of arteritis; but is really a simple effect of post-mortem imbibition, as above stated.

12. MELANINE.-This is the blackish-brown coloring matter which is found in the choroid coat of the eye, the iris, the hair, and more or less abundantly in the epidermis. So far as can be ascertained, the coloring matter is the same in all these situations. It is very abundant in the black and brown races, less so in the yellow and white, but is present to a certain extent in all. Even where the tinges produced are entirely different, as, for example, in brown and blue eyes, the coloring matter appears to be the same in character, and to vary only in its quantity and the mode of its arrangement; for the tinge of an animal tissue does not depend on its local pigment only, but also on the muscular fibres, fibres of areolar tissue, capillary blood vessels, &c. All these ingredients of the tissue are partially transparent, and by their mutual interlacement and superposition modify more or less the effect of the pigment which is deposited below or among them.

Melanine is insoluble in water and the dilute acids, but dissolves slowly in caustic potass. Its ultimate composition resembles that of hæmatine, but the proportion of iron is smaller.

13. BILIVERDINE is the coloring matter of the bile. It is yellow by transmitted light, greenish by reflected light. On exposure to the air in its natural fluid condition, it absorbs oxygen and assumes a bright grass green color. The same effect is produced by treating it with nitric acid or other oxidizing substances. It occurs in very small quantity in the bile, from which it may be extracted by precipitating it with milk of lime (Robin), from which it is afterward separated by dissolving out the lime with muriatic acid. Obtained in this form, however, it is insoluble in water, having been coagulated by contact with the calcareous matter; and is not, therefore, precisely in its original condition.

14. UROSACINE is the yellowish-red coloring matter of the urine. It consists of the same ultimate elements as the other coloring matters, but occurs in the urine in such minute quantity, that the relative proportion of its elements has never been determined. It readily adheres to insoluble matters when they are precipitated from the urine, and is consequently found almost always, to a greater or less extent, as an ingredient in urinary calculi formed of the urates

or of uric acid. When the urates are thrown down also in the form of a powder, as a urinary deposit, they are usually colored more or less deeply, according to the quantity of urosacine which is precipitated with them.

The organic substances which exist in the body require for their production an abundant supply of similar substances in the food. All highly nutritious articles of diet, therefore, contain more or less of these substances. Still, though nitrogenous matters must be abundantly supplied, under some form, from without, yet the particular kinds of organic substances, characteristic of the tissues, are formed in the body by a transformation of those which are introduced with the food. The organic matters derived from vegetables, though similar in their general characters to those existing in the animal body, are yet specifically different. The gluten of wheat, the legumine of peas and beans, are not the same with animal albumen and fibrin. The only organic substances taken with animal food, as a general rule, are the albumen of eggs, the casein of milk, and the musculine of flesh; and even these, in the food of the human species, are so altered and coagulated by the process of cooking, as to lose their specific characters before being introduced into the alimentary canal. They are still further changed by the process of digestion, and are absorbed under another form into the blood. But from their subsequent metamorphoses there are formed, in the different parts of the body, osteine, cartilagine, hæmatine, globuline, and all the other varieties of organic matter that characterize the different tissues. These varieties, therefore, originate as such in the animal economy by the catalytic changes which the ingredients of the blood undergo in nutrition.

Only a very small quantity of organic matter is discharged with the excretions. The coloring matters of the bile and urine, and the mucus of the urinary bladder, are almost the only ones that find an exit from the body in this way. There is a minute quantity of organic matter exhaled in a volatile form with the breath, and a little also, in all probability, from the cutaneous surface. But the entire quantity so discharged bears but a very small proportion to that which is daily introduced with the food. The organic substances, therefore, are decomposed in the interior of the body. They are transformed by the process of destructive assimilation, and their elements are finally eliminated and discharged under other forms of combination.

CHAPTER V.

OF FOOD.

UNDER the term "food" are included all those substances, solid and liquid, which are necessary to sustain the process of nutrition. The first act of this process is the absorption from without of all those materials which enter into the composition of the living frame, or of others which may be converted into them in the interior of the body.

The proximate principles of the first class, or the "inorganic substances," require to be supplied in sufficient quantity to keep up the natural proportion in which they exist in the various solids and fluids. As we have found it to be characteristic of these substances, except in a few instances, that they suffer no alteration in the interior of the body, but, on the contrary, are absorbed, deposited in its tissue, and pass out of it afterward unchanged, nearly every one of them requires to be present under its own proper form, and in sufficient quantity in the food. The alkaline carbonates, which are formed, as we have seen, by a decomposition of the malates, citrates and tartrates, constitute almost the only exception to this rule.

Since water enters so largely into the composition of nearly every part of the body, it is equally important as an ingredient of the food. In the case of the human subject, it is probably the most important substance to be supplied with constancy and regularity, and the system suffers more rapidly when entirely deprived of fluids, than when the supply of solid food only is withdrawn. A man may pass eight or ten hours, for example, without solid food, and suffer little or no inconvenience; but if deprived of water for the same length of time, he becomes rapidly exhausted, and feels the deficiency in a very marked degree. Magendie found, in his experiments on dogs subjected to inanition,' that if the animals

'Comptes Rendus, vol. xiii. p. 256.

were supplied with water alone they lived six, eight, and, even ten days longer than if they were deprived at the same time of both solid and liquid food. Chloride of sodium, also, is usually added to the food in considerable quantity, and requires to be supplied with tolerable regularity; but the remaining inorganic materials, such as calcareous salts, the alkaline phosphates, &c., occur naturally in sufficient quantity in most of the articles which are used as food.

The proximate principles of the second class, so far as they constitute ingredients of the food, are naturally divided into two groups: 1st, the sugar, and 2d, the oily matters. Since starch is always converted into sugar in the process of digestion, it may be included, as an alimentary substance, in the same group with the sugars. There is a natural desire in the human species for both saccharine and oleaginous food. In the purely carnivorous animals, however, though no starch or sugar be taken, yet the body is maintained in a healthy condition. It has been supposed, therefore, that saccharine matters could not be absolutely necessary as food; the more so since it has been found, by the experiments of Cl. Bernard, that, in carnivorous animals kept exclusively on a diet of flesh, sugar is still formed in the liver, as well as in the mammary gland. The above conclusion, however, which has been drawn from these facts, does not apply practically to the human species. The carnivorous animals have no desire for vegetable food, while in the human species there is a natural craving for it, which is almost universal. It may be dispensed with for a few days, but not with impunity for any great length of time. The experiment has often enough been tried, in the treatment of diabetes, of confining the patient to a strictly animal diet. It has been invariably found that, if this regimen be continued for some weeks, the desire for vegetable food on the part of the patient becomes so imperative that the plan of treatment is unavoidably abandoned.

A similar question has also arisen with regard to the oleaginous matters. Are these substances indispensable as ingredients of the food, or may they be replaced by other proximate principles, such as starch or sugar? It has already been seen, from the experiments of Boussingault and others, that a certain amount of fat is produced in the body over and above that which is taken with the food; and it appears also that a regimen abounding in saccharine substances is favorable to the production of fat. It is altogether probable, therefore, that the materials for the production of fat may be

derived, under these circumstances, either directly or indirectly from saccharine matters. But saccharine matters alone are not entirely sufficient. M. Huber1 thought he had demonstrated that bees fed on pure sugar would produce enough wax to show that the sugar could supply all that was necessary to the formation of the fatty matter of the wax. Dumas and Milne-Edwards, however, in repeating Huber's experiments, found that this was not the case. Bees, fed on pure sugar, soon cease to work, and sometimes perish in considerable numbers; but if fed with honey, which contains some waxy and other matters beside the sugar, they thrive upon it; and produce, in a given time, a much larger quantity of fat than was contained in the whole supply of food.

2

The same thing was established by Boussingault with regard to starchy matters. He found that in fattening pigs, though the quantity of fat accumulated by the animal considerably exceeded that contained in the food, yet fat must enter to some extent into the composition of the food in order to maintain the animals in a good condition; for pigs, fed on boiled potatoes alone (an article abounding in starch but nearly destitute of oily matter), fattened slowly and with great difficulty; while those fed on potatoes mixed with a greasy fluid fattened readily, and accumulated, as mentioned above, much more fat than was contained in the food.

The apparent discrepancy between these facts may be easily explained, when we recollect that, in order that the animal may become fattened, it is necessary that he be supplied not only with the materials of the fat itself, but also with everything else which is necessary to maintain the body in a healthy condition. Oleaginous matter is one of these necessary substances. The fats which are taken in with the food are not destined to be simply transported into the body and deposited there unchanged. On the contrary, they are altered and used up in the processes of digestion and nutrition; while the fats which appear in the body as constituents of the tissues are, in great part, of new formation, and are produced from materials derived, perhaps, from a variety of different sources. It is certain, then, that either one or the other of these two groups of substances, saccharine or oleaginous, must enter into the composition of the food; and furthermore, that, though the oily matters may sometimes be produced in the body from the sugars,

Natural History of Bees, Edinboro', 1821, p. 330.

2 Annales de Chim. et de Phys., 3d series, vol. xiv. p. 400.