Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/741

NUTRITION 683 cheeks or escape at the nose, are reabsorbed from the nasal mucous membrane. With respect to the alimentary secre tion, it is merely a small proportion of the bile which remains in the faeces that is to be regarded as truly excretory in the sense of being utterly lost to the organism. If excreta are those substances which the body rejects utterly, then we may reckon as excreta (1) the urine, (2) the sweat and oily secretions of the skin, (3) the milk, (4) certain elements of bile contained in the faeces, (5) the gaseous and watery losses at the lungs, (6) the exuvi ated horny scales of skin and nails and hair, (7) the pro ducts of the generative organs, but not those constituents of faeces which are but the undigested remains of food. Among these (if we except the milk and the generative secretions whose elaboration is so peculiar and exceptional) the urine and the excretory products of the lungs and ,skin are those of paramount importance. The excretion from the lungs will be treated under RESPIRATION ; we shall therefore here concern ourselves with the excretion of kidneys and skin. Kidneys and their Excretion. Urine is a clear amber- coloured fluid, somewhat acid in reaction, with a peculiar aromatic odour and bitter saline taste. Its specific gravity varies, consistently with health, within wide limits, being affected very greatly (a) by the quantity of liquid consumed by the individual in a given time, (6) by the greater or less activity of the secretion of sweat. Whilst the average specific gravity may be stated to be about T020, it is often temporarily much lower and occasionally considerably higher. As a rule the specific gravity is higher in summer than in winter. The average quantity of urine passed by a healthy adult may be reckoned at 52 fluid ounces, though it is affected by the same causes as those which influence the specific gravity no less than by individual peculiarity and other circumstances. The urine is essentially a watery solution of certain organic matters, of which much the most abundant and important is urea, and of mineral salts, of which the most abundant is common salt (sodium chloride), Urea. A well -nourished man passes, on an average, about 33 grammes (500 grains) of urea in twenty-four hours. This body, which has the composition CH 4 N 2 O, is looked upon by chemists as carbamide, i.e., it is the amide of carbonic acid, and on this view may be written CO(NH 2 ) 2 . It is isomeric with ammonium cyanate. This body represents the chief product of the metamorphosis of albuminous or proteid substances in the body. Even during starvation the destruction of proteid matter continues, and the amount of urea may reach about 230 to 310 grains. The quantity is, however, specially and directly affected by the amount of proteid matter contained in the food ; so that, for instance, by consuming a sufficient quantity of meat certain persons have excreted as much as 1540 grains of urea. The excretion of urea, which is a measure of the metamorphosis of proteids in the body, is not, as was formerly supposed, directly influenced by the amount of mechanical work done by the body, the greater activity of the muscles not being necessarily accompanied by their greater waste ; indirectly, however, the excretion of urea is increased by bodily exertion, inasmuch as man and other animals appear instinctively to desire and to need a larger quantity of proteid food when much work is to be done than when the body is comparatively at rest. It was formerly believed that urea was a direct result of the oxidation of proteids in the body. The view actually held is, that it is the product of a synthesis occurring in the body in which a body or bodies resulting from the oxidation of the proteids take part. It has recently been shown that when salts of ammonia are introduced into the system the nitrogen of the ammonia is excreted as urea ; in this case it is obvious that a synthesis must have occurred in which the ammoniacal salt introduced has ^ taken a part no less than some other body. It has, for instance, been surmised that in the oxidation of the proteid molecule cyanic acid arises, and that in the presence of ammonia ammonium cyanate is formed, which in the organism, as outside, may be changed into urea ; or, again, that cyanic^acid is formed, which, uniting with the elements of water, gives rise to urea, carbonic acid being separated in the reaction. Uric acid combined with bases exists in small quantities in the urine of man, about 7 to 10 grains being passed by an adult in twenty-fear hours, though individual peculiarities appear to affect in a remarkable manner the excretion of this constituent (maxi mum limits of quantity 3086 to 15 43 grains). In some animals, as in snakes and birds, uric acid and its salts represent the chief nitrogenous excretory product, i.e., take the place of the urea of the urine of man and carnivorous animals. Uric acid has the composition C 5 H 4 X 4 3 . Its constitution is yet involved in great doubt, in spite of the most profound and ex tensive researches which have been made on its more or less imme diate relations, and in spite of the fact that its synthesis lias recently been effected. It may be looked upon in all probability as a derivative of cyanic acid, CNOH, and glycocin, C 2 H 5 N0 2 , and there are many facts which seem conclusively to show that in the system it does not arise directly from a decomposition of proteids, but is the result of a synthesis in which derivatives of these bodies take a part. Xanthin, C 5 H 4 5sT 4 2 , hypoxanthin, C 5 H 4 N 4 0, and giianin, C 5 H 5 H 5 0, are immediate derivatives of uric acid, of which the first is a constant constituent of urine, and the second occurs in certain states of disease. Hippuric acid, C 9 H a N0 2 , benzo-amido-acetic acid, is present in small quantities in the urine of man (about 15 grains a day), but is the chief nitrogenous constituent of the urine of herbivorous animals. It is formed whenever benzoic acid, or a body which can yield benzoic acid in the organism, is ingested. In addition to the organic matters already enumerated the urine contains small quantities of other bodies, amongst which may be mentioned (1) colouring matters, of which one alone is known with any degree of accuracy, and is termed urobilin, a derivative proxi- mately of the biliary colouring matters, and more remotely of the blood-colouring matter, and (2) minute traces of ferments. As has already been said, the chief salt of the urine is common salt ; there are, however, also excreted in the urine considerable quantities of phosphates and sulphates. The two latter salts re present in part similar salts introduced as such into the system, but in part they are the products of oxidation of organic bodies which contain sulphur and phosphorus respectively; to the former belong the proteids, to the latter certain phosphorus -containing fatty bodies of great complexity, which occur widely diffused throughout the body, but particularly in the white matter of the great nerve-centres and in the nerves. The changes which urine undergoes after it is passed may be briefly referred to. The faint acid reaction proper to freshly-drawn urine is increased, probably in consequence of an acid-fermentation process started by the mucxis accidentally present. The visible effect of this is the deposit of acid urates or free uric acid. In a short time, especially if the urine be kept at a warm temperature, the acid-fermentation is overwhelmed by a fermentation for which organic germs are needed, and which leads to the resolution of urea into carbonate of ammonia. The reaction becomes alkaline, and the smell strongly ammoniacal. Precipitates of ammonia, urate, and ammonio-magnesian phosphate are deposited, the odour becomes putrefactive and multitudes of micro-organisms develop whose germs have been derived from the surrounding air. Kidney. The urine is excreted continuously in the kidneys, two organs situated at the back of the abdominal cavity. The fluid is continuously poured by two ducts called ureters into a common reservoir situated in the pelvis, and known as the urinary bladder. From this reservoir the urine is intermittently ejected by the urethra^ _ The two kidneys never secrete symmetrically; they exhibit an alternation of vascular and secretory activity. Similar variations have been observed in the different portions of one kidney, first one and then another region of the kidney will be found to be in full activity. Nevertheless, when one kidney is extirpated or unfitted for its function, the other may be capable of the whole work of excretion. The excretory portion of the kidney, like the secretory