing putrefaction. Vivid red gills and fulness and brightness of the eye are a good test of freshness.

It is a popular fallacy that fish constitute a good "brain food" on account of their containing a large percentage of phosphorus, a prominent ingredient of nerve tissue; but in reality many fish contain less of this element than meat, and neither Eskimos nor other aboriginal tribes who live largely upon fish are noted for intellectuality.

Some fish contain different species of tapeworm, but they are seldom if ever transmitted to man.

Fish having white meat constitute an excellent food for invalid diet, and when cooked by boiling or broiling (not frying) they may be given to convalescents and to those with feeble gastric powers.

The most digestible fish are fresh sole, whiting, bluefish, white-fish, bass, red snapper, fresh codfish, halibut, shad, and smelt. Pavy says: " Of all fish, the whiting may be regarded as the most delicate, tender, easy of digestion, and least likely to disagree with a weak stomach. The haddock is somewhat closely allied, but has a firmer texture and is inferior in flavour and digestibility".

Crimping is a process sometimes applied to fresh fish, like the cod, by which the firmness of the flesh, as well as its flavour, is increased. As soon as caught, the fish is incised transversely by numerous deep cuts. On being plunged into ice-cold water, the muscle fibres contract firmly and so remain.

As a rule, dried, smoked, or pickled fish should not be given to invalids, although thoroughly boned and desiccated or " shredded " codfish is quite tender. The latter process is now conducted by machinery, and thus prepared the fish requires less prolonged soaking and cooking.

Fish roe is not very nutritious, and it serves mainly as a relish. Shad roe, thoroughly cooked, is not objectionable, but sturgeon's roe or caviare, which is sometimes used as an "appetiser," is capable of arresting digestion, especially when old, black, or rancid. Caviare contains, according to analyses by Konig and Brimmer, water, 45.05; proteids, 31.90; fat, 14.14; salts, 8.91 per cent.

It must not be forgotten that some persons cannot digest fish of any kind, or at most can take but one or two varieties without provoking an attack of dyspepsia or biliousness. A few tropical fish are poisonous, and constitute an exception to the general edibility of vertebrate animals. (See Fish Poisoning).

There are no diseases in which a fish diet possesses specific value, but often in chronic Bright's disease, lithaemia, gout, or other conditions in which it is undesirable to give much meat, it is very serviceable as a compromise.