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nitrogen in excess appears to be eliminated by the kidneys in the shape of a superabundance of urea, of uric acid, or urate of ammonia; the carbon in excess, by the deposit of fat in the tissues, or by a superabundant secretion of bile. The urea normally found, under all circumstances, in the urine, of which it may be termed the principal component, is no doubt principally the result of the normal wear and tear, or disintegration of tissue. Uric acid and urate of ammonia are also present in normal urine, but in minute proportion. Their presence in excess, as also that of urea, may be looked upon as the evidence either of an over-supply of nitrogenous food material or of diseased nutritive action.

In the process of normal nutrition, the different tissues of the economy have the power of extracting from the blood the various salts which they require. Thus it is that the bones and the brain extract the phosphate of lime which forms so important a part of their composition, and that other tissues select the salts which predominate in their structure. In each case the selection is made by their own inherent force of vitality. When these tissues have lived their time and are disintegrated, the chemical elements of which they are composed are thrown out of the economy to make way for new molecular formations.

The carbon and hydrogen, as we have seen, are mostly consumed in the production of heat, by their combination with oxygen to form carbonic acid and water. The carbon which has contributed to form the nitrogenous tissues is probably eliminated, in part at least, by the liver in the shape of bile.

The nitrogen is eliminated in the urine by the kidneys, the great nitrogen emunctories or purifiers of the blood. Whether healthy or morbid, the urine principally contains salts and nitrogenous products, such as creatinine, urea, and its compounds. The kidneys may be said to be large secreting filters, the object of which, in health, is to throw off the surplus fluids taken into the system, and to eliminate the soluble products of the disintegration of tissue. To the kidneys are also intrusted, as we have seen, the duty of eliminating from the blood the nitrogenous elements of food, imperfectly digested, imperfectly chylified, and, as such, unfit for assimilation. When we consider that all the blood in the body passes through the kidneys every few minutes, we shall better be able to appreciate the very great preservative powers which they exercise in this respect,-powers which are very generally overlooked, and to which we shall at a later period devote careful attention

FLUID FOOD, ITS ELABORATION AND DESTINATION.

We have said little as yet of beverages or of fluid food. Water, the basis of all, is composed of oxygen and hydrogen, and may no doubt be decomposed, and yield its elements to the animal economy in its passage through the system. It is probable, however, that to a great extent, it retains its chemical integrity during the different phases of its journey, and acts merely as a solvent. In this latter capacity it is all-important, holding in solution the solid components of the blood, contributing to form the tissues constructed by the molecular nutritive process, and again acting as the solvent of the used-up elements of our tissues on their disintegration and definitive elimination. Moreover, there is a constant demand on the part of the animal economy to supply various excretions and secretions. The lungs eliminate, during respiration, as we have seen, a considerable quantity of fluid daily. The same may be said of the skin, from which insensible perspiration is constantly carrying off vapour. Moreover, there are many glands, the lachrymal, the salivary, the intestinal, which make large demands on the system for water as the basis of their various secretions.

Water is contained in great abundance in all kinds

of solid food, but in much greater proportion in vegetable than in animal substance.

Thus water is abundantly introduced into the system whenever solid food is taken.

The supply thence obtained is, however, quite insufficient for the wants of the economy, a fact which explains the universal craving for fluid. The quantity of water required by the adult human economy has been calculated at about two pints during the twenty-four hours, independently of that which is contained in the solid food. This amount may be considered to represent the fluid requirements in cold or temperate weather. In warm weather, the insensible cutâneous exhalation is very sensibly increased, and the craving for fluid rises with the temperature. When the heat is very great, and approaches to or exceeds that of the body, the cutaneous exhalation and the desire for fluids are both extreme. Nature adopts this means of getting rid of the superfluous heat generated by the economy, which is no longer carried off by the cool circumambient atmosphere. The vaporization of the cutaneous perspiration on the surface of the body carries off, in a latent form, the extra heat, and keeps the body at its usual temperature. Thus is explained the relief which the free perspiration of a moist skin affords in warm weather, and the uncom

fortable, burning sensation which attends dryness of the cutaneous surface under similar circumstances.

It is worthy of remark, that the absence of fluids is much more difficult to bear than that of solid food, so necessary is water for the vital processes. Life may be sustained for weeks without solid food, if fluid is supplied; but in the entire absence of the latter, death closes the scene in a few days. In a case of tetanic hysteria which I attended a few years ago, the patient, a young lady of eighteen, was at last seized with convulsive vomiting every time she attempted to take food. For five weeks she apparently rejected every particle of solid food that she endeavoured to swallow, and yet remained in the full possession of her faculties, although much emaciated. After that time, the vomiting extended to fluids, which she could previously swallow in small quantities, when she sank rapidly, dying on the fourth or fifth day.

The animal creation are satisfied with water; indeed they show dislike and repugnance to all other beverage. Man alone, especially in a civilized state, seeks to combine food and nervous stimulants with his beverage: the latter may thus be divided into nutritious or stimulating.

Nutritious beverages such as milk, soups, cocoa, &c., contain in solution the same elements of nutri

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